“Reduced Engine Power” or “Engine Power is Reduced” is a warning message that continues to baffle and frustrate many General Motors (GM) vehicle owners.
Is your Chevy or other GM vehicle displaying this warning on the instrument cluster? Maybe the check engine light is on as well? Don’t panic and trade-in your car just yet—there may be a relatively easy (and affordable) remedy for your concern.
In this article, we’ll explain the meaning of GM’s “Engine Power is Reduced” warning, as well as the resulting Reduced Engine Power Mode. We’ll also cover the warning’s common causes, and explore the related service information.
What Does “Reduced Engine Power” Mean?
In order to help you further understand the concept, let’s discuss what the “Reduced Engine Power” warning means and what can happen once your vehicle enters Reduced Engine Power Mode.
“Reduced Engine Power” Message
In most cases, the Engine Power Reduced warning indicates that your car’s performance has been intentionally limited. Your car’s primary computer, often known as the powertrain control module (PCM), triggers Reduced Power Mode when it detects a system failure.
There are also some instances where the light may turn on when the PCM detects a noticeable reduction in the vehicle’s performance due to an underlying concern.
If you’ve got the owner’s manual to your GM vehicle (Chevy, Buick, etc.) handy, you can flip to the section about the driver information center (or instrument cluster). There, you’ll find a general overview of what the Engine Power Reduced message means for your particular year and model.
Reduced Engine Power Mode
As previously mentioned, your vehicle’s PCM may trigger the Reduced Engine Power Mode upon detecting a system failure. GM’s Reduced Power Mode can inhibit your vehicle’s ability to accelerate. Even if power isn’t reduced immediately, performance may be limited the next time you go to drive the car. In some instances, the PCM may actually cut off fuel delivery to the engine, rendering the vehicle undrivable.
When this happens, GM’s Engine Reduced Power Mode is being implemented as a “fail-safe” mode (also known as a “limp” mode). But GM isn’t the only automaker to equip its vehicles with a fail-safe mode—all modern cars have some type of fail-safe strategy built into them.
What Causes the “Reduced Engine Power” Message?
Asking what engaged reduced engine power warning message is somewhat like asking what turned on your check engine light—there are too many possibilities to list. Plus, GM has changed its fail-safe strategies over the years, so exactly what can engage the message depends on the year and model of your vehicle.
Still, there are some problems that trigger the Engine Reduced Power message more often than others. Here are some of the issues that are best-known for causing the warning to illuminate:
Electronic Throttle Actuator (Throttle Body) Problems
One of the most common triggers is a problem with the electronic throttle actuator control (TAC) system. Modern GM vehicles use this layout in place of a traditional, mechanical throttle body and linkage. The technology was first introduced on the 1997 C5 Corvette.
In the TAC system, the PCM monitors two accelerator pedal position (APP) sensors to determine the driver’s desire for acceleration. Then, the device calculates the appropriate throttle response from a pair of throttle position (TP) sensors. Once the module has the necessary information from the sensors, the PCM uses an actuator motor (integrated into the throttle body) to operate the throttle, thereby controlling airflow into the engine.
When the throttle body fails or becomes dirty, it can trigger Reduced Engine Power mode.
Bad Pedal Position Sensor
A problem anywhere in the TAC system can easily trigger the Reduced Engine Power warning on the dash. For instance, the problem could be one (or both) of the APP sensors, which are usually integrated into the accelerator pedal.
Faulty Throttle Position Sensor
The other primary input to the TAC system is a pair of TP sensors. When one (or both) of the sensors goes bad, the vehicle will enter Reduced Engine Power Mode.
Circuit issues, such as damaged wires and poor connections, can prevent the TAC system from operating properly. This can cause an array of problems, including a vehicle that enters Reduced Engine Power mode.
PCM or Data Network Problems
Most late-model vehicles contain dozens of computers (also known as modules) that communicate with one another over a data network. If the network gets interrupted, or the PCM is having issues, the vehicle may enter Reduced Engine Power mode.
Idle Relearn Procedure Needed
On many GM vehicles, an idle relearn procedure is required whenever the throttle body is cleaned or replaced. Failing to do so can result in the vehicle entering Reduced Engine Power Mode.
Cooling System Issues
Some GM vehicles will enter Reduced Engine Power Mode when the engine begins to overheat due to cooling system issues. The PCM triggers the mode to protect the engine from further damage.
