Wednesday, August 19, 2015

EPC Q & A CONTINUED

EPC Q & A CONTINUED

It appears that more and more VAG drivers are experiencing Limp Mode with their VW, Audi , SKODA and SEAT vehicles and it appears that service agents are not as competent as expected. This urged me to present 3 more limp mode cases on a VW Beetle, VW Golf and a Golf TDI for the benefit of those with similar vehicles.

Question?

Hello my name is Mariiam Lopez. I have a 2002 VW beetle. I had been having problems with my car for more than a year. First started with the EPC LIGHT on, I had it on for a few weeks and it came and went, but then my car started to lose power while driving. I had to park and turn it off. I would wait a few minutes and then I started it again and the light went off . But then the EPC light stayed on and the speed didn't go up 10 miles (limp mode). I went to a mechanic and he said it was the throttle body. I had it changed. Two days the EPC LIGHT got on again. I took it again to another mechanic and he said it was the accelerator sensor. I change it and I even got the pedal. Next week got the EPC LIGHT on. It was driving regularly until it didn't want to turn on. I let my car rest for more than two months. I stared using it for a few weeks now and the EPC LIGHT goes on and off when I turn off. Now while driving in the freeway my car lose power and I have to park and let it rest for a few minutes and it drives fine but after a few minutes EPC LIGHT turns on. Now my car turns off in the middle on the freeway and all the lights on the dashboard are on. I park and turn on again but I have to accelerate in order to make my car work. Also the red oil light goes on and beeps when EPC light is on. My car has the same radio. Thanks

Answer!

Hola Mariam como estas? I so sorry to hear that you've had so much trouble with your VW Beetle and for such a long period of time. I can relate to what you are going through every time you need to go somewhere, especially at night when you have to drive a car that is completely unreliable. Considering that two mechanics, both of whom guessed that your throttle body and your accelerator sensors were faulty and that both parts were  replaced and your Beetle is still faulty, makes you saint with the patience of biblical Job. But your mechanics are not alone, so many VW service agents replace parts on Beetles and other VAG vehicles on a regular basis which often doesn't fix the problem, making car owners constantly despondent with VW and their "technical personel". At this point in time  I would be very hesitant to suggest that you replace any component without a diagnostic scan of your Beetle. It would have been great if you can get a copy of your Beetle's scan from one of your mechanics and upload it, so that I can  properly analyse it. Mind you, the data in group 60 - group 66 would  really be helpful to determine whether or not the adaptation was successful.  The EPC light problem can be caused by some many things and in your case its not your radio. So I suggest you check the fluid levels of your oil and coolant and make sure the coolant isn't leaking from reservoir bottle onto the wiring harness, if it does, the instrument panel is bound to malfunction. Also check that your fuel cap seals properly and check the vacuum hoses from the brake booster to the secondary air inlet valve and the hose to the combi valve. If this doesn't solve your problem, read through some of my other blog pages to get a better idea of what must be checked. I don't wish to scare you but Beetles especially MY2000 are well know for electrical problems and catching alight with the flames originating from underneath the firewall,  burning its way up the wiring harness, melting the fuse block on top of battery. So do take care.

________________________________________________________________

Question?

Hello my friend. I'm Charles Gunn and I have a VW golf mkv fsi 1.4 2005, which I bought in December of 2007. Since I bought her, she's been going into limp mode when I least expect her to, and have had this problem for the past three years. In my endeavours to fix it, I've tried so many garages, non of whom  could help. Worst of all a VW dealer charged me well over £1200, for replacing the wrong components, making me go back and forth to them, yet they never sorted out the problem. Furious... My VW golf goes into limp mode regardless of weather conditions and especially on long drives and on the motorways whenever I exceed 100kph. I found switching off the engine and letting it cool before staring helped but this was temporary because when it becomes hot, limp mode returnes. She frequently lost power when going up hill or on an incline to a bridge. I've done lots of internet  research and tried most forums but no luck. Thus far I've replaced the NOx sensor, MAF sensor, four coil packs and a fuel rail pressure sensor and an air filter. I spent over £1500 and still have a limp mode problem. Can you please help me? The fault that showed up on the scan is; 

1 Fault Found 
16575 - Fuel Pressure Sensor (G247): Implausible Signal 
P0191 - 008 - Implausible Signal - Intermittent 
Readiness: 0000 0000

Answer!

