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What is Engine Power Reduced message on GM vehicles?

“Engine Power is Reduced,” this message can be seen on the dashboard of GM vehicles, which have an electronic throttle body (ETB). Those vehicles have no mechanic cable connecting the throttle plate and accelerator pedal. The electric motor inside the throttle body opens and closes the throttle plate, and a vehicle computer fully controls the movement of the plate by sending a signal to the motor. When the computer detects something wrong with the engine, it shows “engine power reduced” message and may put the engine into “limp home” mode.

What to do when Engine Power Reduced message comes on?

It depends on the problems. Interestingly, the vehicle acts differently depending on the type of the problem. Some obvious problem, overheat, for example, the message shows up, and a coolant-temperature-warning light illuminates as well. The acceleration and speed are heavily restricted. In this case, you have to stop the vehicle right away and cool it down. Sometimes, however, you don’t see any reduction in performance although the message says “Engine Power is Reduced.” In this case, you may drive to the destination. But, the engine performance may be reduced next time you drive. So, whenever you see this message, consider getting off the road and out of traffic for an emergency stop, or going to see a mechanic or straight home to diagnose the problems as soon as possible.

Common Causes of Engine Power Reduced

If you have the message with the check engine light on, get the diagnostic trouble codes with an OBD scan tool. You may see P0120 thru P0124, P1516, P2135 thru P2140. All these codes are related to an electronic throttle body (ETB). If your vehicle has a lot of mileage, the inside of throttle body may be dirty and sticky. An excessive carbon built-up affects idle speed, and a sticky throttle plate can cause a hesitant acceleration. Both of them confuse the computer finding the right throttle position, and it sets one or more of the trouble codes above. So, if it’s dirty, clean it up with an appropriate cleaner and perform the idle relearn procedure.

Read Also: How to clean the Throttle Body?

Sometimes, a throttle position sensor (TPS) can be faulty and set the codes above. While most TPSs for the other manufacturers comes in one assembly with the throttle body, TPS for GM can be separate from an ETB and replaced relatively easily. You can save money by replacing the TPS only instead of replacing the whole assembly. TPS alone may cost around $100 less than the whole assembly, and it may come with a new harness.

But, please make sure that you may still need to clean up the throttle body if it’s dirty and perform the idle relearn procedure. And, be aware the electric motor which opens and closes the throttle plate is integrated into the throttle body. So, if the problem lies on the electric motor (as well as gears and pinions), replacing TPS won’t fix the problem.

Read Also: GM Idle Relearn Procedure

Another common and possible cause related the ETB is a damaged harness or wires attached to an ETB. Most of GM’s ETBs have either 6 or 8-pin connector. These connectors are not that reliable. If you wiggle each wire, and the engine shows any kinds of reaction like an RPM surge, drop, or even an engine stall, you need to fix the harness. Fortunately, you can buy the new harness alone. It costs much less than the TPS or ETB. But, remember you still need the idle relearn procedure after the replacement.


Engine Power Reduced is the message that the vehicle computer informs you that something wrong with the engine. At the same time, the computer can fully control and limit performance thanks to modern vehicle’s technology. You might think it’s annoying, but it actually protects the engine from further damages. So, whenever you see the message, act appropriately. Sometimes, you may not feel much reduction in performance. Even so, you should diagnose and find the cause of the problem as soon as possible.

GM Idle Relearn Procedure

After replacing a throttle body of late models of GM vehicle, a vehicle computer must relearn the idle position. Late models of GM vehicles equip an electronic throttle body (ETB), which eliminates the need for an idle air control valve and a physical cable between the throttle plate and accelerator pedal. The ETB is fully controlled by the vehicle computer, and it adjusts the opening to maintain the idle speed with specification and respond to accelerator pedal movement. In the event of losing the communication between the ETB and the computer such as a dead battery, cleaning or replacing an ETB, etc., the computer must relearn the new throttle position. Here is GM idle relearn procedure.

Perform the following relearn procedure for the on-board computer to adjust to the new idle position on the electronic throttle body.

1) Start and run the engine in PARK for 3 minutes.
2) Turn the engine OFF for a minimum of 60 seconds.
3) Start the engine and run for another 3 minutes.
4) Engine should idle normally after 3 minutes.

After having performed the relearn procedure and the engine still does not run within normal idle speed, it will be necessary to drive the 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 idle relearn procedure.


Please note that you need to clear the diagnostic trouble codes (DTCs) before performing the procedure. If you don’t have a scan tool, you can clear by disconnecting a negative battery cable for around 15 minutes.

“Engine Power is Reduced” message

Many General Motors vehicle owners have experienced the “Engine Power is Reduced” message on the dashboard. When the computer detects the problem, it puts the vehicle into the limp mode for safety. It literally reduces the engine power and disables the traction and stability controls. There are many possible causes for the message to show up. A dirty and sticky throttle plate can be the cause, and a faulty throttle position sensor can be another.

You don’t have to panic when you see this message. If the DTCs are set (=check engine light is on) with this message, just scan the computer and get the codes. You may get P0120 through P0124 or manufacturer specific code P1516,P2135, or P2140. All these codes are related to the electronic throttle body. A sticky throttle plate and/or a faulty throttle position sensor are the most common cause of the problem. Keep that in mind, when you see this message, take a look at the throttle plate. If it’s dirty, clean it up and perform the idle relearn procedure, and it may fix the problem.

Related Posts

GM Fuel Pump Wiring Diagram

This is a story from my personal experience as an auto mechanic. I worked on 1996 Chevy Tahoe V8 5.7L. The owner of the car complained that the car was hard to start after he parked for a while. Once it started, it drove fine and never had problems such as the engine stalls and shakes. After the engine shuts off, it could start right back if the car did not sit for more than 5 minutes.

First, I suspected a fuel leak. 5 minutes are enough time to lose the pressure in the fuel system if there is a leak. I thought there was too small leak to affect the engine performance. I thoroughly checked by leaving the car on the lift for more than an hour, but I could not find any leak. I scratched this possible cause from the list.

My next target was a fuel pump: the pump delivers fuel and creates fuel pressure. I thought the fuel pump did not work hard and fast enough. I installed a fuel gauge and measure the fuel pressure, and that was it! The pump built up the pressure just before and after the engine started. When there was no pressure, the engine would not start. That forced me to think of a leak, but I had not found anything earlier, and I soon noticed that the pump was not working when experiencing the problem.

At that time, I found it a little odd. Because the symptom indicated a physical problem like a leak, but it is an electrical problem. I came up with some things, but none of them accounted for all the symptoms. Time was up. I told the customer to come back tomorrow, got off work. As far as I know, the customer never came back to the shop. The problem was not solved until I happened to see a wiring diagram of the same car.

That was a long time ago. I was a newbie as an auto mechanic, and the shop I worked for never had auto repair manuals or wiring diagrams. If I go back to the past and the same situation, I would go straight to the library and try to look for a repair manual instead of wait for more than 5 minutes every time I tried to locate the problem. There would have been enough time to fix the problem and give the car back to the customer on the same day.

Looking up wiring diagrams is a short cut for finding the cause of problems. Reading repair manuals before you start a job saves your time. This is one of my experiences that remind me how important auto repair manuals are.

In case you wonder what was the cause of the problem on Chevy Tahoe, look at the diagram.

1996 Chevy Tahoe GM Fuel Pump Wiring Diagram

A relay for fuel pump was bad whole time. On older GM vehicles, an engine oil pressure switch is giving power to a fuel pump while the engine is running. ECM sends a signal to the relay at the item of only start-up.