Engine Performance Problems
In some instances, the Reduced Engine Power warning may illuminate to inform the driver that the vehicle lacks power due to an underlying condition. For example, a faulty high-pressure fuel pump might reduce engine power, thereby causing the warning to illuminate.
Diesel Engine Performance Problems
If you’ve got a Duramax diesel, the Reduced Engine Power warning could Illuminate for a variety of reasons. There are many different technical service bulletins (TSBs) for Duramax-powered vehicles that mention Engine Reduced Power mode. Each document lists something different—ranging from a damaged MAP sensor connector to an air intake leak—as being a potential cause.
How to Fix Reduced Engine Power Mode
Getting rid of the “Reduced Engine Power” warning light on your vehicle isn’t a straightforward affair—you’ll have to perform a proper diagnosis before you can figure out the appropriate fix.
To further complicate things, there are countless reasons why this message may pop up on your dash, and the potential causes for entering Reduced Engine Power Mode will vary depending on the year and model of the vehicle.
If you’re troubleshooting a car that’s in this mode, here are some steps you can take:
Check for Diagnostic Trouble Codes (DTCs)
The first thing you’ll want to do is check for DTCs with a scan tool or code reader. DTCs can point you in the right direction for further diagnostics.
Keep in mind that codes aren’t a complete diagnosis—but they will tell you where to begin your analysis.
As was mentioned, one of the primary causes of Reduced Power Mode is a problem with the TAC system. Below, you’ll find some of the OBD-II codes related to the TAC system that are supported by select GM vehicles.
You may find one or more of these codes stored in the PCM when the Reduced Engine Power light is on.
OBD-II Codes Commonly Associated With the TAC System
It’s important to note that the codes below are the generic definitions, as defined by the Society of Automotive Engineers (SAE). GM’s definition of each code may vary.
Consult a repair manual or repair database to learn the exact code definition for your vehicle.
- P0120 Throttle/Pedal Position Sensor/Switch “A” Circuit
- P0121 Throttle/Pedal Position Sensor/Switch “A” Circuit Range/Performance
- P0122 Throttle/Pedal Position Sensor/Switch “A” Circuit Low
- P0123 Throttle/Pedal Position Sensor/Switch “A” Circuit High
- P0220 Throttle/Pedal Position Sensor/Switch “B” Circuit
- P0221 Throttle/Pedal Position Sensor/Switch “B” Circuit Range/Performance
- P0222 Throttle/Pedal Position Sensor/Switch “B” Circuit Low
- P0223 Throttle/Pedal Position Sensor/Switch “B” Circuit High
- P0638 Throttle Actuator Control Range/Performance
- P0639 Throttle Actuator Control Range/Performance
- P2100 Throttle Actuator Control Motor Circuit/Open
- P2101 Throttle Actuator Control Motor Circuit/Open
- P2105 Throttle Actuator Control System – Forced Engine Shutdown
- P2119 Throttle Actuator Control Throttle Body Range/Performance
- P2120 Throttle/Pedal Position Sensor/Switch “D” Circuit
- P2122 Throttle/Pedal Position Sensor/Switch “D” Circuit Low Input
- P2123 Throttle/Pedal Position Sensor/Switch “D” Intermittent
- P2125 Throttle/Pedal Position Sensor/Switch “E” Circuit
- P2127 Throttle/Pedal Position Sensor/Switch “E” Circuit Low Input
- P2128 Throttle/Pedal Position Sensor/Switch “E” Circuit High Input
- P2135 Throttle/Pedal Position Sensor/Switch “A”/”B” Battery Correlation
- P2138 Throttle/Pedal Position Sensor/Switch “D”/”E” Battery Correlation
- P2176 Throttle Actuator Control System – Idle Position Not Learned
Look Up Technical Service Bulletins (TSBs)
Checking technical service bulletins (TSBs) issued by the vehicle manufacturer can also be very helpful. TSBs address known issues by documenting a repair procedure that will (hopefully) fix the problem on certain makes and models. For instance, there are TSBs that address Reduced Power Mode on select GM vehicles.
In some cases, you’ll find a TSB that documents the correct repair procedure for your vehicle. Applying that procedure from the beginning can save you both time and money.
ALLDATA and Mitchell 1 have single-vehicle subscriptions for DIYers that provide detailed factory repair information, including most TSBs.
