check engine light, but no codes

Discussion in 'GM Powertrain' started by powerhaulic, Sep 29, 2012.

  1. powerhaulic

    powerhaulic New Member

    1994 Sub k2500 454

    check eng light comes on, plug in no codes are stored. running or long term.
    Send clear codes command light goes out but not for long.
    Any Ideas??????
  2. jimbean

    jimbean New Member

    which code reader
    some r more sensiitve than others
    did you config the scanner with the right vehicle ,engine make and model , it matters :}
    some codes on some of my code reader`s will not come up unless u r driving down the road
    and usually if the code reader does not connect to truck`s computer it has to do with the ecm/pcm not working right not getting power {fuse?} and or broken wire and if the broken wire has been shorting out it could be the computer on vehicle
  3. powerhaulic

    powerhaulic New Member

    OTC pro tool

    I can view all data streams

    I suspect ecu failure as well.......
  4. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    I don't know about the '94 Sub, but my '05 Envoy would light the CEL when there were codes that were not related to the PCM.

    A lot of scanners will only read Pnnnn codes (P for PCM), codes for ABS, BCM, Traction control, etc are there but can not be read by most scanners.

    You may need a GM TECH II tool to read the code.
  5. Rockstar 100 Posts

    X2 what RayVoy said, there are several other computers on your vehicle besides the ecm. I use an old Snap On mt2500, and it scans abs, bcm, airbag, tcm, and ecm
  6. powerhaulic

    powerhaulic New Member

    Certainly do not a tech II tool for a '94 ........
    I have an OTC scan pro 3499 and it covers ABS and anything this antique i am asking about has on board.
    There is no BCM on this old of a vehicle, and body modules is about all this tool wont read
    Trans control is in the ECU in this vehicle
  7. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Of course, sorry to mislead. So, perhaps a problem with the ECU. Can you reset the ECU, with the scan tool, or by disconnecting the battery?

    After that, if still on, I'd suggest a ground on the lamp lead going to the ECU. I don't have a '94 wiring diagram, but a lot of times, GM will provide battery on one side of the lamp (a lot of the lamps, not just the ECU) and a switched ground on the other side.

    The ECU would (I'm just assuming here) ground the lamp lead when there is a code.

    The reason for the switched ground, is to provide a warning lamp if the switch fails; if the hot side was switched, a switch failure could cause the fuse to blow, or a fire.
  8. powerhaulic

    powerhaulic New Member

    Installed used ECU swapped in old prom good to go........
    Runs great more power and no light

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