Help! Reduced Engine Power And We Have Tried It All!

marynfairtrade · July 28, 2011

Our truck is a 2004 Sierra 2500hd and for the past year, the reduced engine power message with the check engine light has been coming on and causing issues. We have tried these methods to fix it:

1. replaced the accelerator and the sensor

2. Replaced the wiring harness to the throttle body

3. Replaced the throttle body entirely

4. Checked the grounds behind both heads

I have been researching this and have found that others experience the same message when snow gets into their air box filter (forgive me, I'm not car-savvy) or during extreme heat. The truck has experienced the reduced engine nonsense in all weather and temperature extremes. Aside from considering the computer itself is at fault, we are totally out of other options. Any suggestions?! Thank you!

demech23 · July 28, 2011

A regular handheld code reader will not clear the "Active Code" because its not a "Stored Code." You'll need to take it to a dealer or someone with a TechII or equvilent to clear. It is a real PITA...good luck...

ChevyTech77 · January 11, 2012

This really sounds like whoever is fixing this for you does not really know what they are doing. All of the fixes you posted all sound like someone thinks the engine is not getting enough "throttle" and they are throwing parts at it, hoping to hit the right part. Do not think that the computer is at fault. There are thousands more things it can be, you are not out of options.

You need to start at the beginning, and check (check does not mean replace parts) the codes, and start there. Do not do more than one change at a time.

Have you checked the fuel filter?

Have you checked the fuel pump itself?

How about plugs?

Air filter?

Actually, a reduced power message and loss of throttle control are both directly tied to the TAC, or throtlle actuator control system on this vehicle and all of the repairs he has made are all known causes for the concern he is having. The message, and system, have nothing to do with basic engine operation or concerns with the fuel or air delivery systems as you've listed them. Seems odd but true.

To the OP, I know that you have replaced all the wires from the TAC motor to the TAC module but have you replaced any of the terminals in any of the connectors? That is where most of the "wiring" issues reside in this system fault.

marynfairtrade · July 28, 2011

A regular handheld code reader will not clear the "Active Code" because its not a "Stored Code." You'll need to take it to a dealer or someone with a TechII or equvilent to clear. It is a real PITA...good luck...

Thanks for your response! We have actually had the codes cleared twice - once by a family member tech and just as recently as two days ago by the dealer. We replaced the TB yesterday and it still goes into limp mode and drops down to 15-20mph...just happened on the way to buy milk actually. We are going to go over the grounds and double check the copper wires again...any further ideas would be great to try!

marynfairtrade · July 30, 2011

Hi - I think you are most certainly onto something with your post. We have begun to realize this ourselves, and therefore checked the grounds (again), cleaned them very well (which caused reduced power to happen more frequently and within a very short distance.

We then had the codes checked again and got p1125 - bad accelerator pedal position sensor. This confused us, as the aps was one of the very first things replaced when the problem started (which was worse and the truck would not move). However, after thinking about it, the replaced aps did not fix the problem and the cause was found to be bad wires (this was about a year ago). Things were fine for a month or more, then the reduced power started up intermittently.

But then we read about after market ap sensors and how they don't work properly...after having a voltage test done on a few wires, we found that the current aps we have is bad. So, our next move is to get a GM brand aps and give that a shot. Will post with updates in case anyone else is in the same boat.

fantabear · June 4, 2016

Wow - I'm having this same problem - frequent "Reduced Engine Power" on my 2005 Chevy Silverado 2500HD giving the P1125 accelerator code. My mechanic has been going nuts trying to figure it out. So far:

> Replaced throttle body/throttle position sensor (most common cause from what I've read online)

> Replaced alternator (old one was outputting erratic voltages that he thought might be mucking up the computer)

> Full tune-up (needed it anyway)

> Replaced computer and computer module for throttle (desperate times call for desperate measures)

> Replaced throttle cable harness.

Did you ever get a resolution to your problem?

Doug_Scott · July 29, 2011

This really sounds like whoever is fixing this for you does not really know what they are doing. All of the fixes you posted all sound like someone thinks the engine is not getting enough "throttle" and they are throwing parts at it, hoping to hit the right part. Do not think that the computer is at fault. There are thousands more things it can be, you are not out of options.

You need to start at the beginning, and check (check does not mean replace parts) the codes, and start there. Do not do more than one change at a time.

Have you checked the fuel filter?

Have you checked the fuel pump itself?

How about plugs?

