In this guide, we will be answering a frequently asked question by vehicle owners: how to bypass reduced engine power or limp mode? I know you were quite frustrated when you have to urgently reach a certain place or drive your vehicle to hilly areas, but suddenly it goes to reduced power or limp mode. Reduced engine power may happen at various times in the driving cycle. Most likely you have noticed that your car’s engine was running at a lower RPM.
Bypassing a reduced engine power is not a solution and it should be fixed in any way. This usually occurs when the engine has to work harder to keep up with the demand on the engine to maintain proper speed. The ECU puts the vehicle in a reduced power mode when a vehicle has a damaged wiring harness, loose harness connectors, bad battery or its terminals, bad engine sensors (accelerator pedal sensor, MAF sensor, throttle position sensor, O2 sensor), low oil pressure, bad throttle body, bad air filter, and bad fuel delivery. So, you can only get rid of or bypass a reduced engine power by actually fixing a problem in your engine. So, you should read error codes stored in the engine’s memory to bypass reduced engine power. After fixing the issue, make sure to clear codes usign the scan tool.
A few days back, when I was driving my car, I felt that my car had suddenly stopped picking up speed and was showing a ‘Reduced Power Mode’ instruction on the dashboard. I came to know that something was wrong with my engine. So, I immediately drove my car to my garage and pulled the OBD2 codes using my Scan Tool. The scan tool pulled codes related to the throttle control system of my engine. When I dived in further, the issue was a faulty throttle position sensor (TPS) and Mass Airflow Sensor (MAF). Thus, by replacing the throttle position sensor and mass airflow sensor and clearing error codes using a scan tool, I was able to bypass reduced engine power.
However, there are several other fixes to bypass a reduced power in your engine. That’s why I have written this definitive guide on bypassing a reduced engine power so that you could prevent your engine from further damage.
You can also read my guide on the cost to fix a reduced engine power to get an idea of how much you would need to spend to bypass reduced engine power. In that guide, I have also explained possible causes of reduced engine power.
Table of Contents
What Does Reduced Engine Power Mean?
Reduced engine power is a ‘Loss of Engine Power’ or stopping the engine to produce more power so that the engine and its transmission can be prevented from further damage.
The reduced Engine Power mode is a safety feature deployed in vehicles so that you do not continue to run your vehicle at higher speeds or use your vehicle for towing when a fault is detected in your engine. Reduced Power Mode is also called a ‘Limp Mode‘ which means that the vehicle’s engine power has been limited by the engine control unit (ECU) so that you bring it to your dealership or nearest mechanic as soon as possible.
The reduced engine power mode is not only limited to damage to the throttle system, emission system, or combustion system. If the ECU even detects the problem in the transmission system or vehicle’s driving safety features, such as traction control, ABS, and Stabilitrak, it will put your engine in reduced power mode to prevent any accidental damage.
A check engine light can also alert the driver about any possible damage to the engine’s components. But a reduced power mode will add further precaution by stopping the driver from speeding up the vehicle.
If your engine is running rough at idle or shaking while driving, you can read my guide on check engine light flashing and car shaking.
How To Bypass Reduced Engine Power?
You can bypass reduced engine power by performing the following tasks:
- Checking wiring harness or loose connections
- Checking mass airflow sensor
- Checking throttle body
- Checking throttle position sensor
- Checking accelerator pedal sensor
- Cleaning or replacing the air filter
- Replacing fuel filter
- Replacing fuel injectors
- Replacing catalytic converter
- Fixing low oil pressure
- Check transmission fluid or fix components of the transmission system
- Fixing traction control and stability system
- Check coolant system
Check Engine Wiring Harness and Loose Connections
The most common reason for reduced engine power is loose connectors, damaged wiring harness, or bad ground connections.
So, to bypass reduced engine power mode, you should:
- Visually inspect the engine wiring harness and see if there is chaffed wire at some point.
- Locate all ground connections in your engine by taking help from the owner’s manual. These ground connections are linked with the negative terminal of the battery. You should see if the ground connections are cleaned and properly bolted down.
- Check if there is any ground strap in your engine (some engines have one). If it is damaged, replace it.
- Check harness connectors of throttle body (keep in mind that your engine may not have an electronic throttle body so it won’t have a harness connector), ECU, fuel injector, ignition coil, and engine sensors. Tug on each wire inside the connector and see if it comes out loose.
If you have GMC Chevy trucks, your scan tool might show the P1516 code when the reduced power mode is on. You can read my guide on P1516 causes and solutions to learn more.
In my guide on car won’t start after replacing battery, I have explained how to check ground connections in the engine.