Hi Charley, I must commend you for your perseverance and for your gusto to kick limp mode's ass. Three years of limp mode is probably more than most people can bear. Anyway I see that you replaced a NOx sensor even though your Readiness is 0000 0000. A faulty NOx sensor would have interfered with your short term fuel trim, which would have resulted in a 0010 0000 readiness. A faulty MAF or air filter would also  have also interfered with your readiness which would have resulted in 0001 1000 reading. I'm so sorry to say that you replaced these items, because according to your readiness monitors they are not he cause of your troubles. The coils you replaced can cause limp mode but would also causes the engine to vibrate and idle poorly which would have also given you a P0300 (misfire) error code, so I think they were also replaced unnecessarily. The fact that you replaced the fuel rail pressure sensor says that your were definitely on the right track as pointed out by theP0191 fault, which may have been faulty hence sending intermittent signals to the ECU. However I am more inclined to think that your submersible fuel pump inside the tank is the culprit since it is responsible for creating the fuel pressure, which  is absolutely necessary to drive the injectors. But before you replace the pump, monitor the voltage supply to the pump since corrosion on the fuse in the fuse box that supplies the fuel pump voltage could cause a very similar symptoms.


 ________________________________________________________________

Question?

Hi, I need your advice  and please go easy on me because I'm a woman, who  knows very little about cars.  I have a Vw Golf GT TDI 2.0 with 81000 miles on the clock, which I bought about a month ago and it goes into limp home mode, whenever I get to about 70 mph and 3000rpm's but it comes right  when the engine is turned off and back on again. The local garage  ran a dianostics check and they suggested that I either put some diesel cleaner into my tank or use cleaner diesel as they think it's the turbo sensors that  gets clogged-up with soot! I've done some internet research and forums suggested I check the MAF.  So yesterday i drove the Golf untill it went into limp mode, then I switched off the engine and unplugged the MAF.  I could feel that the turbo wasn't working, not as responsive as it usually is. Anyway I drove upto 75 mph, when limp mode usually kicks in. Nothing happened and I increased my speed to 85mph and limp mode still didnt make an appearance. I stopped the Golf, switched off the engine  plugged MAF back in and sure enough limp mode kicked in at 75mph, do you think I need to change the MAF or could it be something else causing the problem?


Answer!

Hi there. It really would have been great if you had a diagnostic scan from which to work but since you don't have one, analysis is bound to be difficult and probably inaccurate. A MAF sensor can cause a limp mode home fault but it doesn't not mean it is faulty in your case. By disconnecting the MAF sensor the OBD System Component Monitor would immediately determine that the  mass airflow  sensor circuit voltage is outside an acceptable range. Thus One of two things could happen:-

1. The ECU will prevent the car from starting.
2. The engine will start but may trigger the malfunction indication lamp (MIL) and the car would also smoke profusely. Since the ECU cannot achieve the required stoichiometric ratio, it will trigger a DTC P1101 in memory. If the ECU supports "failure mode effects management" (FMEM) it will  default to a ‘safe’ mass airflow value, allowing the the vehicle to be driven to workshop for repair. Somehow I think you may not have unplugged the MAF but it is worth your while cleaning the MAF but be careful not to damage the wire inside.  Since  your car is Diesel I somehow doubt that is would be the MAF .


Question?

Nazeem Sterris. Hi.I have a polo 1.4 16v bby.problem is epc light stays on. It starts well.revs up well but has a slight miss fire when idling.after couple of minutes of idling it goes into a limp mode where revs go up to 1500rpm and pedal don't work. Pls help

Answer?

