Since the 1980s, cars have featured an engine management control unit, or ECU. This unit, now featured on almost all new cars, is a computerised system for optimising the engine performance and detecting faults. If the ECU detects a fault that may cause engine damage, it puts the engine into limp mode.
When an engine experiences a fault, the ECU can typically detect the location and nature of the fault. In many cases, a warning light or warning message will appear to alert the driver to the problem. For example, if one cylinder in one bank has blown, the ECU can shut down the entire bank to prevent damage to the other cylinders.
Limp mode deliberately hampers engine performance. The ECU permits only the minimum level of engine performance required to get the car home or to a garage. This means that acceleration will be laboured, the turbo will be disabled and in some cases, ancillary functions such as climate control will shut down. In some cases, your engine’s ECU will restrict the transmission, meaning you can only select the lower gears. If your car goes into limp mode, don’t continue to drive it for any longer than necessary. You may notice that during limp mode, your fuel consumption goes up dramatically. This is due to the inefficiency of the engine during limp mode.
A variety of engine issues can cause a car engine to enter limp mode. Damage to fuel system parts, such as air filters and lambda sensors can often cause an ECU to switch to limp mode. While not necessarily serious problems, driving with an under-performing fuel system can cause lasting damage to your engine. Gear box problems can also bring on limp mode. Sometimes, a damaged sensor will make your car’s ECU wrongly detect a problem. For example, a faulty sensor could send inaccurate readings to the ECU, causing it to record a fault in the associated part. In such cases, replacing the sensor, rather than the part, is a sufficient fix.
Although having a car in limp mode, especially due to a minor fault, can be a inconvenient, the upside is that your engine’s ECU will store an error code, making diagnosis quick and simple for the mechanic. All cars manufactured in Europe since 2001 are fitted with on board diagnostics. Such cars have a socket into which a mechanic can plug a machine, which takes a reading from the ECU in order to determine the nature of the error. Once the error code is read, the mechanic may have the option to turn off limp mode and all associated warning lights, returning the car to normal without conducting any repairs. Your mechanic may offer to do this if the ECU recorded a minor fault.