Reduced Engine Power: What Causes It? And How Can I Fix It?

Nowadays, it feels like modern vehicles have become more like driving computers than actual cars or trucks. They’re incredibly technologically advanced, with computers and sensors controlling just about every aspect of your driving experience.

While all of this aims to make it as safe, convenient, and easy as possible for you to drive, anything that upsets the balance maintained by each sensor and module could cause a fault in the drive system, and make any number of your error lights come on.

Car dashboard with reduced engine power light coming on

Some can be particularly unnerving, especially the reduced engine power light, which is what we’re going to be discussing in detail throughout this article.

An engine is meant to run a certain way. If it isn’t performing as it should be, it’ll probably be pretty obvious to you. Just in case it isn’t, though, the reduced engine power light lets you know when something isn’t right.

In the following sections, we’re going to discuss why reduced engine power is bad, why that light is so important for you and your vehicle, and how you can take steps to either fix it yourself or have a professional mechanic fix the problem for you.

How Can I tell When My Vehicle Is Running On Reduced Engine Power?

You live with your car almost every day, so you’re most familiar with how it functions under normal circumstances.

For starters, your familiarity with it will probably be the best indicator if you’re running on reduced engine power, even if there aren’t any warning lights displayed in your gauge cluster at the time.

However, just in case you don’t pick up on a problem, your car will. After all, not everyone’s an expert that knows what they need to look out for.

The computers and sensors that make your vehicle run are programmed with a backup setting, sometimes called “limp mode” or “safe mode”, which will automatically decrease your vehicle’s power to try and prevent damage to the engine.

Depending on the vehicle, this reduced power backup system could shut down accessory components, limit your speed, and sometimes completely immobilize your vehicle.

In most cases, your vehicle will give you just enough power to get to a shop or back home in case you happen to be driving somewhere and a problem arises unexpectedly.

What Exactly Does the Reduced Engine Power Light Mean?

The reduced engine power light comes on when the vehicle’s computer detects a fault in any of the systems that allow the engine to run optimally, such as airflow sensors, throttle sensors and oxygen sensors.

This light isn’t found in all vehicles out there, nor is it always a light in the vehicles that do have this warning mechanism.

In some vehicles, it can be a warning displayed on the screen portion of your gauge cluster, if your vehicle has a screen.

General Motors vehicles often have this light. Other vehicles, especially luxury and performance vehicles, don’t have this warning light, but do have a “limp home” mode.

It should be fairly obvious if a vehicle enters “limp mode” as discussed above, especially if the check engine light is displayed and your vehicle is severely limited in power and capability.

What Should I Do If The Reduced Engine Power Light Comes On?

If you’re driving and the reduced engine power light comes on, the first thing you should always try to do is get the vehicle back home or to a shop immediately. If you’re already at home and the light comes on, don’t drive the vehicle.

After the light comes on, the first thing you need to do is have the vehicle scanned with and OBD II scan tool.

Because of all the computer modules in vehicles, error codes and faults that occur are stored within the systems of your car or truck, even if the scan occurs after the light goes off or before it turns on.

Back in the day, older vehicles didn’t have the computer systems that newer vehicles do now. As technology grew in the 1980’s and 90’s, scanning a vehicle was possible, but didn’t always produce specific and accurate results as to the error. It could also be extremely expensive to do.

In 1996, the EPA mandated that every vehicle needed to have OBD II compatible software rather than manufacturer specific or expensive scan tools. OBD stands for “onboard diagnostics”, and OBD II standardized all the information your vehicle stores.

If you don’t have an OBD II reader, you can usually go to an auto parts store and they will scan your vehicle for free. If you do have an OBD II reader, you can see the errors for yourself.

They usually range from about thirty dollars to thousands of dollars, depending on the scanner’s capabilities.

What Might Cause The Reduced Engine Power Light To Come On? And How Can I Fix It?

There are several different reasons the reduced engine power light might come on, and all of them should be cause for concern.

After scanning your vehicle, it should be fairly easy to pinpoint exactly where the issue is coming from, especially when using a high quality scan tool.

There’s a Loose Wire, Harness, or Clamp

Your vehicle is full of wires and connectors that power everything and make it run. A short in the electrical system, a loose ground wire or a loose clamp can all trigger a fault in your vehicle.

This is usually the easiest and most recognizable issue, should it occur. However, if nothing is loose, no wires are exposed or broken and everything is correctly hooked up, you’ll more than likely need to consult a professional mechanic.

What Should I Do to Fix This?

Whenever you’re dealing with a vehicle’s electric systems, be sure to disconnect the battery first before fiddling with any wires or sensors.

Make sure all connections are in place. Check for loose wires or grounds that have become unhooked. Look for exposed or damaged wiring.

Reconnecting anything that’s loose can usually be easily done, but any exposed or damage wiring will need to be replaced.

Oxygen Sensors Are Failing

Oxygen sensors measure the amount of oxygen exiting the vehicle through the exhaust system.

They aid in adjusting the air-fuel mixture for proper combustion. If one is faulty, the reduced engine power light or check engine light will illuminate.