Hi Nazeeem. The slight miss you are describing is most likely the cause of your EPC problem. Sorry, I cannot pinpoint the problem for you because  you don't have a diagnostic scan attached, I'm guessing that the miss could be associated with your Ignition Coils, your Fuel Injectors,  or your Throttle Body, or your Fuel Pressure Regulator Valve or even your Wastegate Bypass Regulator Valve. You need to work through them systematically but first check that your fuel cap seals because if it doesnt, the EVAP pressure cannot stabilize  and can also trigger the EPC circuit.  


Tuesday, August 4, 2015

POWER STEERING FAILURE

POWER STEERING FAILURE 

STIFF STEERING / STEERING WHEEL STIFF

It was a perfect sunny day  with hardly any breeze, when my wife and I decided to go for a Sunday outing. We loaded the  VW Polo 2.0L Highline, with picnic umbrella, and packed a picnic basket filled with food and refreshments.  I mounted the bicycle holder on the tow-bar ball and attached our 2 bicycles, and off we went. But our joyous mood  was  abruptly interrupted about halfway to our destination when the Polo had  a sudden steering problem.  This is probably the worst thing that can happen to any unsuspecting driver whilst driving in a relaxed state. As I was entering into a long but sharp curve, my power steering kinda "froze for a second" .  I was steering with one hand — which is all it normally takes  — because the steering wheel is so light. But before I could respond to grip the steering with two hands, I also instinctively braked and with screeching tyres, I hit the pavement so hard that both front airbags popped.

The steering wheel had suddenly become hard to turn / stiff and tight /difficult to turn.  Kinda difficult to explain.  A stiff hard to turn steering wheel as if there is no power steering.  It was light to steer when suddenly it was hard to turn the steering wheel, because the steering wheel went stiff.  A stiff steering when turning. The steering wheel becomes stiff and hard to turn. I hope you know what I mean. Fortunately there wasn't a another car involved and even more fortunate nobody got injured except my pride. The first thing that went through my mind after calming the wife down and confirming that she was ok, was to determine the extent of the damage. 

As the airbags deflated I looking at the dashboard and saw all the light on the dashboard glowing. The yellow EPC light was on, the yellow steering wheel light (K161) was on, the ABS light was on, the red battery light was on, and the yellow brake pad light was on. The engine had switched off  and I realized the ignition was still on.  So I tried started the engine and it wouldn't take, I switch the ignition off and tried again, all the light went out and the car idled. On further inspection, I saw the rim that hit the curb was deformed and the tyre was flat. I figured I could just fit the spare and we would be on our way. As I reversed away from the pavement,  the steering wheel  was back to normal, it was light again.  After fitting the spare wheel, I  turned the steering wheel two full turns in either direction with one hand or rather my fingertips.  This was very odd, even peculiar, because the Polo just came back from its 100k service / cam belt replacement a short while ago. I'm certain if there was any problem with my steering wheel my machy would have told me but he said absolutely nothing. Anyway, so we eventually get home and as I turned into my driveway the steering goes hard/stiff/difficult to turn once again. As I drove forward the steering wheel returned to normal. So I popped the bonnet, checking for anything out of the ordinary. I rolled under the Polo, checked the steering control module, looked at the steering pump (V187), yet it all checked out fine. Out came VCDS and I scanned the Polo and found the battery connection to be intermittent. Look at the 18010/P1602 error below.

