Code P2138 Buick reduce engine power

2007 Buick Lacrosse Code P2138I have replaced the gas pedal unit which includes the sensor 2 months ago on my 2007 Buick Lacrosse 3.8L. Has been working fine. Then the other day the check engine light came back on with the same code P2138 that says (reduce engine power) which means gas pedal and senor unit again. The problem I’m having the check engine light comes on, I drive it a few times it goes away, but then it will come on again a few days later back and forth, back and forth. What could be causing this to happen please.


The Engine Control Module (ECM) has detected that the APP sensor 1 and APP sensor 2 disagree more than 4.5 percent for less than 1 second. This turns on the check engine light. It is possible that the wiring or connections are faulty. Also possible that the parameters set in the ECM need to be reprogrammed. It is also possible that the “New” part is bad. I would anticipate the repair and part are covered under warranty and as such you should return the vehicle to the repair shop. There is a TSB – Technical Service Bulletin for the Code P2138 with reduced engine power which identifies a repair instrument panel to body harness connector. (see below)

CODE P2138 BUICK – Accelerator Pedal Position Sensor 1-2 Correlation

Code P2138 Possible causes

– Faulty Accelerator Pedal Position Sensor
– Accelerator Pedal Position Sensor harness is open or shorted
– Accelerator Pedal Position Sensor circuit poor electrical connection
– Faulty Electric Throttle Control Actuator

The Accelerator Pedal Position (APP) sensor is made up of 2 sensors that are housed inside one assembly. The Engine Control Module (ECM) supplies a separate 5-volt reference circuit and a low reference circuit for each of the sensors. The 5-volt reference for APP sensor 1 is supplied from the same source in the ECM as the 5-volt reference for the mass air flow (MAF) sensor and the fuel tank pressure (FTP) sensor. The 5-volt reference voltage for all of the sensors is supplied on separate ECM terminals, but the terminals are connected internally to a voltage supply. The APP sensor 1 sends a signal from the sensor to the ECM indicating the accelerator pedal position. The ECM actuates the throttle plates based on this information. If the ECM detects that the APP sensors are not within a predicted value from each other, DTC P2138 sets.

TSB 07-06-04-019E

Intermittent malfunction indicator lamp(MIL) illuminated, DTC P2138 with reduced engine power (repair instrument panel to body harness connector)

Affected models: 2005-2015 GM passenger cars and light duty trucks


  • Some customers may comment on an intermittent malfunction indicator lamp(check engine light) being illuminated with a message or an indicator that displays Reduced Engine Power.
  • The technician may observe on a scan tool DTC Code P2138 – Accelerator Pedal Position (APP) Sensor 1-2 Correlation set as Current or in History.

The condition may be caused by water intrusion into the instrument panel to body harness connector, which carries the APP Sensor signals to the ECM/PCM. The water intrusion results in a voltage difference between APP Sensor 1 and APP Sensor 3 that exceeds a predetermined value for more than a calibrated period of time, setting Code P2138.

Some examples of potential water leaks are: A-Pillar seals, Sunroof drain lines and windshield /cowl sealing.

The Fix:

  1. Verify that aftermarket equipment is not electrically connected to any of the APP sensor signal or low reference circuits or to any other ECM/PCM 5v reference or low reference circuits.
  2. Perform the diagnostic system check – vehicle.
  3. Locate the Instrument Panel to body harness connector, which may be located in and around the left hand kick panel area or inside the instrument panel. Depending on the vehicle and model year.
  4. Inspect for water leak in area. If necessary use a water hose to determine the source of the leak.
  5. Inspect the instrument panel to body harness connector terminals for corrosion and debris.
  6. If any corrosion or debris is observe, repair as needed.
  7. After completing the repair, verify the proper operation of the system.