P2138 GMC - Only when engine is cold

Question

I have a 2004 Envoy with the infamous P2138 diagnostic trouble code (only; no other codes come up). Most info I can find on this issue have led me to believe that replacing the Accelerator Pedal Position (APP) sensor is not the right thing to do, at least initially. Many of the causes seem to be related to wiring (bad harnesses/connectors or wet intrusion into them). A lot of the folks reporting this seem to encounter it after a rain or car wash, but in my case it seems to be temperature related. Ie: every time I drive with a cold/cool engine (ie: after sitting overnight), I encounter this. As soon as I put in reverse/drive, and touch the gas pedal, the Reduced Engine Power and Service Engine Soon lights illuminate. Once I let the engine warm to 150 or so, the Reduced Engine Power light goes off (and eventually the SES light, after a few shutdown/start cycles)

So I'm wondering if my issue is related to something faulty with a component that measures temperature in some way? Ie: could it be related to the fan or fan clutch? Or some type of choking mechanism on the throttle body?

I've seen some TSBs (#07-06-04-019A, #08-06-04-041B), that talk about inspecting the "Instrument Panel to Body Harness Connector", but I could never find it (nothing behind the driver's side kick panel). I've look at all of the other harness connectors that seem relevant (the ones on/near the throttle body, the one next to the radiator/fan, etc.)

O'Reilly Auto Parts also provided some info on P2138, and two of the reported fixes are replacing the fan and replacing the fan clutch, which is another reason for my suspicion about something temp related...

Thanks very much!

Answer 1

P2138 APP sensor 1-2 correllation

What you have is the famous mediator problem with APP1 and APP2  The TPS. You have two seperate sensor each with its own ground. If TPS sensor 1 has a dropout or a glitch you go to reduce power mode, you can replace the TPS as a seperate unit, and here the TSB diagram

Comments

Thanks Reynaldo.  I continue to have the problem, but 95% of the time, it happens when I first start it in the morning.  If I let it warm up (roughly 160 or higher), it does not occur.  The only 2 other times it has happened  was when I was rounding a sharp corner.  

I've read the TSB, and it says to look for water leaking into the "Instrument Panel To Body Harness Connector".  I have no water leakage problems though; it happens when the vehicle has been dry for days.   I've also read that other components that share the 5v reference/return lines could be triggering this (ie: fan clutch sensor, fuel pressure sensor, etc.), but why would it only/typically happen before the engine is up to operating temp?  Unless there is a pressure related issue?

Thanks!
Bryan

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You have two 5volts reference that the ECM monitor, if the fan clutch or sensor ect. shorted, You will have a code that state 5 volts reference low, Your not loosing the 5 volts at all, The TPS does not have to code up at all,any time you get a reduce power its either due to a misfire,or the APP sensor or TPS itself.On your throttle body you have 2 TPS sensor they are potenoimeter,both have separate ground and 5 volts reference. One of the TPS sensor is regulated by the PCM for diagnostic purpose,If either TPS sensor glitches you go to reduce power, Both of the TPS being a potentiometer do wear out over time,just by starting a vehicle during a cold start demands more fuel,a lack of fuel will not set a reduce power,you will have a lean code. The engine coolant temperature is monitor by the PCM by a timer, if it doesn't reach closed loop within a set amount of time by given factors,you will code up a temperature issue.A dirty or comaminated MAF can set a reduce power but this is only at warm up temperatures , (very rare conditions)and  the vehicle will stall when coming to a stop,a MAF cleaning will solve that problem.You have a TPS problem,You do not need to replace the APP, just the TPS,

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OK, thanks again.  So is it possible then, that when the engine gets to temp (and thus, the TPS assembly heats up), the heat is causing the TPS to expand and be less likely to throw the P2138?  I'm still not clear on why this usually happens only when the engine is cold.

Also, if I were to replace the TPS, is it part of the motor unit mounted on the throttle body, that controls the throttle?  Or is it somewhere else?