Air filter?

gmtech4 · January 12, 2012

more self diagnoses,, burns more people every day,

makes me nuts ,but saving money is important and inexperience ,lack of mechanical knowledge burns people constantly

no offence to the OP at all,he had the right idea ,he is not the first and not the last either ..

i wish parts stores had a half a brain but making money is their first priority,deadly combo

dealer and corner garage door rates per hr have climbed so high that people cant afford to take it in and get burned anyways the cost of the diagnoses

not a rant just a sad reality these days,

no offence to ANYONE please

Rob

marynfairtrade · July 28, 2011

Update:

1. Checked the continuity of all of the wires - all of them checked out fine

2. Checked the grounds - all seemed fine

3. Forgot to mention that the battery has been replaced as well

Getting the truck looked at in 30 min, will post any extra info. That way, anyone else with these issues has more stuff to check - it's very frustrating!

redjewel2ss · July 28, 2011

check your fuel pressure as well.

another guy in another active thread has similar issues and his turned out to be a bad fuel pump.

cfitz · August 10, 2016

Just a quick question, have any of you having this problem had a performance tune done?

pkeagle · July 22, 2016

New to this post. I've the same issues of Reduce Engine Power. (REP) This will likely be eratic information with the way I write.

I've had this issue for 2 years now and slowly trying to work on it. (Mainly b/c I'm stubborn)

Had some fuel system issues (leaks) - all repaired & codes cleared.

More Code reads suggest TBPS, TB, Pedal Position Sensor. Replaced all w/ OEM parts.

Purchased and replaced pigtails to TBPS & MAF Sensor.

Cleaned MAF Sensor w/ intake cleaner (probably not the best but removed black soot)

I had codes read again recently and I got all kinds of miscellaneous errors including check engine oil that I've not had before.

Changed Oil Pressure Sensor and MAP Sensor. Diligently check & change oil. Oil Pressure Gage does not work in Instrument cluster but I thnk thats a stepper motor thats bad.

Thinking maybe there is a bad ground or connection. I've check G0103 & G0104. Connections tight w/ no corrosion but loosened and used contact cleaner on them just in case. I opened the engine distribution box and found several wires frayed, and broken. (F'n mice!) Repaired those but still have the issue. Don't know what they wires went to but repair seems to have had zero effect.

So I unplugged every connector in the engine compartment and in the dashboard (methodically) checking for corrosion and to see if REP mode would go away or come on. No ah ha findings.

I've wigged every wire of every connector sitting in driveway w/o REP mode coming on. Drove a mile and bam REP mode.

Sometimes truck would operate fine for a day or two then go into REP Mode.

Truck has gone into REP mode:

sitting at stop light,

driving on the highway,

sometimes as soon as I start the truck.

Morning/afternoon, cold/warm weather does not matter.

Drove 700 miles w/o issue then on return trip 4-5 times.

The check engine light use to stay on ALL the time. Now it comes on after a REP event and a goes off after at different time intervals.

Procuring ETC module (located on leftside firewall) to see if that might be the issue.

FRUSTRATED and STUBBORN to find issue w/o going to dealership.

Any suggestions (other than take it to dealership - lol)

sierradenali11 · July 30, 2011

hello,

aside from making sure if its not a fuel delivery problem, make sure to have the fuel pressure tested because its not like years ago where all you did is unscrew the fuel cap on the fuel rail on top of the engine and press the valve (that looks similar to one on a tire) and fuel will shoot out. check a book to see what the proper psi should be at.

one other off type of thing you can do - i got lucky once with this - check the fuses first to see if any are blown, second all the relays in each fuse box, pull them out and check the prongs (or legs of each one) and check to see if there is any rust or dirt on them, they build up sometimes over the years depending on the age, conditions, etc. if there are any rusted you can slightly sand them down to clean up the prongs, and the most important thing is - make sure you re-insert them in thier respective place properly - because i once had a problem where the engine was in limp mode, i traced it back to a relay that i pulled out and saw that i reinserted it upside or backwards, and that made a world of difference - that happened to my 08 sierra, from the engine fuse box on the drivers side in the engine bay. good luck

but try the basics first, fuel delivery, spark, etc

Deljr · January 11, 2012

I have had the same problem in the past. Check the wires themselves, the dealership had to replace some of the wiring because the cover had been worn off. Problem stopped instantly. Since then I only get it if I happen to accidentally pull up on the accelerator when in cruise control. Now I am getting the P1125 and P2125 codes so I am going to replace the APP sensor and see if that corrects the problem.