P2138 is a generic powertrain code that can affect any make or model of vehicle that has a “drive by wire” throttle system (almost every vehicle made today).
It means that your vehicle’s “D” and “E” throttle position sensors do not agree with one another on how much gas you are giving the vehicle. They should have a similar voltage output. In this case, they do not.
How P2138 is Triggered
- You press the gas pedal.
- The accelerator position sensor sends a signal to the PCM.
- The throttle position sensors confirm how much the throttle is opened. The PCM adjusts the throttle position based on this input.
- The “D” and “E” TPS signals need to AGREE with one another AND the accelerator position sensor.
- If they don’t agree, P2138 is set.
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
- Traction control response
- Cruise Control
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
P2138 Causes + 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 the 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 vehicle and model year by searching at NHTSA. It’s probably faster to just call the local dealer for your manufacturer and ask the service department though.
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 vehicle!