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goodwrench9124
goodwrench9124, ASE Certified Technician
Category: Chevy
Satisfied Customers: 3420
Experience:  Ase Master Certified, GM Master Certified Tech. Awarded GM Top performer, Hybrid certified
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04 2500hd 6.0l gas having reduced engine power

Customer Question

04 Silverado 2500hd 6.0l gas having reduced engine power all wires are in good shape and all ground are tight. replaced throttle body and apps sensor twice. still having issues. auto zone gets the p1125 code everytime it does it. some times it will have a slight hesitation and then will idle high like 1800rpm followed shortly by reduced engine power and check engine light. Any thing else I can do or any ideas? Thanks
Submitted: 4 years ago.
Category: Chevy
Expert:  goodwrench9124 replied 4 years ago.
Hello and welcome, code P1125 is strictly with the pedal sensors and not the throttle body. replacing the throttle body wont repair this problem. what grounds have you checked? and when you say you replaced the app sensors, do you mean the accell pedal assembly?
Customer: replied 4 years ago.
I have replace the app sensor not the entire pedal assembly. I baught my sensors form o reilly and car quest. i checked the grounds on the rear of the block. My 2500hd has 2 grounds in the harness, black w/ white strip that ground to the block
Expert:  goodwrench9124 replied 4 years ago.
ok, I need to pull some info and some wiring diagrams, but my service info system is down at the moment. is this something you need quick? or can you wait until the system comes back up?
Customer: replied 4 years ago.
yea the black/white ground wires are located in the harness going to the tps on the throttle body the one on the left side of the block was cut so it was replaced and the right one is still intact. The main grounding straps (thick i straps) are also in good shape. I also replaced a section of the yellow wire going to the tps that had a pinch in the insolation. About how long until your system is up and running? thanks
Expert:  goodwrench9124 replied 4 years ago.

to be honest it has been up and down all day!!! just come back up. while i am doing some info research, here is what causes the code to set

 

The accelerator pedal position (APP) sensor 1 and the APP sensor 2 are potentiometer type sensors, each with the following circuits:

A 5-volt reference circuit
A low reference circuit
A signal circuit

The control module provides the APP sensors with a 5-volt reference circuit and a low reference circuit. The APP sensors then provide the control module signal voltages proportional to pedal movement. The APP sensor 1 signal voltage is low at rest and increases as the pedal is depressed. The APP sensor 2 signal voltage is low at rest and increases as the pedal is depressed. When the control module detects that the APP sensor 1 and the APP sensor 2 signal circuits are out of correlation with each other, DTC P1125 sets.

Customer: replied 4 years ago.
Do you think with all that I have replaced it would need to have a relearn performed? Also once you get a wiring diagram will it be specific enough to allow me to test the current with a probe and a multimeter for correct ohms?
Expert:  goodwrench9124 replied 4 years ago.

this system is really hard to track down the problem without a scan tool that can read real time data. in most cases this problems turns out to be a bad wire terminal on one of the circuits. this is what you are going to have to do first. I have included a wiring diagram, you will need to check these wires for resistances. none of the circuits should have more than 5 ohms. if all the circuits check ok, you may a problem with the throttle control module

 

 

Conditions for Setting the DTC

 

The PCM detects that the difference between APP sensor 1 and APP sensor 2 is more than the predicted value.

graphic

 

  1. Turn OFF the ignition.
  2. Disconnect the accelerator pedal position (APP) sensor.
  3. Disconnect the TAC module.
  4. Measure the resistance of the following circuits for each of the APP sensors with a DMM:
  5. The low reference circuit
    The signal circuit
    The 5-volt reference circuit

 

 

 

Expert:  goodwrench9124 replied 4 years ago.
no relearn procedure is nec for the app code setting
Customer: replied 4 years ago.
So turn of ignition unplug the sensor at the pedal turn on ignition and test for correct ohms at the plug that connects into the pedal sensor.
Expert:  goodwrench9124 replied 4 years ago.
no, you are going to unplug the app sensor at the pedal, unplug the control module mounted on the firewall under hood on the drivers side. with the key off, check the resistances of all the wires that run in between the app sensor and the module. this will verify that all the wiring is good. there are 9 wires total
Customer: replied 4 years ago.
Ok got yea, so if they all test fine should I assume the module is toast or do i need to check any other wiring on the throttle body sensor and blade motor. If so do you have the wiring diagram for that harness as well. Is it remotely possible that the 2 sensors I have purchased are both bad? And lastly does the install of the new throttle body require a relearn? Thanks so much for the info.
Expert:  goodwrench9124 replied 4 years ago.
on this vehicle no relearn required for the throttle body. and no other circuits to check for the app problem. if the throttle body or the wiring to the throttle body was a problem, the system would have set throttle position codes and not pedal position codes. and yes, if all the wiring checks ok between the app sensors and the module are good, most probable cause is a control module. as far a repeat sensor problems, in GM's world, we replace the entire pedal assembly.
Customer: replied 4 years ago.
Ok lastly do you have the part number for the TAC module incase I need to find one.
Expert:  goodwrench9124 replied 4 years ago.
I dont have the correct part number for the control module, but I can get it for you tomorrow am. just let m me know and I can post you the part number tomorrow
Customer: replied 4 years ago.
Ok I am going to test that harness now and I will get back to you. thanks a lot for all the info.
Expert:  goodwrench9124 replied 4 years ago.

