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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. Usually, this is associated with a fault in the electronic throttle control (ETC) system. However, depending on your particular car's make and model, these faults may include problems with the:

  • Electronic throttle control (ETC) system
  • Accelerator pedal position sensor
  • Clutch
  • Throttle position sensor
  • Circuit fault in the throttle control system
  • Fault in the cooling system
  • Catalytic converter
  • Timing belt
  • Transmission speed sensor
  • Fuel pump
  • Oxygen sensor
  • Data network (module) issues

Some models dealing with this issue, may still be able to move at very low speeds, while others may not not go beyond an idle.

Index

1. What a Reduced Engine Power Light Means

2. ETC Related to Reduced Engine Power Issues

3. How an ETC System Works

Video: A Lexus Electronic Throttle Control (ETC) System

4. Trouble Codes Related to Reduced Engine Power

5. An Automatic Transmission in Limp-In Mode

6. Known Problems Causing REP in Some Models

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

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

9. Cleaning the ETC Body and Valve

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.

Many Reduced Engine Power (REP) warning lights are related to faults in the ETC system.

A fault in this system usually results in the vehicle going into limp-home or fail-safe mode.

As a driver, you may notice one or more of the following driveability issues:

  • Engine speed reduced between 1200 and 1600 RPM.
  • Poor accelerator pedal response.
  • Cruise control system disabled.
  • Check engine light on.

The ETC fault will also trigger the REP message on the dashboard to come on.

However, different manufacturers handle the REP message differently. For example:

  • Reduced Power light on most GM vehicles.
  • Red lightning bolt symbol on some Chrysler models.
  • Wrench symbol light on some Ford model vehicles.

3. How an ETC System Works

The follwoing video gives you an overview of the operation of an electrnoic throttle control system and how it monitors faults. This particular one is from Lexus but other models work under the same general principles.

A Lexus Electronic Throttle Control System

Read More from AxleAddict

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.

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)
  • P1512 Minimum Throttle Position Not Learned
  • 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.

5. 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.

6. 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.

Water Leaks and Aftermarket Equipment in Some GM Models

Some of these models have seen the MIL (malfunction indicator light) turned on intermittently along with a REP message on the dashboard.

The problem usually triggers the DTC P2138 (Accelerator Pedal Position sensor issue).

What triggers a P2138:

  • Water leak reaching the instrument panel to body harness connector.
  • Installation of aftermarket equipment.

Depending on the particular model, a water leak may occur on a:

  • windshield or cowl
  • sunroof
  • pillar seal

A harness connector may rust if water reaches the electrical pins, causing voltage to drop.

Models involved:

  • 2005-2009 Hummer H2
  • Saab 9-7X
  • 2005-2011 GM cars and light-duty trucks
  • 2005-2011 GM Saturn
  • 2006-2010 Hummer H3
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.

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

Broadly speaking, you'll need a scan tool to diagnose a REP issue.

Many auto part stores will download the DTCs for you, if necessary.

However, you still can perform some checks on your own, if necessary.

When diagnosing the ETC throttle as it operates, listen for any noises during idle as you or an assistant depress the accelerator pedal. Pay attention to what it does. Noise during operation is a sign of a bad throttle control assembly.

Also, look inside the throttle body as the valve moves. If there is any carbon buildup, the throttle body and valve have to be cleaned. See the procedure below.

Components to Look Into

On vehicle equipped with the ETC system, REP messages may be triggered by:

  • Low battery voltage
  • ETC actuator motor fault
  • Both throttle position (TP) sensors have gone bad
  • A TP sensor and the MAP (manifold absolute pressure) sensor have failing at the same time
  • The ETC throttle spring has failed
  • Dirty electronic throttle body barrel or valve

The last section in this post goes over the general procedure to check and clean the throttle body, if necessary.

Check any of these components with the help of your vehicle repair manual.

If you don't have this manual yet, you can buy a relatively inexpensive copy through Amazon. Haynes manuals include step-by-step procedures for many troubleshooting and maintenance procedures. It also comes with photographs and images, electrical diagrams, and a service schedule for easier repair and vehicle maintenance. So you'll recoup your small investment in a short period of time.

Faulty Grounds

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.

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

9. Cleaning the ETC Body and Valve

Even those models equipped with an ETC system require a cleaning body and valve job from time to time.

Often, when it becomes dirty, you'll notice one or more of the following symptoms:

  • Rough idle
  • Low idle speed
  • Stalling when coming to a stop

Check your car owner's manual or vehicle repair manual for any warnings and specific procedures to clean the ETC body and valve.

In general, you can follow this cleaning procedure:

  1. Remove the ignition key form the vehicle. Make sure no one has access to the key and accidentally tries to turn on the ignition. If it comes "alive", the ETC motor can cut off one of your fingers if trapped between the valve blade and the throttle body barrel.
  2. Detach the air inlet hose and air duct from the throttle body to gain access to the throttle valve.
  3. Dampen a shop rag with throttle body cleaner.
  4. Use the shop rag to remove varnish and carbon deposits from inside the throttle walls, around and under the throttle valve. Don't spray cleaner directly into the ETC body. You may damage a sensor.
  5. Reinstall the air duct and air inlet hose.
  6. Follow the steps in your vehicle repair manual to do a throttle body relearn procedure, if necessary. This will allow the car's computer to get the correct throttle position and the engine to idle correctly.

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

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