Rick's Free Auto Repair Advice

Reduced Engine Power

Reduced Engine Power light on and P2138 Reduced Engine Power trouble code

The reduced engine power message appears on GM vehicles when the engine controls have detected a problem that can cause a safety issue. For example, the electronic accelerator pedal position sensor (APP) (fly by wire) sends multiple signals to the engine control to tell it how much and how fast to open the electronic throttle body. The APP has multiple potentiometers for redundancy. However, if the signals don’t correlate as the computer expects, it will reduce available engine power and set the reduced engine power message.

GM has issued a service bulletin #07-06-04-019C to address a reduced engine power message, along with a P2138 trouble code. The TSB applies to the vehicles listed below. In addiition to the reduced engine power message and the P2138 trouble code, the bulletin also notes that the service engine soon light may come on intermittently.

2005-2010 GM passenger cars and light trucks
2005-2009 Saab0-7X

P2138 Accelerator Pedal Position (APP) Sensor 1-2 Correlation

GM warns NOT to assume that the problem is a bad APP. Do NOT replace the APP as your first option. GM states that water intrusion into the instrument panel to body wiring harness may be causing the problem. The wiring harness carries the signals from the APP to the ECM/PCM. The water results in a voltage difference between the APP 1 and APP 2 sensor. The change in value exceeds the pre-programmed limit, indicating a problem with the APP.

GM also notes that aftermarket installed electronic equipment can also cause problems with the APP signal. If the vehicle has aftermarket electronics, make sure they’re not connected to any APP signal wires or any voltage reference signal wires.

To diagnose the problem, locate the IP to body harness connector in or around the left hand kick panel or inside the IP, depending on year and model. Check the wiring for water leaks. Use a hose to soak the A-pillars, sunroof drains, and windshield cowling. If you discover any water, fix the leak before proceeding.

Next, check for corrosion in the electrical connectors. Clean the terminals, apply DeoxIT, followed by dielectric grease. Then clear the code and verify that it does not reset.

If the problem does re-occur, isolate the APP and conduct the approriate tests.

©, 2015 Rick Muscoplat

Posted on by Rick Muscoplat


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