GMC Acadia P2138:  Symptoms, Meaning, Causes + Diagnosis

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P2138 GMC Acadia

P2138 is a generic powertrain code that can affect your GMC Acadia’s “drive by wire” throttle system. 

It means that your Acadia’s “D” and “E” throttle position sensors do not agree with one another on how much far the throttle has been opened.  They should have a similar voltage output.  In this case, they do not.

P2138 Meaning

P2138 Definition

 

  1. You press the gas pedal.
  2. The accelerator position sensor sends a signal to your Acadia’s PCM.
  3. The throttle position sensors confirm how much the throttle is opened.  The PCM adjusts the throttle position based on this feedback.
  4. The “D” and “E” TPS signals need to AGREE with one another AND the accelerator position sensor.
  5. If they don’t agree, P2138 is set.

Your Acadia’s drive by wire throttle systems use a sensor on the gas pedal to transmit the pedal position (accelerator position sensor) to the PCM, which controls the throttle body.  

The advantage of operating the throttle this way is that with a traditional gas pedal (drive by cable), most of the engine’s torque will be located at the beginning of the throttle, whereas with drive by wire if you press the throttle down 20%, the computer will give you 20% of the torque.  This allows drive by wire to allow for the torque to come in more gradually.  

This saves wear and tear on the vehicle and prolongs engine life.  Drive by wire also improves:

  • Fuel economy
  • Emissions
  • Traction control response
  • Cruise Control
  • Rev Limiting

GMC Acadia: P2138 Symptoms

P2138 Check Engine: GMC Acadia

Here are the most common symptoms associated with P2138.  They include:

  • Reduced engine power
  • Strange “pedal feel”
  • Engine revving with no throttle input
  • Engine stalls
  • Limp mode

GMC Acadia P2138 Causes + Diagnosis

Acadia P2138 Diagnosis

The four main issues that cause P2138 are the accelerator pedal position sensor, the throttle position sensor, a wiring issue, or a PCM issue.  Of these issues, the PCM is usually the least likely unless there is a TSB for your engine and model year.

1. Use a Multimeter to Examine the Wiring

Use a multimeter to see if the voltage coming from your GMC Acadia’s D and E throttle position sensors and accelerator position sensor are within the manufacturer’s specifications.  If they are out of range, you’ll need to replace the sensor in question.

That’s easy to say but becomes a lot clearer if you can see it.  Try watching this video on YouTube.  It’ll tell you exactly how to test a throttle position sensor.

2. Examine the Wiring for Opens/Shorts

P2138 is often caused by an open or short in the wiring.  Examine the pigtails that plug into the throttle position sensor and make sure that they are not damaged in some way.

3. PCM Issues

While it is not where you should start.  A PCM reflash will correct P2138 on some vehicles.  You can check to see if there are any technical service bulletins related to your Acadia’s model year and engine by searching at NHTSA.  It’s probably faster to just call the local GMC dealer and ask the service department though.

Bottom Line

P2138 can be a tricky code to diagnose, and it’s an important one to get right.  It’s not safe to drive around with a computer-controlled throttle that isn’t sure how much throttle it’s giving your vehicle. Good luck fixing your Acadia!