6.6 Duramax Allison LCT1000 Intermittent DTC P0742 - kw LB7 TCC transmission LLY

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

Intermittent service engine soon light comes on and code P0700 stored in the ECM along with a P0742 stored in the TCM. The condition seems to be most easily duplicated by driving the vehicle in a "float" type condition where the vehicle and engine are neither accelerating nor decelerating. You may notice that the TCC slip speed will read near 0 RPMs but the TCC is not commanded on.

Before any transmission repairs are attempted, all engine driveability concerns must be repaired. A false engine load perception by the TCM will cause the internal diagnostics to mistakenly set a P0742 because it senses the TCC must be stuck on with the engine load it is seeing. Some items that have been found to cause this concern are as follows:

Sticking fuel pressure regulators

Skewed MAF sensors

Fuel pressure sensors

Turbo boost sensors

Baro sensors

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These items can cause various engine symptoms such as:

Idle surge

Excessive smoke

Lack of power

All engine issues must be corrected before proper diagnosis of the transmission controls can be performed.

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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.

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