2003 GMC 1500HD 6.0 "REDUCED ENGINE POWER" ONLY

[OregonASEMASTERTECH]

Here is a good one... First of all, I am ASE Master Technician out of Oregon. I personally own a 2003 GMC Heavy Half, otherwise known as a 2003 1500 HD with a 6.0 Liter and a 4L80E. It has 178,000 miles on her and she looks great and runs (most of the time, great)

 

A few months back I was driving about 65 mph, towing a trailer with a vehicle on it. I was going through a construction zone and my truck started to ever so slightly buck. Kind of like a misfire. My CEL came on and a "Reduced engine power" came on. I lost most power and it felt like I was on 3 cylinders. Now I know how a Ford owner feels most of the time. Anyways, I pulled over, turned off the key. Restarted the engine and the "reduced Power" light was out, but CEL was still on. I started to go, and had all my power back. When i got to the shop, I checked codes and found MAF codes. I checked live data and the correlation was not correct so I replaced it. That did not work.

 

At our shop, we have Identifix, and most people stated they had the following...1.) MAF issues, 2.) Ground faults at the heads and battery grounds 3.) Gas pedal Sensor 4.) TPS Sensor issues 5.) Bad alternator/draw. one or two of these usually fixes the problem.

I replaced the TPS first, Nothing.

Then the MAF as a whole, (includes new TPS on it), Nothing.

Replaced all the TPS wire harness, Nothing.

The redid all grounds, cleaned and rechecked, Nothing

Replaced pedal assy, Nothing.

Alternator is new by 6 months and tests great.

Wiggled the wiring harness while running to see if a short in the system was there. Nothing!

 

So I checked and have replaced all of the above items. I even returned them to the warehouse and got a second part to replace with just in case of a faulty part. Nothing. I checked online and people say a TAC replacement doesn't work and neither does a ECM reflash. Whats its problem? I have seen ALOT of people with this crappy issue and no one has a fix... got one??

 

My truck is now sitting next to the Fords in the lot... what a shame. I went to "just-ask" .com and they wanted me to pay a ASE Technician, not even certified Master status $32 for him to "hunt" for solutions. I have over 30+ years experience and do NOT understand why GM would make someone lose power when they need it, especially in a construction zone with no turn off's for over 2 miles and all I could do was 20 MPH!

 

Dealership has no cures, they just duplicated my above parts list.

 

If I drive down the freeway, I have to coast until 50 mph, then hit the passing gear to keep rpm's up or put on cruise control from 25 mph on up to attempt to keep the light out and the power on. It seems as if it is just under light foot when this occurs and not all the time. Last time, we did the grounds, disconnected the battery for an hour and drained the ECM. Truck worked for 2 hours in the AM, then I shut off. When I returned for another 20 mile run, it would not stop doing this! UGH!

 

My wife says she doesn't like the truck anymore and I do not blame her. She wants me to think about trading in the truck, but if GM had these issues with the fly by wire, then dodge would be my next option.

 

 

[SpankyMcGrits]

Looks like you have a picture of it. Yours looks like this.

 

 

 

Well crap. At least I can take that sensor back and recoup some of the money I'll need for the throttle body.

 

Thanks for the help!!

[00Silverado4x4]

I worked on an '04 2500 awhile back that was having the reduced power going on. I was just replacing the valve cover gaskets when I noticed they were full of crud, so....

 

Cleaned inside of valve covers

Cleaned output of valve cover through PVC orifice

Cleaned throttle body

Cleaned all tubes coming from and to throttle body

Cleaned all tubes coming from and to intake manifold

Cleaned as far as I could reach of intake manifold

 

Everything was so full of crud there was no flow through these areas of the engine. This all took the better part of a day since they were completed plugged up. Once I cleaned all that crud out and got the engine crankcase and throttle body breathing again the truck quit acting up and last I knew still hasn't had an episode of reduced power since.

 

Just a few ideas of what else to check/look for. Keep us updated.

[gmtech4]

We, after looking at it, and my ASE mechanic who works for me did as well... we were told by GM to look at these items:

 

Grounds at back of heads: DONE

Ground at battery cable to block: DONE

New throttle body: DONE

New TAC module: DONE

New wiring to throttle body: DONE

Cut wire going from ground to TAC module and relocated at block near alternator: DONE

 

So after this, we took it to the local GM dealership at T&C Chevrolet in Milwaukie Oregon

 

Their drivability technician looked at it and stated he found a chaffed wire near the headlight switch. He cut, soldered connection and stated it was good. Charged $326

 

We got about 7 miles, and guess what, it still failed.