2. Check Throttle Body
A bad throttle body can cause several trouble codes like P2101, P2175, P2122, P0121, P0122, P2135 or P0222, and put the engine in a reduced power mode. This happens in engines that have electronically controlled throttle bodies. If your throttle body has a motor and harness connector connected to it, it means the throttle body is electronically controlled.
The throttle body has a butterfly valve that opens as you press an accelerator pedal. The opening of the butterfly valve is controlled by a 5V DC supplied by the ECU.
The following conditions indicate the healthy butterfly valve of the throttle body:
- The throttle valve should be open 20-25% at zero throttle position i.e. when the accelerator pedal is not pressed.
- The throttle valve should not be completely closed or completely opened more than 25%.
- The throttle butterfly valve should move freely to the open and closed position under the normal spring pressure without being bound.
- The throttle should not be free to move open or closed without spring pressure.
You can check the above condition of a throttle valve by removing it from the engine, inserting your finger into the throttle body, and trying to manually open the valve.
You should also check if there are any signs of soot, grease, or carbon deposits inside the throttle body, especially at the ends of its butterfly valve.
You can watch the below video from 5:35 for a better understanding.
I would recommend CRC Throttle Body cleaner to clean the throttle body. Moreover, make sure that you do not use any wire brush to clean the throttle body. Paper towels are recommended. Also, use this Nylon brush to get into the tiny areas of the throttle body.
If the issue persists after cleaning the throttle body, it means it has an internal wiring issue. The best would be to completely replace your throttle body.
When you reinstall the throttle body, make sure to perform the throttle relearn procedure.
To relearn throttle body:
- Start and idle the engine in PARK for 3 minutes.
- With a scan tool, monitor desired and actual RPM.
- The ECM will start to learn the new idle speed and the desired RPM should start to decrease.
- Ignition OFF for 60 seconds
- Start and idle the engine in PARK for 3 minutes
- After the 3-minute run time, the engine should be idling normally
After performing relearn procedure for the new throttle body and the engine still does not run within the normal idle speed, it will be mandatory to drive your vehicle above 45 mph (70 Km/h) including several decelerations.
Also, ensure that after driving the vehicle, let the engine idle for a minimum of 5 minutes. If the engine has returned to normal idle speed, the relearn procedure is completed. If the idle speed is still erratic or incorrect, restart the idle relearn procedure.
3. Checking Throttle Position Sensor
Throttle position sensors monitor how far the throttle body is opened and send signals from the throttle body to the ECU. If the ECU does not find a correlation between the readings sent by the throttle position sensor and the mass airflow sensor, it will put your engine in reduced power mode.
A vehicle might have two throttle position sensors. One reads from low (less than 0.9V) to high voltage (around 5V) and the other from high to low voltage as you press the accelerator pedal.
You can watch below YouTube video to check throttle position sensors using a voltmeter.
4. Check Accelerator Pedal Sensors
Fixing malfunctioning accelerator pedal sensors can also bypass reduced engine power.
The accelerator pedal position sensor is a component of the electronic throttle control (ETC) system used in most vehicles today. The sensor is located under the floor mat, near the accelerator pedal. It sends signals to the throttle control computer when the driver depresses the pedal. In this way, the ECU decides how much the throttle valve should be opened.
A vehicle can have two or three accelerator pedal sensors. The voltage across APP sensors changes in the same as the voltage across throttle position sensors.
5. Checking or Replacing Engine Air Filter
Air filters remove small particles from the air and prevent them from entering the engine. However, they can clog up over time, causing a reduction in airflow. This will reduce the amount of oxygen flowing into the engine, leading to a drop in power.
A good air filter will clean out dust particles, pollen, hair, and other debris from the air. You can find an air filter in the air box that is present in the engine bay. You should always replace the engine air filter after every 20,000 to 30,000 miles.
6. Clean or Replace Mass Airflow Sensor
Malfunctioning of Mass Airflow Sensor (MAFS) can cause a reduction in the amount of air flowing through the engine. The airflow sensor sends a signal to the ECU (Electronic Control Unit) which then adjusts the fuel-air mixture to match the requested RPM.
If the sensor malfunctions, the engine may be running leaner than normal. As a result, the engine may not be producing enough power and the car may not accelerate.
To clean the MAF sensor, follow these steps:
- Disconnect the negative terminal of the battery.
Remove the MAF sensor, it is located on the intake tube between the air filter box and the throttle body facing upwards.
- Spray all of the little sensors you see inside of the MAF carefully with this MAF sensor cleaner.
Reinstall and reconnect the battery terminal.
- Start the engine and let the ECU relearn idle for 15 minutes.
You can check out the below video to test the mass airflow sensor:
7. Check Transmission Fluid and Components of Transmission System
If you have an automatic transmission system, you should check if there is a low transmission fluid level to bypass reduced engine power.
