help have u and p codes what to do

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by johnpimentel72, Jan 30, 2008.

  1. johnpimentel72

    johnpimentel72 New Member

    hi, im not an expert on vehicles, but my check engine lighted up so i went to a dealer and had it read. P1875, cleared it coz they said it was history. drove for a while and the check engine lighted up again. went back to the dealer and showed P1875 and U1026. I know from searching here that these are P1875 - 4WD Low Switch Circuit Electrical and U1026 - Loss of ATC Class 2 Communication. can anyone interpret these for me and how do i repair these?

  2. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    Did the dealer give you those code definitions or did you get them from a generic OBD-2 code list? Just want to be sure, because I expect those are manufacturer specific codes.

    If those code definitions are correct, it appears that communication has broken down between the PCM and the transfer case control module (TCCM). I don't have immediate access to anymore information right now. If it were me, the first thing I'd do would be to find a good manual and find the specific diagnostic information that goes with those codes. Sometimes you can get them at a library.

    In the absence of a complete diagnostic tree, about all I can suggest is to trace the wires between the TCCM and the PCM and make sure there are no loose connections, breaks, or shorts.
  3. johnpimentel72

    johnpimentel72 New Member

    we (GM technician and me) got the codes after hooking it up with tech2. could it be that the tccm is bad? before this we cleaned the transmission solenoids and valve body; changed the transmission fluid and filter, transfercase fluid, differential fluids engine oil and filter, fuel filter. the previous owner didnt used the truck for about 2 years just turned on the ignition for a couple of minutes then off. when we drained the transmission fluid it was very dirty and so with the solenoids and valve body.

    after everything was done and used it for a couple of days it was doing fine. drove it today after one day of rest the shifting problem came back again (from first gear the rpm goes up and down then jumps to fourth and from fourth sometimes it doesnt dowshift) with out check engine yet. having this experience before i know that the codes will come back soon.
  4. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    Yes, a bad TCCM is one possibility. However, I don't know the specifics of testing the TCCM to determine if it is at fault.
    The reason we do diagnosis is to sort through all of the possible causes to find the specific cause for the specific problem. Does your technician have access to the right information? I know they are often loathe to share any information with us ("heaven forbid we should empower the consumer in anyway beyond writing checks"), but maybe your tech is an exception. What has your technician done beyond pulling codes?
    Vehicle specifics (year, engine, transfer case, etc.) might help someone who has access to some good information post details.
  5. johnpimentel72

    johnpimentel72 New Member

    i was driving today with still the same problematic shifting then one of my front wheels hit a small bump and guess what, the check engine lighted up. went back to GM and got the codes P0470, p1860, p1875 and u 1041.
    i trust this GM tech that he won't hold back anything from me. all they did before was to clean the 4x4 switch due to the service 4wd light. i never came back for additional jobs coz they charge too much per hour and sometimes it takes them too long to look for solutions in their TSBs/SIs which they still charge per hour. Im having my repairs in another shop whom the owner owns a tahoe, suburban, jeep, cherokee and other 4x4s which he uses for competition and some american/japanesen 4x4 owners come here for repairs. there's not much GMCs here so some problems are first encounters and hit or miss solutions.
  6. johnpimentel72

    johnpimentel72 New Member

    oh i forgot mine is a 2000 stepside extended cab 4x4 silverado, 4.8l, 4l60e (i think), all are stock. before i bought this, the previous owner didn't use the truck for about two years coz he was out of the country. his brother just starts the engine a couple of min a day then off. the odometer has 33k kms (20k miles) on it.
    when we got the codes all of them indicated that they all passed and there were history. could it be that the bump triggered the codes? there's also the possibility that there is a loose connection somewhere or probably poor grounding. problem is where do i start?
    Last edited: Feb 5, 2008
  7. ronbo123

    ronbo123 New Member

  8. johnpimentel72

    johnpimentel72 New Member

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