Close

What Limp Mode Is, and Why Cars Use It

Alex Palmeri
June 29, 2018

Your car has a lot of safety features to protect you, but it also has ways to protect itself. 

Limp mode, also called "limp-home mode," is activated when the car’s computer system detects an issue that could damage the engine or transmission. It reduces power to lessen the load on these crucial parts, but still allows you to drive the car home or to the closest repair shop

The vehicle can cut power by limiting the amount of throttle used, the ignition timing and the RPM, but it can also put the car in one fixed gear — usually second or third — to limit how fast you can go. 

Let’s look at how to know if your car is having this problem, or you can skip ahead to common causes and how to get your vehicle back to normal.

Signs your car is in limp mode, and what to do

Limp mode is meant to be a safety feature. But depending on where and how you’re driving when it starts, it can be scary and even dangerous.

The first thing you’ll notice when limp mode starts is that the check engine light is on. If the car is misfiring, it may shake, and the check engine light might flash. If there’s a transmission issue, you’ll likely be locked into one middle gear, so the car will feel very sluggish when accelerating from a stop. 

If you’re almost home or close to a repair shop, it’s best to drive straight there. But make sure you pay attention to the engine temperature gauge, if your car has one, or watch for temperature warning lights. If the car is overheating, shut off the engine as quickly as you safely can and call for a tow to avoid further internal damage. 

If you’re on the highway, it may be best to pull over to the shoulder and turn the engine off, as your speed will be reduced and your car may become a traffic hazard. If you’re on city streets, remember that your vehicle won’t accelerate quickly from a red light or when turning. Give other drivers plenty of room and don’t make any quick maneuvers. 

» LEARN MORE: Track down your symptoms and learn what they mean

Problems that can cause limp mode

Sometimes, limp mode will start completely unexpectedly. But other times, there may be warning signs. If your car is running rough or shifting differently, limp-home may be just around the corner. 

Here are the most common reasons why cars go into limp mode. 

  • Engine misfire: If an engine is misfiring badly enough, it can damage the catalytic converter, as well as itself. This is especially true if fueling is the issue. You will feel this as a rough idle or uneven acceleration. There may be black smoke coming from the exhaust, or the car may stall at idle. Limp mode can help prevent damage.
  • Overheating: Many cars will activate limp mode when overheating is detected. The engine’s temperature may spike for a number of reasons, usually due to problems with the cooling system. The car will shut off fuel to some cylinders, allowing fresh air to enter the engine and help cool it down. But you’ll still be able to drive it, slowly, to get help. 
  • Bad sensors: Modern vehicles have many sensors that monitor and report back data to multiple computers. That information is used for things like proper fueling, ignition timing and controlling the automatic transmission. 

If a transmission or engine sensor fails and isn’t able to report information to the computers, limp mode can be activated. In this case, a secondary programming is used to estimate what the failed sensor would be reporting. The amount of power and number of gears available is determined by which sensor fails.

» LEARN MORE: Get an estimate for your car repair

Get it diagnosed by a professional
 

How to get your car back to normal 

If your car goes into limp-home mode, you can attempt to shut it off and restart it. Sometimes, this will temporarily reset the system and restore normal driving so that it’s easier to get to a repair shop. But this doesn’t always work.

After getting the vehicle to your mechanic, the technician will connect a scan tool to find trouble codes logged by the car’s computers. These will point the tech in the right direction to find out what went wrong. 

A bad sensor will typically be a relatively easy and inexpensive fix. But if your car overheated long enough to damage the engine’s components, or if your automatic transmission has failed, the price to repair or replace these parts will likely be quite high

In this case, it’s time to get an estimate and then take a hard look at how much the vehicle is worth. Compare the two figures. If the repair costs more than the car’s value, it might be time to get a new ride. Talk with your mechanic, do your homework and decide what’s right for you.


Alex Palmeri

About the Author

Alex Palmeri worked nine years as a master technician at Mercedes-Benz of Chicago and is currently the foreman at a large fleet garage. He writes about automotive news, maintenance and racing, and runs a YouTube channel called Legit Street Cars.

No comments yet...

Sign in to comment