Subaru Forester:  P2135 DTC:  Meaning, Causes, and Diagnosis

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P2135 Subaru Forester

P2135 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).

P2135 TPS Switch “A/B” Voltage Correlation

P2135 Meaning

If your Subaru Forester has P2135 it is considered “drive by wire”.  Which means that it uses sensors and actuators to control the throttle body.  Here’s how this method of operating the throttle works:

  1. You press the gas pedal.
  2. The accelerator position sensor sends a signal to your Forester’s PCM.
  3. The throttle position sensors (A/B and C/D) confirm the position of the throttle blade and the PCM adjusts the throttle position based on this input.
  4. The two throttle position sensor signals need to AGREE with one another AND the accelerator position sensor.
  5. If they don’t agree with one another, P2135 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
  • Emissions
  • Traction control response
  • Cruise Control

P2135 Symptoms:  Subaru Forester

Subaru Forester P2135 Check Engine

Here are the most common symptoms associated with P2135.  They include:

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

Subaru Forester: P2135 Causes + Diagnosis

P2135 Causes + Diagnosis

When your Forester has P2135, there are four main issues that cause it.  They 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 (technical service bulletin) for your engine and model year.  Checking the wiring harness at the pedal and throttle body would be the easiest thing to check.

1. Use a Multimeter to Examine the Wiring

Use a multimeter to see if the voltage coming from the A/B throttle position sensor and accelerator position sensor are within Subaru’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 actually 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

P2135 is often caused by an open or short in the wiring.  Examine the pigtails that plug into your Forester’s 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 P2135 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 Subaru dealer and ask the service department though.

Bottom Line

P2135 can be a tricky code to diagnose, and it’s an important one to get right.  It’s not safe to drive your Forester around with a computer controlled throttle that isn’t sure how much throttle it’s actually giving your vehicle. Good luck!