What Can I Do to Fix This?

At the very least, you can attempt to clean your oxygen sensor, but if this doesn’t work it may need replacing.

Most modern vehicles have at least two oxygen sensors – one before and one after the catalytic converter – but some vehicles can have up to eight. You will need to identify, via an OBD II scan, which sensor is throwing the fault.

Replacement oxygen sensors aren’t too expensive – they may cost up to about $500 – but most replacements require a special tool to complete the job. This tool is essentially a special wrench and is relatively inexpensive, but most people don’t have one.

You can attempt to clean your oxygen sensor, you can buy a replacement oxygen sensor online and have a mechanic install it for you. This will cut down, at least a small amount, on replacement costs.

The Throttle Position Sensor is Faulty

The throttle position sensor is usually located within the throttle body near the butterfly valve that opens up to allow air into the engine.

This sensor measures the position of the accelerator pedal as you step on the gas, and it tells the computer how much to open the valve, therefore letting a proportionate amount of air into the engine.

This sensor also aids in transmission shifting as air enters the engine. If this sensor can’t communicate with the vehicle’s computer systems to regulate engine revving, the vehicle will go into limp mode and trigger the reduced engine power light.

What Can I Do to Fix This?

The throttle body sensor, similar to the oxygen sensor, isn’t exactly an incredibly costly fix, nor is it an inexpensive one. A new sensor can range anywhere from $100 to $500, depending on the make and model of your car.

A well-versed do-it-yourself-er should be able to replace it themselves, but more often than not, a mechanic will need to replace the sensor and possibly the entire throttle body.

At the very least, you can buy a new sensor to avoid any kind of price gouging that may occur.

A Throttle Body Issue

The throttle body is comprised of more than just the throttle position sensor.

A butterfly valve opens and closes within the throttle body to allow air to enter the engine. If this valve is damaged or broken, it may trigger the reduced engine power light.

Additionally, carbon buildup from the engine may clog the opening or the valve and cause the engine to think that less air is being received, thus cutting engine power and setting off the warning light.

What Can I Do To Fix This?

Generally speaking, it’s easier to just replace the entire throttle body altogether, rather than each individual piece of it. Most replacement throttle bodies can cost between $100 and $200 before labor.

You may not need to replace the throttle body, though. Before you spend any money on this, try cleaning it out yourself first. Constriction of air due to dirt and soot can be easily remedied.

Even if you have to replace the throttle body, it still is a fairly easy and straightforward process.

The Mass Airflow Sensor is Faulty

The mass airflow sensor is located between the air filter and intake manifold.

It measures things like air density and pressure entering a direct injection engine, and tells the computer how to deliver and mix the fuel with the incoming air to provide optimal combustion within the engine.

What Can I Do to Fix This?

Like the throttle body, you can clean the mass airflow sensor of any debris that may accumulate from a loose or clogged air filter. Nowadays, they also make sprays that aid in cleaning out the MAF.

A replacement can cost up to a few hundred dollars, but is generally similar to the cost of a throttle body or oxygen sensor.

It’s also not a must for you to have a professional work on this for you. Doing the replacement yourself is fairly simple, especially with the numerous charts and how-to videos available online nowadays.

The Vehicle’s ECU is Faulty

The ECU, short for Engine Control Unit, is the computer with which all of these sensors communicate. It controls almost every aspect of how a car runs and will even try to compensate for unusual conditions introduced within the engine.

A faulty ECU can be one of the most serious and costliest issues that will cause the reduced engine power light to illuminate.

What Can I Do to Fix This?

Unfortunately, because the ECU is essentially a computer, the only real way to tell if it’s causing the warning light is to run a scan with an OBD II scanner.

You can buy a replacement ECU, but it may require tuning or a re-flash to adapt properly to the specific vehicle and environmental factors.

This is better done by a professional mechanic and not on your own, unless you are confident in your own tuning abilities and computer skills.

The Catalytic Converter is Clogged

The catalytic converter is mounted underneath the vehicle after the exhaust manifold. Its job is to convert carbon monoxide into carbon dioxide through reduction and oxidation, thus reducing harmful emissions.

As of this writing, it is currently illegal to drive a car without a catalytic converter (unless it is a pre-1975 vehicle), and will fail an emissions test if it becomes clogged.

This could trip the reduced engine power warning light, but is more likely to trip the check engine light.

What Can I Do to Fix This?

Like the ECU, a catalytic converter replacement is a costly repair because of the way it’s made and the materials used. A new one, with labor, can cost more than $2000.

Though a tech savvy do-it-yourself-er can replace the catalytic converter themselves without much hassle, a jack stand and other tools will be needed as you will have to get underneath the vehicle to make the repair.

There’s a Battery Problem

In most cases, a battery problem – be it a dead/damaged cell or a loose ground – won’t cause the reduced engine power light to come on.

There will likely be other symptoms of a battery issue before a warning light comes on.

What Can I Do to Fix This?

The easiest way to solve a battery issue is to make sure all the connections are in place and not corroded.