Chassis Type: 9N - VW Polo
Scan: 01,02,03,08,09,15,17,19,25,37,44,45,46,56,76
---------------------------------------------------
Address 01: Engine       Labels: None
Controller: 06A 906 032 PB
Component: 2.0l ME7.5.10       0305
Coding: 00071
Shop #: WSC 01120
AAVZZZ9NZ7UXXXXX     VWZ7Z0G52XXXXX
1 Faults Found:
18010 -  Power Supply Terminal 30: Voltage too Low
P1602 - 35-10 -   - - Intermittent
Readiness: 0000 0000
---------------------------------------------------
Address 15: Airbags
Control Module Part Number: 6Q0 909 601 F
Component and/or Version: 05 AIRBAG VW5  0010
Software Coding: 12341
Work Shop Code: WSC 31414
6 Faults Found:
00595 -  Crash Data Stored
35-00 -   -
01214 - Crash Data for Belt Tensioner Stored
35-00 - -
00588 - Airbag Igniter; Driver Side (N95)
32-00 - Resistance too High
00589 - Airbag Igniter 1; Passenger Side (N131)
29-10 - Short to Ground - Intermittent
00589 - Airbag Igniter 1; Passenger Side (N131)
32-10 - Resistance too High - Intermittent
01280 -  Airbag; Passenger Side; Disabled
35-00 -   -
---------------------------------------------------

Thereafter I decided I was going to take the Polo back on the road. So I reversed out of the driveway and suddenly the problem was back. The only thing I did was climb the 30 mm coping at the edge of the driveway so I drove back into the driveway and as the  back wheels hit the driveway copping the steering was fine once more. Since the steering pump (V187) actually stops working for the second or so, it had to be something to do with the power. So I pulled on every visible cable that goes into the wiring harness to the steering pump (V187). I even checked the enclosed fuses on top of the battery when I noticed that the negative battery terminal wasn't properly tightened.  As I tightened the nut I discovered the nut was stripped and wouldn't tighten any further. Then realized this had to be the problem. I scratched around in the garage, found a screw that would work, tightened the battery terminal and reversed out of the driveway and drove it back in a few times and the problem didn't reoccur. So I called my wife and asked her move the steering wheel to and fro with her hand while standing outside the Polo with her hand  through the open window. I loosened the battery terminal and disconnected the battery while the polo was idling. When it was disconnected my wife couldn't move the steering wheel and when I replace the terminal she could. So I was totally convinced that the intermittent battery contact caused intermittent power to the pump which is exactly what happened a second before I fit the pavement.  
So now I had to track down the cowboy who stripped by battery terminal nut because the  last time I took the battery out to replace my headlight bulbs I personally tightened it and am darn sure it wasn't stripped. This was roughly a week before it went for that major service. It just had to be one of the workshop guys that was responsible so like three days later I went to my VW /Audi machy's workshop to complain and demanded compensation for the damage to my car but got the saddest news. The same Sunday that my wife and I want for our picnic my VW/Audi machy was out biking and was killed in a head-on collision and died on the spot. 

Here are some VW, Audi power steering problems listed below, all of which seem to have a (G85) sensor issue.

Address 03: ABS Brakes
Controller: 7L0 907 379 G
Component: ESP ALLRAD MK25 0203
Coding: 0006402
Shop #: WSC 3141
00778 - Steering Angle Sensor (G85)
000 - -

Address 34: Level Control
Controller: 7L0 907 553 F
Component: LUFTFDR.-CDC- 3C3P1 3081
Coding: 0015521
Shop #: WSC 02631
00778 - Steering Angle Sensor (G85)
004 - No Signal/Communication

Address 03: ABS Brakes
Part No: 6Q0 907 379 M
Component: front H02 0001 ESP 5.7
Coding: 0000000
Shop #: WSC 00000 000 00000
00778 - Steering Angle Sensor (G85) 
005 - No or Incorrect Basic Setting / Adaptation

Address 44: Steering Assist
Part No: 1K1 909 144 J
Component: EPS_ZFLS Kl.5   D04 1606
Shop #: WSC 00000 000 00000
00778 - Steering Angle Sensor (G85)
000 - -

Address 03: ABS Brakes    
Part No: 1K0 907 379 K
Component: ESP FRONT MK60      0104
Coding: 0021121
00778 - Steering Angle Sensor (G85) 
005 - No or Incorrect Basic Setting / Adaptation

