P2135 Code — What it means, how to fix

Learn what a P2135 code means and how to diagnose and fix the problem

A P2135 code is defined as Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation. A throttle position sensor tells the ECM exactly how far to open or close the throttle plate. This code is used almost exclusively in engines equipped with an electronic throttle body and an electronic accelerator pedal (gas pedal). In these “drive by wire” systems, the ECM receives input from the accelerator pedal and transmits the commands to the electronic throttle body. Both the accelerator pedal and the throttle body have multiple sensors to ensure redundancy.

How the dual Throttle Pedal and Throttle Body A/B sensors work

Since this is a drive-by-wire system, the ECM must take extra care that the intended acceleration from the pedal opens and closes the throttle body by the exact amount. The sensors are potentiometers that vary voltage as the driver presses or releases the pedal or the throttle plate opens or closes. To ensure accuracy, the accelerator and the throttle each contain two sensors. One sensor drops voltage as the driver presses the pedal and the other sensor rises as the same time and same rate. The same thing happens in the throttle body; as the motor opens or closes the throttle plate, one sensor raises voltage, while the other sensor drops voltage.

The ECM compares the rising/dropping voltages from the accelerator pedal and they must match. If they don’t, the ECM sets a P2135 code, while also limiting the engine RPM and vehicle speed to prevent a runaway event. In addition, some carmakers present a Reduced Engine Power message on the dash. At this point, the vehicle is in “limp in” mode, allowing the driver to get to a shop, but not continue driving normally.

Symptoms of a P2135 code

• Check engine light and possible dash message
• Limited power
• Stalling or hesitation
• Stalling after coming to a stop
• Engine revving when you aren’t pressing the gas

What are the most common causes of a P2135?

• Broken, frayed, open or intermittent wiring to the accelerator pedal position sensor.
• Damaged or worn out potentiometers in the accelerator pedal assembly (not individually replaceable)
• Corrosion in the electrical connector that alters voltage readings to the ECM

How to diagnose an accelerator pedal assembly

Check visually

Inspect the wiring harness and connector to the accelerator pedal assembly. Check for signs of breakage, chaffing or opens.

Disconnect the electrical connector and check for signs of corrosion.

Operate the pedal and check for ease of movement with no binding

With a scope or DVOM

Using shop manual voltage specifications and a multimeter, monitor the at rest voltages of both sensors. Then move the pedal and watch the voltages as you move the pedal. Ideally, you’d be using a scope to see a visual representation of the two corresponding voltage swings. If the swings aren’t equal the problem is the accelerator pedal assembly

Wiggle the wires as you conduct the testing to check for an intermittent open condition.

Fix the cause of the P2135

If the wiring checks out but the voltage readings are off, replace the accelerator pedal assembly.

Repair any wiring faults

© 2012 Rick Muscoplat