How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask John Your Own Question
John, GMC Technician
Category: GM
Satisfied Customers: 9176
Experience:  ASE MASTER TECHNICIAN/40 years experience
Type Your GM Question Here...
John is online now
A new question is answered every 9 seconds

2004 GMC c2500: stays..throttle..degrees..voltage drops to 1.5 volts

Customer Question

Can you answer questions about a 2004 GMC C6500 with and 8.1 L engine.

When you press on the throtle it goes up to 3000 and then immesiately falls to 2000 and stays there. The throttle positioning sensor drops to 22 degrees and the voltage drops to 1.5 volts. We have replaced the throttle positioning sensore and the APP peddle and this made no difference. The APP goes to 100% and stays but the voltage drops to 2.5 volts. If the truck is not running and the key is on the throttle positioning sensor goes to 100% angle and the voltave goes to 4 volts however, when the truck is started it goes to the above mentioned settings. Is this the sign of a default or limp mode. It is also a manual transmition.
Submitted: 6 years ago.
Category: GM
Expert:  John replied 6 years ago.
Chat Conversation Started
john :

Hello & Welcome to JA,

Expert:  John replied 6 years ago.
Chat Conversation Ended

If you are still working with your Expert on this question, you can post additional replies and receive additional information on this page.  If you received a satisfactory answer in Chat, you can accept the chat conversation above.
Customer: replied 6 years ago.

The only response we have recieved to date is Hello and Welcome to JA.


What additional information do you need?

Expert:  John replied 6 years ago.

What code are you getting ?


Does this vehicle have a PTO ?

Customer: replied 6 years ago.

We have it hooked up to a Genesis and are getting engine readings but no trouble codes.


Yes there is a PTO it operates the dump bed.


Expert:  John replied 6 years ago.

Ok,check the data parameter for the PTO. If it shows that the PTO is active ,the computer will trim the engine rpm to a max lower rpm.


Here is a GM bulliten on PTO switch

PIP3187A: Engine Has Lower Power Check PTO Switch - keywords 6.6 6.6L 8.1 8.1L allison automatic diesel fuel L18 lack LB7 LBZ LLY LMM MAF no pressure RPM sluggishness sponginess speed - (Jun 9, 2009)

Subject:Engine Has Low Power, Check PTO Switch

Models:2001-2007 Chevrolet Silverado Classic
2001-2007 GMC Sierra Classic
2003-2009 Chevrolet Kodiak
2003-2009 GMC Topkick
2007-2009 Chevrolet Silverado
2007-2009 GMC Sierra
Equipped with the 6.6 diesel or 8.1 gasoline engines


This PI was superseded to update model years. Please discard PIP3187.


The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

A customer may have a concern of low power. The dealer may notice that the engine will not go over 2000-2300 RPM.

A low power concern can be induced if the Power Take Off (PTO) switch is on.

If a low power concern has been duplicated, complete the current SI diagnostics for any trouble code or symptom found. During diagnosis be sure to check the PTO switch. When the PTO is enabled, it may limit the engine speed between 2,000 and 2,300 RPM (RPM that is reached may be dependent on actual PTO settings).

If the PTO switch is on, lower than expected readings on the FRP Regulator Fuel Flow Command, Turbocharger Boost Pressure, and Mass Air Flow readings may be noticed.

It may also cause changes in transmission operation such as long or delayed upshifts.

Evaluate low power after PTO has been disabled.

John, GMC Technician
Category: GM
Satisfied Customers: 9176
Experience: ASE MASTER TECHNICIAN/40 years experience
John and 4 other GM Specialists are ready to help you
Customer: replied 6 years ago.

Thanks for your time it turned out to be that the traction control button was on.

Expert:  John replied 6 years ago.
Good to hear you found it,hope you have a good evening.

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

  • I was having a terrible problem with the wheel bearings on my 98 Chevy Cavalier and I suspect that using this website saved me plenty of time and money. Todd L Rochester NY
< Previous | Next >
  • I was having a terrible problem with the wheel bearings on my 98 Chevy Cavalier and I suspect that using this website saved me plenty of time and money. Todd L Rochester NY
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
  • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
  • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY
  • Thank you so much for taking your time and knowledge to support my concerns. Not only did you answer my questions, you even took it a step further with replying with more pertinent information I needed to know. Robin Elkton, Maryland

Meet The Experts:

  • GM Tech (Cam)

    GM Tech (Cam)

    GMC Mechanic

    Satisfied Customers:

    GM Grand Master Technician 2007. 14 years experience.
< Last | Next >
  • GM Tech (Cam)'s Avatar

    GM Tech (Cam)

    GMC Mechanic

    Satisfied Customers:

    GM Grand Master Technician 2007. 14 years experience.
  • Gary's Avatar


    GMC Technician

    Satisfied Customers:

    15 years gm master tech/15 years ASE master tech
  • car tech's Avatar

    car tech

    ASE Certified Technician

    Satisfied Customers:

    25 year master tech and shop owner
  • Paul M ASE Master's Avatar

    Paul M ASE Master

    ASE Master Tech

    Satisfied Customers:

    ASE Master Tech 20+ Years
  • Chris (aka- Moose)'s Avatar

    Chris (aka- Moose)

    GMC Technician

    Satisfied Customers:

    16 years of experience
  • Anthony's Avatar


    GMC Technician

    Satisfied Customers:

    GM Certified
  • Amedee's Avatar


    GMC Technician

    Satisfied Customers:

    ASE certified tech with advanced level specialist. Wisconsin certified emissions state inspector