Address 44: Steering Assist Labels: 6Q0-423-156.lbl
Part No: 6R0 423 156 B
Component: LenkhilfeTRW V277
Coding: 11221
Shop #: WSC 06402
00778 - Steering Angle Sensor (G85) 
49-00 - No Communications

When I surfed the net, I discovered that several other VW owners experience similar problems. Here a dude who says:- "The dealership keeps giving my car back to me unfixed yet still replacing parts! The dangerous part of this is the steering locks up when the car dies and i'ts still moving. I am fearful of driving it and I haven't gotten anywhere with VW.  I am not alone and that it seems to be the same particular year and engine as mine in most owners replies.  I would like to help find a solution before any serious accidents happen or anyone is hurt. Thanks".

The Steering Angle Sensor (G85) is an opto coupler fitted in a collar under the steering wheel airbag. It sends sends the  steering wheel position to the Power Steering Control Module (J500) at the base of the steering column interfacing with the Electro-mechanical Power Steering Motor V187 all of which need power to operate properly. If the power disappears, the pump doesn't work and the steering goes stiff until the power is restored.


steering wheel stiff, steering wheel stiff hard, Steering wheel hard to turn, steering wheel becomes stiff and hard to turn,  steering is very tuff and its hard to turn, Steering stiff and hard to turn, Stiff Steering,  stiff steering when turning, stiff hard steering, stiff hard steering wheel stiff and tight, hard to turn the steering wheel, hard to turn your steering wheel,

Monday, August 3, 2015

VAG FUEL TRIM PROBLEMS

SHORT TERM FUEL TRIM ISSUES

Modern day OBD-II systems can be described as high-end electronic systems that "sort of" took the automotive industry by surprise. As such , there are so many motor mechanics that have not made the transition from technologies prevalent in older model cars to the technologies pervasive in newer model cars yet, and understandably some never will.  Several of these motor mechanics don't even own a scan tool and even fewer of them are able to interpret the DTCs and the results of a diagnostic scan. With the  result, that many VW owners took it upon themselves to become ODB-II savvy and even do their own automobile repairs. Many of whom have invested in low end scan tools like VAG COM, Actron, Nextech Carmen,  Foxwell, OBD Scan, Altar, etc, and others invested in high end scan tools like G-Scan, VCDS, Xtool, Launch, AutoHex, Autel and Auto Boss etc. Even though the VW service departments are equipt with the best diagnostic scan tools they tend to lack the technical personnel with the requisite expertise to understand them and effectively repair clients VW, Audi, SEAT and SKODA cars. Electronics has taken over the modern day car and being knowledgeable about electronics is key however not every car owner  is therefore not equipt to to analyse the diagnostic printout. As an example, lets take a look at the infamous VAG fuel trim problems list below. All of them very loudly shouts that the stoichiometric  ratio is off. Generic Short Term Fuel Trim (STFT) DTCs range from P0170- P0175 which are generic government required codes and Long Term Fuel Trim (LTFT) range from P1123-P1130, P1139-P1139,  P1151-P1152, P0166-P1167 and all are manufacturer specific.

SHORT TERM FUEL TRIM (STFT)


16554 - P0170 - Fuel Trim, Bank1 Malfunction
16555 - P0171 - Fuel Trim, Bank1 System too Lean
16556 - P0172 - Fuel Trim, Bank1 System too Rich
16557 - P0173 - Fuel Trim, Bank2 Malfunction
16558 - P0174 - Fuel Trim, Bank2 System too Lean
16559 - P0175 - Fuel Trim, Bank2 System too Rich

LONG TERM FUEL TRIM (LTFT)

17531 - P1123 - Long Term Fuel Trim Add.Air, Bank1 System too Rich
17532 - P1124 - Long Term Fuel Trim Add.Air, Bank1 System too Lean
17533 - P1125 - Long Term Fuel Trim Add.Air, Bank2 System too Rich
17534 - P1126 - Long Term Fuel Trim Add.Air, Bank2 System too Lean

