Limp mode is a failsafe mechanism that is built into electronic engine control units (ECUs). When the ECU detects a problem with the transmission or engine, it puts the vehicle into limp mode to lower power output and prevent further damage until the driver can take their vehicle in for necessary repairs.
Some of the most common reasons for a car’s ECU to force the engine in limp mode include low fluid levels, a failing clutch/transmission, or malfunctioning sensors that fail to report necessary information to the ECU. When this happens, the vehicle will drop to below 3,000 RPMs and lose power to avoid overstressing the engine.
On diesel engines, this issue is usually referred to as an engine derate
If your vehicle has entered limp mode while you are driving, it’s best to stop as soon as possible and seek help from a mechanic. This guide will explain what causes limp mode, what signs indicate that your vehicle is stuck in limp mode, and how you can fix the problem to restore your vehicle’s full power output.
What is Limp Mode?
Limp mode is a failsafe mechanism that limits the amount of power an engine can produce in order to prevent further damage. It’s also sometimes referred to as “limp-home mode,” because it allows drivers to slowly get their vehicle home or to a nearby service station even if the engine has sustained serious damage. On BMW cars, it is usually referred to as: “TRANS. FAILSAFE PROG.”
Generally, limp mode is activated when the ECU detects an issue with the transmission or engine. When this happens, the ECU will automatically reduce power output from these systems in order to avoid any further strain on the engine.
This usually works by limiting RPMs (revolutions per minute) and capping torque levels at around 25-30% of what the car is normally capable of producing. In some cases, limp mode may also engage other safety features like traction control and stability control which work to limit wheel spin and keep your vehicle pointed in a straight line – even if you are flooring the accelerator.
What Causes a Car to Go Into Limp Mode?
There are a number of reasons why your vehicle’s ECU might force the engine into limp mode, but the most common causes include:
- Low Fluid Levels: If the ECU detects that coolant or oil levels are low, it will put the engine into limp mode in order to prevent overheating and physical damage.
- Transmission Defects: If your vehicle’s transmission control unit detects a fault in operation, it can place the engine into limp mode, helping to reduce strain on the system. This way, you can continue driving to a safer location without damaging the engine or transmission and then check for bad solenoids or poorly adjusted linkages.
- Malfunctioning Sensors: In some cases, your engine’s MAF sensors may become damaged or malfunction altogether. This can lead the ECU to make inaccurate assumptions about what is happening with your car. As a result, the ECU may put your car into limp mode as a precautionary measure.
In most cases, simply topping up your vehicle’s fluids can fix them problem. By checking your coolant levels every six months and your oil once a month, you can prevent future problems and keep your vehicle running at peak performance.
How to Tell if a Car is Running in Limp Mode?
Although it’s usually easy to tell if your vehicle is stuck in limp mode, it’s certainly possible to confuse the safety feature for other mechanical issues. To help you better identify the problem, here are a few tell-tale signs that your vehicle is operating in limp mode:
- Reduced Power Output: One of the most obvious signs that something is wrong is if you suddenly feel as if your car has lost power. This will be especially noticeable when accelerating. Even if you have your foot pressed all the way down on the accelerator, your vehicle will struggle to reach high speeds and may eventually stop altogether.
- Check Engine Light: Another tell-tale sign of an issue is if a red check engine light comes on while you’re driving. This indicates that there is some sort of problem with your engine’s performance and it should be checked by a mechanic as soon as possible.
- Transmission Warning Light: If you see this warning light, it means that there could be an issue with your transmission, which will likely place your car in limp mode. As such, it’s best to drive to the nearest service station to have the codes checked and make any necessary repairs.
- Decreased Fuel Efficiency: When limp mode is engaged, your vehicle will use more fuel than usual to maintain power output. This can lead to decreased fuel efficiency and may eventually cause you to run out of gas sooner than expected.
The transition into limp mode is usually quite sudden so, if your vehicle seems to lose power for no reason, it’s likely shifted into limp mode. Check the dashboard for any check engine lights and head to your nearest service station. If necessary, pull over and call a tow truck.
How Do I Fix the Problem?
If you think that your vehicle is stuck in limp mode, the first thing you should do is pull over and turn off the engine. Once the engine is turned off, check fluid levels (coolant, oil) and make sure they are topped up if necessary – this could be all that’s needed to fix the problem and get your car back up and running properly again.
If topping up fluids doesn’t work, there may be an issue with sensors or other components within the ECU itself. In this case, it will likely need to be replaced in order to fix the problem. If your car is still under warranty, it’s best to take it into a dealership so they can diagnose and repair the issue free of charge.
However, if your vehicle is no longer covered by a warranty, you will likely need to pay for repairs out-of-pocket. In either case, it’s always best to consult with a professional before attempting any sort of repairs on your own. Improperly removing or replacing damaged electrical components can cause bodily harm and major damage to the ECU, leaving your vehicle in far worse condition.
Is it Possible to Temporarily Override Limp Mode?
If your vehicle suddenly shifts into limp mode but you’re several miles away from the nearest town, you can temporarily override the safety protocol by turning the engine off, leaving it to cool for five minutes, and then restarting your car. This is sometimes enough to reset false alarms, allowing you to drive on to the nearest town.
If this does not work, you can also try disconnecting and reconnecting your vehicle’s battery. By disconnecting the battery, you’ll automatically reset your vehicle’s transmission history, bringing the engine back out of limp mode.
This is only a temporary fix and you should take your car in for an inspection as soon as possible.
The Bottom Line
Although driving in limp mode can be tedious and stressful, the safety precaution was designed to help you get your vehicle to a safe location without damaging the engine. If your car suddenly shifts into limp mode, be sure to check fluid levels and consult with a professional before attempting any repairs on your own.