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The "Reduced Engine Power" Message and Fixes for it

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

A number of issues can cause a Reduced Engine Power message.

A number of issues can cause a Reduced Engine Power message.

Your car's computer can throw your engine into Reduced Engine Power (REP, limp-in mode), when detecting one or more potential faults. Depending on your particular car's make and model, these faults may include problems with the:

  • Throttle control system
  • Accelerator pedal position sensor
  • Clutch
  • Throttle position sensor
  • Wiring in a specific circuit
  • Catalytic converter
  • Timing belt
  • Transmission speed sensor
  • Fuel pump
  • Oxygen sensor
Index

1. What a Reduced Engine Power Light Means

2. Trouble Codes Related to Reduced Engine Power

3. An Automatic Transmission in Limp-In Mode

4. GM and Ford Models With "Reduced Engine Power" Issues

5. Can You Fix a Reduced Engine Power Issue at Home?

6. Video: Fixing a 2007 Chevy 2500 HD with a "Reduced engine power" message.

Throttle control systems are a common source of Reduced Engine Power issues.

Throttle control systems are a common source of Reduced Engine Power issues.

1. What a Reduced Engine Power Light Means

When your engine or transmission goes into limp-in mode, it operates with limited functionality and speed. This limited mode is intended to help the driver get to a service shop when the ECM or transmission control module (TCM) detects a serious problem with a vital sensor, circuit, engine control or transmission component that requires immediate attention.

Usually, a REP light shows along with a check engine light (CEL), and, in some cases, a Service Traction Control message as well.

You can use a scan tool to download trouble codes stored in the electronic control module (ECM) of your car. To download trouble codes related to the traction control system (TCS), you will need a scan tool that can connect with the anti-lock brake system (ABS) controller.

Download trouble codes that can guide you to the source of the problem.

Download trouble codes that can guide you to the source of the problem.

2. Trouble Codes Related to Reduced Engine Power

Depending on your particular vehicle model and the fault detected by the computer, a Reduced Engine Power message may appear along with a particular trouble code in the ECM's memory:

  • P0120-P0124. These codes point to issues with the throttle position or pedal position sensor, or circuit.
  • P1125. Faulty system or component may vary depending on make and model (additional codes may include: P1276, P1281, P1286)
  • P1516. Chevrolet - Throttle Actuator Control Module Throttle Actuator Position Performance
  • P1518. Faulty system or component may vary depending on make and model.
  • P2111. Throttle actuator control system, stuck open
  • P2135. Accelerator pedal position (APP) / Throttle position (TP) sensor/switch A/B - voltage correlation

On drive-by-wire systems, pay special attention to DTCs that point to the throttle actuator. Make sure the harness connector and wires are not loose or damaged. This fault will trigger the Reduced Engine Power light.

Automatic transmission problems can also lead to reduced engine power.

Automatic transmission problems can also lead to reduced engine power.

3. An Automatic Transmission in Limp-In Mode

When an automatic transmission goes into limp-in mode, limited functionality may vary, depending on the particular vehicle make and model. Your function may be limited to:

  • Only first and third gear available while in Drive.
  • Only Park, Neutral, Reverse and Second gears.
  • Being able to drive in second or third gear at a low speed.
  • The transmission being locked in third or second gear.
Some Ford F-150 models were reported with reduced engine power issues.

Some Ford F-150 models were reported with reduced engine power issues.

Known Problems Causing REP in Some Models

If necessary, check with your local dealer for any service bulletin or recalls on your particular vehicle make and model. You can also check for recalls specific to your vehicle by going to this NHTSA database site.

Throttle Assembly Problem in Some GM Models

Some GM models have suffered from the REP problem because of a fault affecting the throttle assembly. Usually, you'll get DTC P2135 along with the REP message.

The following models may need a new throttle position sensor (TPS) cover, or have the TPS and throttle body replaced as a single assembly. Furthermore, some models (2008-2010) may need to have the ECM reprogrammed.

The following models fall under this category:

  • 2008-2009 Chevrolet TrailBlazer
  • 2008-2011 Chevrolet Tahoe Two-Mode Hybrid
  • 2009-2011 Chevrolet Silverado Two-Mode Hybrid
  • 2009-2011 Cadillac Escalade Two-Mode Hybrid
  • 2008-2009 GMC Envoy
  • 2008-2009 HUMMER H2
  • 2008-2010 HUMMER H3
  • 2008-2011 GMC Canyon, Savana, Sierra, Sierra Denali, Yukon, Yukon Denali, Yukon XL, Yukon XL Denali
  • 2008-2011 Cadillac Escalade, Escalade ESV, Escalade EXT
  • 2008-2011 Chevrolet Avalanche, Colorado, Express, Silverado, Suburban, Tahoe
  • 2009-2011 GMC Sierra Two-Mode Hybrid
  • 2008-2011 GMC Yukon Two-Mode Hybrid

Engines affected include:

  • L94, LY5, L20, LMG, LC9
  • LH8, LH9, L76, LZ1, LS2
  • L96, RPO, LH6, LY6, L9H
  • LY2, L92, LFA, LMF

Charge Air Cooler in the Ford F-150 (2001-2013)

Similar concerns were investigated by the National Highway Traffic Safety Administration a few years ago, after some drivers of the popular Ford F-150 pickup truck reported the vehicle would suddenly experience reduced engine power while moving at highway speeds. Models included 2001 through 2013, equipped with the 3.5L, turbocharged direct injection engine.

Company service bulletins indicated some problems with the Charge Air Cooler (CAC). It seems to have a tendency to accumulate moisture while cruising at highway speeds for a period of time under humid conditions. The repair includes replacing the CAC and reprogramming the vehicle's computer.

Carefully check harness connectors and wires that might have triggered a Reduced Engine Power message as reported by the ECM.

Carefully check harness connectors and wires that might have triggered a Reduced Engine Power message as reported by the ECM.

5. Can You Fix a Reduced Engine Power Issue at Home?

Usually, the Reduced Engine Power message appears with a DTC in the computer's memory that points to a fault in a specific system or sensor. However, don't immediately assume that something is wrong with a particular system or component described by the DTCs. The fault can be with a circuit, harness, or connector, or even in a different system or component altogether.

Faulty engine grounds are sometimes to blame for many apparently faulty parts or wires that connect the many sensors and actuators to the electronic control module (ECM). So one of the first checks you want to make is the condition of engine grounds, including battery connections.

A faulty ground connection will cause voltage, and current, to drop, leading to all kinds of driveability issues. So check the engine grounds in your vehicle with the use of a digital multimeter. This will make it easier to detect loose, corroded or damaged connections that might be affecting sensors, actuators or system circuits.

Then, if you found any DTCs, test the particular component or circuit that the code points to, before replacing components, to make sure it is faulty.

When checking electrical components, remember to turn the ignition key off. Then:

  • Unplug the connector and check for dirt or corrosion.
  • Tug at the wires on the harness connector to check for loose wires.
  • Check the connections on both ends of the wires to verify good connections.

Watch the following two videos that show how a damaged wire caused a Reduced Engine Power message to appear in a 2007 Chevy HD.

Fixing a 2007 Chevy 2500 HD With a "Reduced engine power" Message

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 Dan Ferrell

Comments

Mario Gamez on June 15, 2020:

Thank you so much for making this video. Very helpful!!

Nederland, Texas

Larry Slawson from North Carolina on September 05, 2019:

Very informative! Thank you for sharing!