17535 - P1127 - Long Term Fuel Trim mult, Bank1 System too Rich
17536 - P1128 - Long Term Fuel Trim mult, Bank1 System too Lean
17537 - P1129 - Long Term Fuel Trim mult, Bank2 System too Rich
17538 - P1130 - Long Term Fuel Trim mult, Bank2 System too Lean

17544 - P1136 - Long Term Fuel Trim Add.Fuel, Bank1 System too Lean
17545 - P1137 - Long Term Fuel Trim Add.Fuel, Bank2 System too Lean
17547 - P1139 - Long Term Fuel Trim Add.Fuel, Bank2 System too Rich

17559 - P1151 - Bank1, Long Term Fuel Trim, Range 1 Leanness Lower Limit Exceeded
17560 - P1152 - Bank1, Long Term Fuel Trim, Range 2 Leanness Lower Limit Exceeded

17573 - P1165 - Bank1, Long Term Fuel Trim, Range 1 Rich Limit Exceeded
17574 - P1166 - Bank1, Long Term Fuel Trim, Range 2 Rich Limit Exceeded

17582 - P1174 - Fuel Trim, Bank 1 Different injection times

THE DEFINITION OF A BANK

Before we continue, it is important to get certain definitions right. In 8 cylinder (W8) engines and 12 cylinder (W12) engines, 4 or 6 cylinders are staggered  aligned at a V-angle,  72 degrees in relation each other, thereby making the engine more compact.  Each staggered row of either 4 or 6 cylider has its own head,  which is called a "Bank",  hence Bank 1 and Bank 2. On the other hand VR6 engines cylinders are also staggered but has a single cylinder head, however the three left most side cylinders (odd numbers) are called "Bank 1" (passenger side left-hand drive) and the right most cylinders (even numbers) are called Bank 2. Four cylinder engines normally have 4 in-line cylinders but here as well, the odd cylinders are called Bank 1 and the even cylinders Bank 2 as can be seen in the "Chassic type: Skoda Fabia  1,2l/4V" below. But this configuration does apply to all 4 and 5 cylinders engines because in some engines all cylinders are referred to as bank 1. Looking at  the above P-Codes  P0170-P0175 it can clearly be seen that  P0107 refers to Bank 1 and is common to both  P0171 and P0172. And likewise P01703 refers to Bank 2 and is common to both  P01704 and P0175. This information allows us to determine which cylinder is bank is running rich or lean. Intermittent signals may be due to continuous STFT occurrences or the bad electrical connections at the o2 sensors. However, every time the engine is started the OBD-II system  does a self test on the O2 sensors,  and should the Check engine light not remain on, then the O2 sensor is probably not the culprit.
______________________________________________________________________

Chassis Type: 6N - VW Polo
16554 - Fuel Trim: Malfunction: Bank 1
P0170 - 35-10 - - - Intermittent

Chassis Type: 6N - VW Polo
16555 - Fuel Trim; Bank 1: System Too Lean
P0171 - 35-10 - - - Intermittent

Chassis Type: 6Y - Skoda Fabia
16555 - Fuel Trim: System Too Lean: Bank 1
P0171 - 35-10 - - - Intermittent

Chassis Type: 3B - VW Passat B5
16555 - Fuel Trim: System Too Lean: Bank 1
P0171 - 35-10 - - - Intermittent

Chassis Type: 9N - VW Polo
16555 - Fuel Trim; Bank 1
P0171 - 35-10 - System Too Lean - Intermittent

Chassis Type: Audi 3.2
16556 - Fuel Trim; Bank 1
P0172 - 35-10 - System Too Rich - Intermittent

Chassic type: Skoda Fabia  1,2l/4V 
16557 - Fuel Trim: Malfunction: Bank 2
P0173 - 35-10 - - - Intermittent

Chassis Type: 6N - VW Polo
16556 - Fuel Trim; Bank 1
P0172 - 35-10 - System Too Rich - Intermittent

Chassis Type: 3B - VW Passat B5
16556 - Fuel Trim; Bank 1
P0172 - 35-10 - System Too Rich - Intermittent

