Some drivers may have experienced something rather curious in their cars, which is the vehicle suddenly slowing down and a number of its key functions deactivating.
The “Check Engine” light may have appeared in the dash display, and a now concerned and confused driver is faced with a car apparently crippled. In all likelihood, the car has entered a special safety setting that is known simply as “limp mode.”
In today’s blog, we’re exploring this “limp mode,” looking at what it is exactly, why it happens, what it means for drivers and where it has been used in various brands.
What is Limp Mode?
Put simply, limp mode is a car safety feature that typically activates when the control unit of either the engine or the transmission detects a major fault within the system. It then proceeds to shut down what the car deems to be “less important” systems like air conditioning, for example.
By shutting down these features, and limiting the car to how fast it can go, the idea is to minimize the ways that the car could fail and break down, allowing it to limp on to its destination. The alternative name given to this system is “limp home mode.”
As the car’s ECU receives signals from all over the vehicle, it will activate limp mode in response to serious abnormalities that the signals reveal. It is designed as a protective measure to prevent further damage from occurring in the vehicle.
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If you’re a science fiction fan, it’s not unlike those moments where a starship captain orders his team to “shut down all unnecessary systems” to conserve power and allow it to be channelled elsewhere that it is needed.
Symptoms and Causes of Limp Mode
If you’re driving along and experience any of the following symptoms, they could point to limp mode being activated. It’s not always 100 percent, but all of these are strong indications of your limp mode coming online:
- You see a “Check Engine” light on the dash
- Your RPMs are limited to 2500-4500
- Your car is suddenly performing much more poorly
- Functions of your car have been greatly reduced/restricted
- Transmission doesn’t shift above third gear
As we said, what happens is that the ECU receives signals about these problems and then activates limp mode in response. Causes can be anything from wiring problems to malfunctioning engine boost control; defective engine components to faulty or problematic sensors.
The main trouble people have with limp mode is that it typically doesn’t give any clear answer to such questions. The symptoms above however point clearly to limp mode coming online.
If you have a lot of mods or a custom exhaust , it's possible incorrect configuration or installation could trigger issues in the ECU by not responding or acting as the OEM module would expect.
How Should You Respond to Limp Mode?
Let’s first deal with the wrong way to respond to limp mode. Some people look at this function as a hindering force that is just stopping them from getting to their final destination. The fact is, however, that limp mode is, first and foremost, a safety feature on your car.
It is designed to protect your car from further damage and allow you the chance to get it to an auto shop for inspection. Therefore, trying to force the car out of limp mode or viewing it as a problem feature or an annoyance is wrong-headed.
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The correct response to being put into limp mode is taking your car to a mechanic that will help you diagnose the problem in the vehicle and then get it fixed. The limitation of limp mode as it currently stands is that it just lets you know that there are problems, but it doesn’t say specifically what the issue is.
You can use the symptoms you experience to determine what it is that may have engaged limp mode. For example, if your RPMs become limited to 2500-4000, then the most likely fault that was detected by your ECU is that boost control in the engine has a malfunction. A reduction in performance in particular also indicates that the ECU has received signals that indicate faulty engine sensors.
In any event, the correct response remains the same: get the car (carefully) to an auto shop. Limp mode still allows you to drive, even if just at slower speeds and/or with fewer additional features. Remember that these systems being shut down and the reduced performance of the car are all part of the proper function of limp mode. You shouldn’t view them as obstructional.
Case in Point: BMW Limp Mode
A number of BMW models come with limp mode installed. One of the benefits of BMW limp mode compared to some others is that you can get more details on your central touchscreen that should shed light as to why your BMW limp mode has activated.
For example, the top of the screen might indicate “Drivetrain malfunction.” It doesn’t go into detailed specifics, but it informs you that you should “Drive moderately” and that “Maximum drivetrain output is not available.”
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By limiting the drivetrain output, BMW is able to preserve the mechanical integrity of the engine and transmission, allowing you a much better chance of reaching an auto shop to inspect and/or repair the malfunction. BMW has gone to some lengths to try and educate their customers about this feature so that they will respond in the right way when confronted with it.
Those with some mechanical knowledge themselves can take limp mode’s activation as a cue to pull over and perform some simple checks on the car themselves to see if there are leaks, signs of damage, or any other indications as to what is wrong with the car.
Limp Mode: FAQs
What does limp mode mean?
In short, limp mode is a protective safety feature on your car designed to preserve both the engine and transmission from harm after various faults are detected by the vehicle’s ECU. It shuts down unnecessary systems and reduces the engine output, limiting RPMs. In doing so, it allows the driver to “limp” to an auto shop for inspection and repair.
Why has my car gone into limp mode?
There are a number of things that could trigger limp mode in your car. First, it could be elements of the engine like wiring or sensors that are faulty. It could also be a reduced engine boost control, or even an excessive boost control. In addition, there may be issues with wiring in the brake system and transmission that are causing limp mode to be activated.
Can you still drive in limp mode?
Yes, you can. Limp mode is not a feature meant to stop you from driving, but it is designed primarily just to buy you time and mechanical integrity enough to get to an auto shop for a proper diagnostic test and subsequent repairs of any faults. It’s not intended for you to drive for protracted periods of time while in limp mode.
Is limp mode ever wrongly activated?
Yes, but it shouldn’t change your final response. There could be issues with sensors and other components that trigger the ECU to erroneously believe that there are serious faults to address. The fact is, however, that limp mode activating, even “wrongly,” is a sign of a malfunction in your car’s system that is in need of repair. So, while limp mode can be activated erroneously, it’s still the right thing to take it to a mechanic to get looked over.