Thanks,
Bryan

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The TPS is on the throttle body, You can do this as a fix, GM recommends this, You can use their brand they sell it at O reillys, Its MAF sensor cleaning, Just follow the instruction on the can,And there is a throttle body cleaning,again follow the instruction on the can, If you think about P2138 its always about a intermittent conection that has occurred between APP1 and APP2 for less than 1 second, Belive it or not most electrical intermittent was repaired by disconnecting and reconnecting a componenet, You can get a small tube of dielectric grease and a small paint brush at a hobby store,(the small one) just dab a tiny wipe in the terminal of the APP (the gas pedal) and install the connector, then go to the throttle body this is where the TPS and TAC motor are disconnect the connector and do the same dab there again not to heavy just a light coating, by the way there is a TSB about GM putting a heavy amount on their connection causing dash gauges to go intermittent, and their instruction for repair is to use electrical cleaner to make better contact,and using very light dielectrical lube,again connection need this dielectrical grease why corrosion will build there, where electricity flows so does electrolytes,and it attracts moisture.So 3 things clean the MAF sensor why,The DOT has made it mandatory that all airfilter have fire retardant sprayed on the filter due to transportation reason, and what happens is a lot of MAF gets contaminated, and when they get contaminated it leans the engine system out,and there is the famous P0171 and P0172 codes,As for the throttle body the motor oil has change world wide, If you get one tablespoon of todays motor oil and throw it on a hot exhaust you will see very light fumes,you get oil from 5 years ago and spill 1/4 tablespoon you get a big blue cloud,so todays oil does not leave a trace but leaves a gummy film on the throttle body, So do a throttle body cleaning, You do all the three you will not only save money on fuel,The vehicle will idle smoothly,and above all may even resolve P2138

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The TPS is on the throttle body, You can do this as a fix, GM recommends this, You can use their brand they sell it at O reillys, Its MAF sensor cleaning, Just follow the instruction on the can,And there is a throttle body cleaning,again follow the instruction on the can, If you think about P2138 its always about a intermittent conection that has occurred between APP1 and APP2 for less than 1 second, Belive it or not most electrical intermittent was repaired by disconnecting and reconnecting a componenet, You can get a small tube of dielectric grease and a small paint brush at a hobby store,(the small one) just dab a tiny wipe in the terminal of the APP (the gas pedal) and install the connector, then go to the throttle body this is where the TPS and TAC motor are disconnect the connector and do the same dab there again not to heavy just a light coating, by the way there is a TSB about GM putting a heavy amount on their connection causing dash gauges to go intermittent, and their instruction for repair is to use electrical cleaner to make better contact,and using very light dielectrical lube,again connection need this dielectrical grease why corrosion will build there, where electricity flows so does electrolytes,and it attracts moisture.So 3 things clean the MAF sensor why,The DOT has made it mandatory that all airfilter have fire retardant sprayed on the filter due to transportation reason, and what happens is a lot of MAF gets contaminated, and when they get contaminated it leans the engine system out,and there is the famous P0171 and P0172 codes,As for the throttle body the motor oil has change world wide, If you get one tablespoon of todays motor oil and throw it on a hot exhaust you will see very light fumes,you get oil from 5 years ago and spill 1/4 tablespoon you get a big blue cloud,so todays oil does not leave a trace but leaves a gummy film on the throttle body, So do a throttle body cleaning, You do all the three you will not only save money on fuel,The vehicle will idle smoothly,and above all may even resolve P2138

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Thanks again Reynaldo.  I really appreciate the time and thorough answers.

I should have mentioned before, some of the things I've already done, which include the items you mention above.  I've put a small amount of dialectic grease on all of the connectors I can find (APP, TPS, ECM/PCM, Electric Fan, etc.).  I've also made sure there is no moisture entering the body and flowing down the A pillar (something the TSB called out).  Last weekend I cleaned (did not replace) the MAP with MAF cleaner; mine is the L6 and apparently only the V8 has the MAF?  I also took the throttle body off and sprayed thoroughly with cleaner.

So at this point, I'm pulling my hair out, because it still seems temp (or pressure?) related.  I feel like all I can do now is start the process of elimination by replacing [expensive] parts (ie: TPS, accelerator pedal/sensor assembly, fan clutch, etc.)

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Remember the code , The ECM see something we cant,You don not need a fan clutch, or a accelerator pedal, Even if you had a labscope, or a dealer scantool,you will not see it or catch it, satisfy the code, The mediator is the TPS, nothing else can do this if its faulty during cold weather then that's the personality of the TPS, There is no other sensor that can do this.If you lost the 5 volt reference through such as the fan clutch the ECM will respond with a logic code,(loss of 5 volts reference).You have P2138 APP1 to APP2 correlation.

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OK, thanks very much, again, Reynaldo.  I'm very appreciative of your time and advice!  I'll be ordering a new TPS (or throttle body).

Bryan