You are very welcome. and please let me know if you need any further help with this!!!

 

Thanks for using JA!!

Customer: replied 4 years ago.
Ok so I tested the harness and all checked out fine. We then connected the harness to the app sensor and tested for proper resistance as we pressed and released the pedal. We noticed that the app3 was showing no resistance at all. We then removed the harness from the pedal and re connected it to the tac module and tested for the 5 volt reference and app 1 and 2 were fine but 3 again showed no reference. Is app 3 not an operating circuit on this model truck. (so we tested the harness not connected to anything, the with harness connected to pedal and not tac, then with harness connected to tac and not pedal) Any other ideas?
Expert:  goodwrench9124 replied 4 years ago.
the app sensor 3 is a working circuit and should test out just like sensors 1 and 2. the yellow/ black wire is the 5 volt reference from the tac module. with the tac module plugged in, and the app unplugged, the yellow wire at app should have 5 volts with the key on. are you not getting this reading?
Customer: replied 4 years ago.
I am not getting it through the tac module with app unplugged or with the harness pluged into the app sensor and not the tac. It is like app 3 is a dead circit in both my app sensor as well as the tac module. This problem is also very intermittint, all day today so far no problems.
Expert:  goodwrench9124 replied 4 years ago.
Ok, when the problem only happens at times, it is almost always a connection problem. if you noticed the connector terminals are very small, and it doesn't take much to create a connection problem. I think at this point, instead of throwing more parts at the problem, since the app has been replaced 3 times and unplugged quite abit. I would replace the connector at the pedal. GM sells the connectors with new terminals and wires, the new connector also comes with the correct crimp and seal splice connectors.
Customer: replied 4 years ago.
ok even if the connector tested ok with the dmm for under 5 ohms?
Expert:  goodwrench9124 replied 4 years ago.
yes, what can happen, the wire can partially break at the wire terminal where it is crimp together inside the plastic connector. as you drive the vehicle the wire and terminal can move just enough to cause a open for a second, causing a reduce power mode and code P1125. the system is designed as soon as it sees a fault it will set a code and default to a reduce power mode. even if it only happens for a split second. it is designed that way for safety reasons. so even the smallest connection issue on this system can be real critical
Customer: replied 4 years ago.
Sounds good I will try to get in contact with the guys at the dealer to get the connector. Lastly would this short cause the sometimes seen jump in idle that I get right before I get the reduced engine power message and the check engine light? possible the pcm trying to make up for the momentary fault? Thanks
Expert:  goodwrench9124 replied 4 years ago.
yes very possible. the pcm is pretty quick on picking up faults.
Customer: replied 4 years ago.
Ok thanks again for all the help I will get the harness tomorrow and let you know how that works.
Expert:  goodwrench9124 replied 4 years ago.

great!! keep me posted and let me know how things are going?

 

Have a great evening!!

Customer: replied 4 years ago.
Ok replaced the harness and truck is still acting up. Do you think it could be possible that the sensor from oreilly auto parts could be bad? Also i currently have it at a dealer that is running the engine diagnostics for me but they are taking for ever so I figured I would ask. The dealer mechanic said he has seen 2 like mine and he wants to replace the engine ground black/white wire back closer to the fuse box to resolve the issue. He said that this wire is spliced and tapped into in many locations and on of the splices could have gone bad. What do you think? Thanks
Expert:  goodwrench9124 replied 4 years ago.
I know the grounds on these trucks are real critical, also I am not a personnel fan of aftermarket parts. I have had good luck always using OE parts. and had some issues using aftermarket parts. by the sounds of things, I would trust the GM tech on this one. we are not perfect, but given some time we will usually track down the issue.
goodwrench9124, ASE Certified Technician
Category: Chevy
Satisfied Customers: 3420
Experience: Ase Master Certified, GM Master Certified Tech. Awarded GM Top performer, Hybrid certified
goodwrench9124 and 11 other Chevy Specialists are ready to help you
Customer: replied 4 years ago.
Can you send me the TSB for this issue if there is one. I just picked up from dealer after 5 days and they still have not fixed the problem. Test all circuits and replaced all the grounds, replaced a terminal in the app/TAC harness, and still unable to fix problem. Any other ideas? only code is still p1125.
Expert:  goodwrench9124 replied 4 years ago.
let me see what i can find out today!

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