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If your check engine light (MIL) is on and the code P2138 shows up on the scan tool when you connect it to your car’s OBD port, you’ll want to know what it means and its possible causes. This short guide will help you figure out its causes, symptoms, and other information you need to know about this specific error code.

throttle position sensor image
Code P1238 indicates that the signals from the two throttle position sensors (or two accelerator pedal position sensors) do not correlate.

What Does the P2138 Code Mean?

Diagnostic trouble code (DTC) P2138 stands for “Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation.” It is triggered when the vehicle’s engine control module (ECM) or powertrain control module (PCM) detects that the signals from the two throttle position sensors (or two accelerator pedal position sensors) do not correlate.

When that happens, the ECM/PCM can’t be sure of the actual accelerator position and will typically illuminate a warning light (wrench or throttle blade light, etc.) and limit throttle response to just under half throttle or, in extreme cases, the engine might not accelerate at all.

The PCM determines ETC motor operation based primarily on input from a pair of throttle position (TP) sensors mounted on the throttle body and a pair of accelerator pedal position (APP) sensors connected to the accelerator pedal. Pairs of sensors are used in both places as a solidly dependable way of detecting faults. Remember, unlike the redundant circuits and hydraulics in military aircraft, these sensors don’t come in pairs so one sensor will still work if the other one fails. If one sensor doesn’t work or doesn’t agree with the other sensor as it should, fail-safe mode kicks in, reducing or eliminating throttle response for safety purposes.

Depending on the vehicle manufacturer, code P2138 can indicate either the car’s two TP sensors don’t agree, or it’s two APP sensors don’t agree.

P2138 is a common issue among the following makes: Chevrolet (especially on a Chevy Malibu), Nissan, Honda, Subaru, and Acura.

An In-depth Look at Accelerator Pedal Position Sensors

Accelerator pedal position (APP) sensors provide information to the ECM/PCM concerning driver throttle requests. The ECM/PCM processes that information very rapidly, mirroring pedal movement by moving the throttle plate. The throttle body has a motor that drives the throttle plate open and closed (both ways are powered, so keep your finger out of the throttle body). There is a throttle position sensor (TPS) on the throttle body for feedback purposes – the ECM/PCM needs to know what the throttle plate is doing at all times.

The driver’s perception of acceleration with properly operating electronic throttle control (ETC) is virtually the same as it would be if there was a throttle cable connected from the pedal to the throttle body plate. No cruise control parts are needed on ETC-equipped vehicles, only software. No idle speed control parts are needed – once again, ECM/PCM software handles it all.

powertrain control module (PCM)
Accelerator pedal position (APP) sensors provide information to the ECM/PCM concerning driver throttle requests.

Redundant accelerator pedal sensors are used to prevent possible runaway conditions, which would obviously be very bad. But one wrinkle is that the two (or three) sensors will put out slightly different signals; it’s that way on every vehicle – some vehicles will have a “mirrored” signal from sensor to sensor and others will have a lower voltage signal on one sensor and higher voltage signal on the second sensor.

This is to differentiate between the two sensor signals for fault detection – if one of the two sensors were to fail on a system designed so that both sensors read exactly the same or if the sensor output wires were to short together, the ECM/PCM would have no way of knowing about the failed sensor or circuit. With the sensor signals slightly different, a fault easily detected.

What are the Possible Causes of the P2138 Code?

All OBD-II codes have multiple possible causes. As for what causes code P2138, there are several possibilities. These include the following:

  • A failed TP or APP sensor
  • Circuit issues, such as damaged wires or poor connections
  • An issue with the PCM, such as software in need of an update
check engine light image
One common symptom of code P2138 is an illuminated check engine light.

What are the Common Symptoms of the P2138 Code?

Below are some of the most common symptoms of the P2138 code:

  • Check engine light is on
  • Loss or lack of power
  • The engine stalls when it stops
  • Vehicle stuck in “limp mode” or “reduced power mode”

How to Diagnose the P2138 Code

Many problems can trigger the P2138 code, from a defective TP sensor to faulty wiring in the accelerator pedal. It usually takes a seasoned mechanic to pinpoint its exact cause.

However, if you are confident with your automotive repair know-how, you can try to diagnose the code yourself.

Here are a couple of videos that demonstrate what the diagnostic process might involve:

How to Fix the P2138 Code

Even though numerous DTCs share causes, symptoms, and affected areas, there is no one-size-fits-all solution for OBD-II trouble codes. To fix the P2138 code, you’ll have to identify its exact cause and determine the right repair process for your vehicle. And here’s a solid piece of advice. Always carefully check wires and connections before replacing any parts. It’s the smart thing to do.

Use online auto repair resources and guides to figure out the right fix. To avoid accidentally worsening the problem with a solution that’s inappropriate for your vehicle’s make and model, consult your owner’s manual. You can also secure an ALLDATA single-vehicle subscription for in-depth factory repair guides.

If you are uncertain of your automotive know-how, it’s best to let a professional diagnose and resolve the P2138 code.

Other Notes About P2138

The P2138 trouble code is triggered when there are problems with the throttle/pedal position sensor/switch. However, it’s not the only code related to the throttle body and its circuitry. The P2135, P2136, P2137, P2139, and P2140 DTCs also indicate issues in the same areas. Although somewhat related and seemingly similar, these codes will differ in their possible causes, symptoms, and solutions.

Products Mentioned in this Guide

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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Sam

Could this code be because of a P1602 in the case of the battery being inserted without being reset correctly ? As all codes are showing a voltage issue on my golf R 2011

CarParts.com

Hi Sam,

It’s possible that low-voltage issues could be causing code P2138 to set. On your car, when the battery voltage drops below 10 volts (or the battery is disconnected, or power to the ECM is lost), the throttle body’s basic setting may be lost. To fix the problem, a basic setting relearn procedure will need to be performed using an OEM-level scan tool.

-Mia, Chief Mechanic @ CarParts.com

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