You can easily clean off any corrosion around the terminals by disconnecting any cables and using baking soda or other cleaner to get rid of the unwanted buildup.

If battery cells are damaged or dead, the electronics – or lack thereof – along with the inability to start your vehicle will probably alert you to a problem sooner than a warning light will.

Replacement batteries are usually relatively inexpensive and are commonly replaced. Though charging your battery might be all that’s needed. The only way to really find out is to use a dedicated battery tester and analyzer with diagnostic features, to tell you the health of your battery.

And if you determine you need a new battery, check out our guide to car battery types, groups and sizes to make sure you get the right one!

There’s a Fault in the Transmission

Another issue that will most likely trip the check engine light before the reduced engine power light is a transmission fault code.

All of the sensors we have talked about so far in this article are so intertwined with correct transmission operation, that reduced engine power may originate from the transmission and any one of its systems.

Low transmission fluid or a slipping clutch could both trigger a fault code.

What Can I Do to Fix This?

You can check the transmission fluid yourself by referring to your owner’s manual or by following our guide on how to check and add transmission fluid. In essence, you find the dipstick, and examine the fluid level and quality. This is similar to checking the engine oil.

Replacing the transmission fluid may sometimes fix the issue, but this is unlikely to solve the problem since it’s usually a more serious issue going on.

Running a scan on the car will be able to provide more detailed information on what is actually going on with any transmission faults.

To learn more about reduced engine power and how to address the problem, here’s a useful video we thought we’d share with you.

What Should I Expect If I Can’t (Or Don’t Want) To Fix My Vehicle Myself?

Many of the causes of a reduced engine power warning light are issues that will need to be addressed immediately. However, this shouldn’t be a cause for panic, since most of these issues can be repaired with a little research and some basic know-how.

Taking your vehicle to a mechanic will cost a little more than making the repair yourself, but the benefit of their advanced scan tools and readily available equipment will save you time and prevent any mistakes you might make.

Expect any mechanic to run a full scan of the vehicle before doing any work on it, to pinpoint the exact location of the issue. If you want to be involved with what’s going on, you can always request that the mechanic show you the error codes presented by the ECU and an explanation of each.

After that’s done, you should receive a quote for parts and labor.

If any of this is unsatisfactory to you, a second opinion is always an option. With that being said, it should be noted that most mechanics will charge a small fee for performing an initial scan.

What Do I Do If My Vehicle Doesn’t Have a Reduced Engine Power Warning Light?

In vehicles that don’t have a reduced engine power warning light, and you can feel that the engine is running poorly or goes into limp mode, you should do your best to find out what the problem is as soon as possible.

You can even preemptively run a scan on your vehicle if you sense or anticipate a problem might arise.

There is a reason your vehicle is programmed to reduce the capability of its driving systems when it detects a fault related to reduced engine power.

It is imperative that a vehicle be driven at a minimum until the problem is resolved. Otherwise, damage could occur and expensive repairs could follow.

Just remember that it only takes one issue with a single system to spill over into other systems as well. An error code that triggers the reduced engine power light may also trigger other error lights within the gauge cluster.

If you think your vehicle is running on reduced power, and there are no warning lights displayed, you can easily check the following components.

If any of these are culprits, you may not need to take a trip to your local mechanic after all:

Air Filter: A clogged air filter won’t generate an error code, and can be easily replaced.

Fuel Pump: A fuel pump problem can be more difficult to diagnose and can be an expensive replacement part. If your engine is running poorly, your car is trying to start but is unable to, and there are no warning lights in the gauge cluster. In this case, you may need to replace your fuel pump.

Tire Pressure: Low tire pressure (in and of itself) won’t cause your vehicle to run poorly, but it can cause added friction on the road surface. This might cause a noticeable difference in the driving dynamics of your vehicle.

Wrapping it Up

Modern vehicles are wonderful pieces of equipment, but they also wear out just like everything else we use on a daily basis.

The reduced engine power light is beneficial to the proper health of your engine, and can alert you to serious issues before the problem gets out of hand. When this light comes on, you should not wait to have it checked, especially if your vehicle goes into “limp mode” and performance is suffering.

Scan tools available at most shops and stores can give you a full read out of exactly what’s going on with your engine so that you can make the repair yourself or take it to a mechanic instead.

Whatever the issue may be, it’s good to know that your car has your back and has built in safeguards to help you and your family return home safely!

Shawn Furman

I've had a passion for cars since 8 years old, and been a subscriber to Auto Week magazine since my 10th birthday.Ever since I turned old enough to drive, I have driven as many vehicles as possible, while teaching myself how to perform maintenance and upgrade work on every vehicle I've owned.For the past 10 years, I've been honing my skills as a vehicle hobbyist, in recent years also enjoying writing car reviews, opinion articles, vehicle how-tos, car-buying guides, and even provide individual consultations for those who need car-buying advice.In addition to writing for Vehicle Scene, I currently write for Autolist, and also own and operate my own vehicle blog website, The Unlimited Driver.

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