DTC P0128 After Cold Start - keywords 6.6 6.6l cool coolant cooling ECT engine heat heater HVAC LMM SES temp temperature thermostat

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

A dealer may encounter a customer concern of DTC P0128. Inspection of vehicle may reveal the vehicle has an add-on rear heater system.

Complete the current SI diagnostics for any symptom or trouble code found. If P0128 is setting only after a cold start, and the vehicle has an add-on heater system, please do not attempt repairs.

P0128 is set when the engine does not meet the ECT target temperatures during an expected calibrated warm-up time. Add-on heater systems could cause extended warm up times and induce P0128 to set.

Engineering is investigating P0128 when it occurs as described in this PI. This PI will be updated when more information becomes available.

If P0128 has set and the vehicle has no heater modifications complete the current SI diagnostic and repair the vehicle as needed.

Note: The customer may be starting the vehicle (with both heaters on) and idling for long period of time to warm up the interior of the vehicle. If the vehicle is setting P0128 when both the factory and the add-on auxiliary heaters are turned on at idle, try starting the vehicle with both heaters in off position to see if DTC P0128 will still reset. This may help verify the extra load on the heater system is inducing P0128.

Please follow this diagnostic or repair 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.