Chassis Type: 1J - VW G/J/B Mk4
16556 - Fuel Trim: System Too Rich: Bank 1
P0172 - 35-10 - - - Intermittent

Chassis Type: 9N - VW Polo
16556 - Fuel Trim: System Too Rich: Bank 1
P0172 - 35-10 - - - Intermittent

Chassis Type: 6N - VW Polo 
16556 - Fuel Trim; Bank 1
 P0172 - 35-10 - System Too Rich - Intermittent

Chassis Type: 8D - Audi A4 B5
16557 - Fuel Trim; Bank 2: system too lean
P0174 - System too Lean

Chassis Type: 4F0 - Audi A6 3.2L
16559 - Fuel Trim; Bank 2
P0175 - 007 - System Too Rich

Chassis Type: 4A - Audi 100/A6 C4
16559 - Fuel Trim: System Too Rich: Bank 2
P0175 - 35-00 - -
16556 - Fuel Trim: System Too Rich: Bank 1
P0172 - 35-10 - - - Intermittent
16557 - Fuel Trim: Malfunction: Bank 2
P0173 - 35-00 - -
16554 - Fuel Trim: Malfunction: Bank 1
P0170 - 35-10 - - - Intermittent

Chassis Type: 8D - Audi A4 B5
16554 - Fuel Trim; Bank 1: Malfunction
P0170 - 92-00 - Unknown Error Elaboration
16554 - Fuel Trim, Bank 1
P0173 - 92-00 - Unknown Error Elaboration
17658 - Fuel Level too Low

Audi A4 B5 96 a4 2.8l
16558 - P0174 - Fuel Trim, Bank 2
P0174  - System too Lean  -detecting lean fuel in exhaust
16555 -  Fuel Trim, Bank 1
P0171  - System too Lean -- detecting lean fuel in exhaust
16554   Malfunction
P0170 - 35-10 - - - reached maximum amount of fuel adjustment
16557 - Fuel Trim; Bank 2
P0173 - Malfunction

Fuel Trim: System Too Rich means / Rich Mixture implying too much fuel, not sufficient air
Fuel Trim: System Too Lean means / Lean Mixture, implying too much air, not sufficient fuel

Correct combustion relies on an air/fuel mixture of 14.7 to 1 ratio. Meaning 14.7 parts air to every 1 part of fuel, but if  the air ratio dips below 14.7 parts, then the mixture is called  "rich", whereas when the air exceeds 14.7 parts, then the mixture it is called  "lean".  To keep the engine running properly, the Engine Control Module measures the oxygen content in the exhaust emissions with the oxygen (O2) sensors in the exhaust system and makes adjustments to the mixture by injecting more or less fuel. The ECU is capable of keeping the STFT air/fuel mix within  specific parameters under normal conditions, and is based on input signals  from the Barometric Pressure Sensor and the Oxygen Sensor and will make minor adjustments to the air/fuel mixture. However when these adjustments it makes exceed a internally stored predetermined level, it sets  a fault code. When DTC P0171 and P0174 are triggered, the oxygen sensors are detecting too much oxygen in the exhaust fumes instructing the ECU to add more fuel in order to maintain a correct  air/fuel mixture.  But when DTC P0172 and P0173 are triggered the reverse is implemented. When DTC P0170 and P0173 are triggered the ECU is unable to compensate for the errors and uses an internally generate signal as compensation.  The main causes of STFT DTCs are due to leaking vacuum  hoses or a poorly functioning  Mass Air Flow Sensor sensing too little air or a faulty Barometric Pressure sensor or insufficient Fuel Pump pressure. Functionality of all these sensors can be checked by verifying there scan data before attempting to replace them. Cleaning the MAF wire with electro cleaner may help but take care not to damage it. Symptoms of STFT DTC may cause the EPC light to come on, make the car go into limp mode, stall, hesitate before acceleration, idle unevenly, backfire and refuse to idle. LTFT will be dealt with in future blog.