 

Took back into dealership... they had for over 30 days. After me calling and inquiring if I was going to see my truck again, they laughed and stated truck was done and tech drove for over a week to see if it acted up again. I picked up. He said a RCA cable from a previous owner was pushed through firewall and wiring was chaffed at that point. He cleaned up wiring, and put back together. At this time, I chose to have new cluster installed since I could not read PRNDL1 any more. (another flaw in this model) This is the 2nd cluster to date.

 

I got the truck back, paid $730. $350 for cluster and $400 for main problem repair. Guess what? 44 miles down the road, same problem. I literally could not get out of a parking lot as the truck kept turning on the light. I took it back to the dealership... they finally gave me a loaner car. I drove the car for 2 days, then just said the hell with it. I went to the dealership, picked up my truck almost $800 lighter in the wallet to have nothing fixed. I am driving the truck in little steps and when the light comes back on, I am taking it to our electrical shop we go to.

 

The tech at the dealership stated he knew nothing on how to fix this. That GM tech support had three things to fix. One, the grounds to the heads...two, replace the APP relays, third...replace wiring harness to engine. If that don't work... it's anyone's guess.

 

So, just like the rest of us, we tried the dealership, and even they don't have a clue on how to fix this...

 

Anyone???

For the newer trucks with the same msg there was a reprogram along with a new TB ,idk ,

 

At the end of every single diagnostic trouble tree is to check connectors and then the last few things are components to replace and the modules , PCM etc etc etc depending on system of course ,

 

I can't get over the fact that they couldn't nail it down for you ,

 

You picking up the truck and hemeroging that amount of money when it's not fixed hurts my head frankly and need to know ,,,,, why ?

[Doug_Scott]

After rechecking everything, I took it to the dealership. Tech stated a p1001 code, no communication to buss via harness behind headlight switch. He stated he found a caffed wire, rewrapped and no more issues. $327 Later, I pickedup the truck. With 7 miles, lights came back on, reduced engine power on again, cel light still has p1126. (I think that was the code)

 

I retook to dealership, they have had for 2 weeks now, must be that hidden troll under the dash?

When they call you to tell you it is fixed, ask them to come pick you up using your truck. This way you will likely know it is actually fixed. If the light is back on when the driver shows up to get you, send him back with the truck asking that you want this fixed before they come get you.

[I actually found the fix]

This is really going to sound crazy, I normally don't respond to posts but this one is pretty important to me. I had struggled with this very same issue for about two years and went through all of the same motions as you did, repalceing parts, testing, repalceing the same parts over and over again to no avail. I would literally ride down the road with my code tester plugged in and when it would go into limp mode I would clear the code repeatedly just so I could keep moving and not cause an accident.

I happen to be at a parts store picking up parts for a different issue when I ran into a 30 year GM mechanic. I figured I would take the opportunity to ask to see if he was familiar with this issue. He was. This was his advice to me which has now worked for the past two years and still going strong. Well here it goes, He and I narrowed it down to one of two things. When replacing the wiring harnness that plugs into the tps, twist and solder the wires rather than crimping, as what was suggested to do by GM. The other, which I kinda think is the primary problem, is applying dielectric grease to every plug terminal that is in correlation to the the throttle. I placed the grease on the plug to the gas pedal, tps, PCM (computer), and I'm sure there were a couple more but you get the point. The reason is that such low voltage is used in communication from one part to another that the slightest resistance causes the computer to read the interruption and produce a faulty code and then go into limp mode. I know this sounds like such a simple fix to such a large problem but what can I say other than it actually was the problem. Hope this helps all who read this post, I know where you're at and it's really very frustrating .. If you've sold the vehicle by now, it was certainly understandable, I almost did the same.

[cashflow50]

I had a similar problem about a year ago.

My problem started off with the DIC stating a 'Service Brake System' flash. Engine sputtered. After a mile or so 'Reduced engine power' comes on. Codes pulled were MAF related. After throwing parts at it after pulled codes ...it was the Brake module all along. This might not be the fix for you, but for me after numerous research on many forums..this was my final conclusion for the fix. 03 yuke runs like a champ now.

[sahls01gmc]

 

What would you do if this was a customers vehicle?