In an automatic transmission, the transmission oil is used to operate the torque converter and engage and disengage various clutches and bands to engage gears. If the transmission fluid level is low or dirty, the ECU will put your vehicle in a reduced power mode to prevent further to the transmission system.
Other components of transmission system that you should check include:
- Transmission speed sensor
- Torque converter
- Oil gear pump
- Valve body
- Clutches and drums
- Transmission solenoid valves
8. Low Oil Pressure
Low oil pressure can cause the engine to lose power. Oil pressure is measured in psi (pounds per square inch). It is the force that pushes the oil through the engine’s system. The oil pressure sensor measures the oil pressure and sends signals to the ECU. If the pressure is lower than the threshold value, the ECU will reduce the amount of fuel being injected into the engine, turn on the low oil pressure light and may also show the message of reduced power mode.
The ideal oil pressure depends on the type of engine. You can read my guide on low oil pressure at idle to learn about the recommended oil pressure and different causes of low oil pressure.
Before diving into the root cause of low oil pressure, you should first check oil pressure sending unit. Chances are that it is malfunctioning. After that, you should check the quality of the oil in your engine. You can read my guide on SAE 30 vs 10W30 to understand the types of motor oils. I have also written an in-depth comparison guide between Pennzoil Platinum and Castrol Edge.
9. Check Traction Control/Stabilitrak System
If your vehicle dashboard is displaying a Service Traction Control/Service Stabilitrak message along with the Reduced Power Mode message, it means that you have to inspect your vehicle’s stability system to bypass reduced engine power. In my guide on what does TC mean in a car, I have explained the working of the traction control system and how it differs from Stabilitrak/Stability system.
Traction Control (TC) is a safety feature that is designed to prevent the vehicle from spinning out. The system utilizes wheel speed sensors mounted on the wheels of the car and works with Antilock Braking System to maintain the stability of the vehicle. When the ECU detects an issue with the stability system of the vehicle, it will put the vehicle into a reduced power phase to prevent further damage and to keep the passengers safe.
To fix the traction control system and bypass reduced engine power, you should check:
- Tire pressure monitoring sensors
- Steering angle sensor
- Wheel speed sensor
- Brake fluid
- Yaw rate sensor/lateral acceleration sensor
- ABS (Anti-lock braking system)
10. Check Catalytic Converter
The catalytic converter is a device that cleans exhaust gas emitted by vehicles to reduce the number of harmful gases in the air. The main function of the catalytic converter is to convert carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC) into carbon dioxide (CO2), water vapor, and nitrogen (N2).
The catalyst can be blocked up by soot particles, oil, gasoline, water and other substances. When this happens, the oxygen molecules in the exhaust are trapped by the catalyst, which prevents them from reacting with other chemicals and reduces the power of the engine.
If your scan tool is showing any OBD2 code related to bad emissions or catalytic system efficiency, you should immediately check the catalytic converter. A clogged catalytic converter also causes the engine to overheat. In that case, you can bypass reduced engine power by replacing a catalytic converter.
11. Check Spark Plugs
Spark plugs can cause reduced engine power due to engine misfire. The spark plug is located in the cylinder head of the engine. It is connected to the distributor via the ignition coil or directly tightened into the ignition coil. It is used to generate a spark that ignites the fuel/air mixture in the combustion chamber of the engine. With time, the spark plug is fouled with carbon deposits, oil film, ash deposits, and corrosion. In that case, you have to replace spark plugs to bypass reduced engine power. This guide (PDF) is good to identify bad spark plugs.
12. Check Coolant System
Any damage to the coolant system can also put the engine into reduced power mode to prevent engine damage.
The coolant system is responsible for keeping the engine cool. This cooling system consists of a radiator, coolant pressure regulator, coolant temperature sensor, a water pump, a heater core, and hoses connecting all these parts. A coolant leak may cause overheating of the engine.
So, if you accelerate the vehicle, more fuel combustion will take place and more heat will produce. To prevent this, the ECU puts the vehicle in a reduced engine power mode so that engine temperature can’t be increased above a certain limit. You can read my guide on P0128 code to learn the coolant system in the engine.
When the reduced engine power warning light comes on, your car is telling you that something is wrong with your car’s computer control system. This means that the engine is not producing the same amount of power that it was capable of producing before the light came on. This usually occurs when the vehicle’s computer system detects a problem with one of the car’s sensors.
The computer control system monitors the engine’s performance and warns you when there is a problem. So, to bypass reduced engine power, you should first scan all error codes and find all possible causes of reduced engine power. After fixing the engine, make sure to clear all trouble codes and reset the ECU.