Allison LCT1000 P1781 or P0802 Setting in the ECM - kw 6.6 8.1 code CEL DTC ECM GMT560 GMT800 intermittent L18 LB7 LLY light M74 MIL P1781 P0802 PCM repeat shift start TCM transmission

The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Some customers may comment that the Service Engine Soon indicator lamp is illuminated. Technicians may find DTC P1781 (model years 01-02) or DTC P0802 (model years 03-05) (Transmission Control Module - Incorrect Voltage Level on MIL Request Circuit) stored in the ECM. This condition will usually occur during engine start up. This code sets when the ECM and TCM do not communicate properly on the initial "hand shake" during power up. One module may be powering down during crank due to a low voltage situation.

Note: For 2001 vehicles refer to TSB 01-06-04-020 for the latest ECM calibration. Check the vehicle for a possible weak battery or a dead battery in dual battery applications. Also check for possible poor battery cable connections, poor grounds, UBEC connections, or possibly an ignition switch dropping out during crank.

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

(01-05 Chevrolet Silverado) and (01-05 GMC Sierra) and ( 03-05 Chevrolet Kodiak) and (03-05 GMC Top Kick)

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