 

 

I can't read the whole crazy thread cuzz as mentioned , no code number AND YOUR A TECH ???

 

come on , follow the code , holy crap

I agree with these guys. I'm sure you're puzzled since you're a tech, but what would you do if this was a customer? I don't have any experience in "Limp mode", so I'm not much help. But the codes that come up ought to tell you where to hunt.

[Jsdirt]

I hear that Rob. If a dealer can't even fix a product it sells, what's that say about the product & manufacturer?

 

It's par for the course today. My neighbor had to sue Ford to get them to take his faulty Ecoboost back that nobody could figure out over the course of 2 painful years. They kept trying all sorts of bandaids - even DETUNED the engine!

 

That would've been the last straw for me right there. Mess with my horsepower and I'm gonna start swingin' ....

 

Long story short, they finally took it back, and sold him the same truck with the V8 in it for $3k, after paying his lawyer fees. Got 2 model years newer with 20k less miles, so guess it's a wash.

[Holy Smokes]

I had the same thing happen to my 2003 2500HD at approx 160,000 miles. Became a reoccurring problem for over a year. Truck spent several days at the dealer with the tech running thru all the same items mentioned above. On the 2nd or third trip, they claimed they found a ground wire with cut insulation and partially corroded conductors. It seemed to work fine for a while and in the end I gave full explanation and disclosure to a buyer and went and bought a 2014. It's not an uncommon problem.

[newdude]

Any codes? History? Current? Have you checked all the modules for codes? With reduced power displaying in the DIC there's got to be some code associated with it.

[SpankyMcGrits]

Ok. That casing is riveted on. Will I have to drill those out or what? I'm not very knowledgeable on the subject.

[Doug_Scott]

I had a similar problem about a year ago.

My problem started off with the DIC stating a 'Service Brake System' flash. Engine sputtered. After a mile or so 'Reduced engine power' comes on. Codes pulled were MAF related. After throwing parts at it after pulled codes ...it was the Brake module all along. This might not be the fix for you, but for me after numerous research on many forums..this was my final conclusion for the fix. 03 yuke runs like a champ now.

 

This is a good example of why people should not try to diagnose something that they do not completely understand. There are always many different things that can go wrong and cause a CEL to come on. Even getting the code does not mean you are heading in the right direction. The manuals generally have good flowcharts for trouble shooting. Problem with flowcharts is non-technical people tend to just skim over them, looking for the easy fix. If the professional needs a diagnostic flowchart to fix something, wouldn't that mean that a plumber or gardener also needs to use those flowcharts when they try to fix the issue?

 

Cars for decades have been using fairly low voltages, with high amperage power to run everything. It would take a very corroded connection to actually fail under those conditions. Once computers showed up, voltages dropped to less than half, and amperage is nearly none now. Goes against everything we were taught about DC power. Lower voltage is supposed to need higher amperage. Computers use all these sensors simply for signals. Poor connections are fatal now. What used to run 12 volts is using 5 volts now. I was taught in trade school that every connection on a 12 volt system would create a 0.1 volt drop, and that was normal. 5 connections would equal half a volt. No big deal on 12 volts. It is a huge deal on 5 volt systems. My point here is that poor connections are the cause of a lot of failures, are the hardest thing to locate and even harder to correct.

[Duelin]

 

Well crap. At least I can take that sensor back and recoup some of the money I'll need for the throttle body.

 

Thanks for the help!!

You can buy that TPS/IAC sensor without buying the whole throttle body.

[00Silverado4x4]

I have not heard of any issues with the drive by wire. It will be a moot point with direct injection anyway won't it? Don't think there is a traditional throttle plate assembly on the direct injection engines. Just a big wide open hole where throttle used to go. Does nothing to help your truck though.

 

This engine won't be direct injection as it's an '03.

Fuel pressure good?

Fuel pressure regulator good?

Throttle body clean?

[Duelin]

I'm having this problem with my 2003 Silverado 5.3 V8. I bought a throttle position sensor but there isn't one on my throttle body! There is even a spot on the throttle body that looks like where it would go.

 

Nothing on the other side either. Pics attached. Any suggestions? I've already cleaned the throttle body a couple times. It was pretty nasty as There are 205,000 miles on it and it has probably never been cleaned.

 

IMG_0171.JPG

 

IMG_0172.JPG

Looks like you have a picture of it. Yours looks like this.