Examples of TSBs Related to Reduced Engine Power Mode
There are many different TSBs that mention Reduced Engine Power Mode. You can consult a repair database (e.g., ALLDATA DIY or Mitchell DIY) to obtain the TSBs for your year and model.
Here are a few examples of TSBs that mention Reduced Engine Power Mode:
- PIP4578B: Mentions that Reduced Engine Power Mode may be triggered if an idle relearn procedure is not performed after servicing the throttle body. The TSB applies to many different years and models.
- PIT3138A: Mentions that Reduced Engine Power Mode may be triggered due to an electrical short inside the cooling system fan clutch. The TSB applies to 2002-2005 Midsize GM SUVs.
- PI0759: Mentions that Reduced Engine Power Mode may be triggered due to a faulty manifold absolute pressure (MAP) sensor connector pigtail. The TSB applies to 2011-2012 diesel-powered trucks and vans.
Consult the Factory Repair Information
It’s also a good idea to consult the factory repair information (from a repair manual or database) before digging in too deep. As was mentioned, exactly what causes Reduced Engine Power mode varies depending on the year and model of your vehicle. If you don’t research your particular application beforehand, you may end up chasing your tail.
Another reason why you may want to obtain access to a good repair manual is to find comprehensive diagnosis and repair instructions for your vehicle. Retrieving DTCs and looking up TSBs may bring you closer to resolving your “Reduced Engine Power” problem, but you’ll still have to perform additional diagnostic steps before you can move on to the repair—and having the appropriate repair information on-hand is key to making the process go as smoothly as possible.
What if You Can’t Fix Reduced Power Mode Yourself?
In some instances, you might get lucky, and a simple procedure, such as cleaning the throttle body, will get your vehicle out of Reduced Power Mode. But all too often, solving the problem involves advanced diagnostic skills and equipment that most DIYers don’t have.
Instead of wasting time and money trying to fix the problem yourself, in many cases, it’s more effective to have a professional tackle the job for you. A reputable repair shop will be able to get the Reduced Power warning to stay off—and that will get you back on the road.
Frequently Asked Questions
Technically, you can (usually) drive slowly when Engine Power Reduced is illuminated on the dash. However, doing so is not recommended. It is best to tow your vehicle to the nearest repair facility, as driving at extremely low speeds can be dangerous. Not to mention, your vehicle is in a fail-safe state because something is wrong, and pushing it further could cause additional damage.
Also, there are instances where a vehicle will shut off completely after entering Reduced Engine Power mode. Then, you’ll have no choice but to tow it in for repair.
Much like your car’s check engine light, the reduced engine power warning cannot be reset with a simple push of a button or flip of a switch. To get the warning to go off and stay off, you (or your mechanic) will need to fix whatever underlying problem triggered it in the first place.
When the warning pops up, the first step towards fixing the problem is to retrieve diagnostic trouble codes from the car’s computer. These codes can serve as a starting point for further troubleshooting that will (hopefully) lead to the vehicle’s repair.
Once the car is fixed and the codes are cleared, the reduced power warning will go away.
There a many possible reasons why your car might exhibit a hesitation and/or lack of acceleration. Some of the most common causes include:
1. Inadequate fuel pressure and/or volume due to issues (e.g., faulty fuel pump or restricted fuel filter)
2. Excessive backpressure caused by restricted exhaust or catalytic converter
3. Sensor issues creating improper air/fuel ratio (mass airflow sensor is a common culprit)
4. Ignition system problems (e.g., bad ignition coil or worn spark plug) causing a misfire under load
5. Internal engine issues leading to a loss of compression
6. Problem in the variable valve timing (VVT) or cylinder deactivation system
7. Problem with turbocharger or supercharger (if equipped)
8. Vehicle stuck in “limp mode” or “reduced power mode” due to an underlying problem
Also, an automatic transmission that’s slipping can lead to an increase in engine RPMs without an increase in vehicle speed. The driver often interprets this sensation as a loss of acceleration.
Although not the most common cause of reduced engine power, there are instances where a low oil level can lead to a lack of acceleration. For example, modern engines feature variable valve timing (VVT) systems that rely on oil pressure to operate. Engine oil that is low or extremely dirty can inhibit the performance of these systems, potentially leading to hesitation and lack of acceleration.
In some instances, the vehicle may even enter a “limp mode” or “reduced power mode” in response to a low oil level that creates a lack of oil pressure.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.