A car is said to be in limp mode when it is traveling with a malfunctioning system and the car has limited the functionality of the system in order to protect it from further damage.
Some people refer to it as limp home mode which implies that it is meant to not leave you stranded but to carry you home where a repair should take place.
The power supply of a car is the source of electrical power to other parts of the car.
Limp mode causes the engine to operate in a limited power mode or causes the transmission to be stuck in one gear or not go beyond certain gear. Limp mode operation enables the car to be driven to a workshop without further damage to the engine or transmission. It is a safety feature that prevents catastrophic, irreversible or expensive failures.
What causes engine or transmission limp mode?
Limp mode can be caused by mechanical or electrical faults. This ranges from sensor, actuators, wirings, oil issues to other mechanical faults.
For transmission; a bad clutch, wrong or worn fluid, incorrect transmission fluid level, bad valve body, faulty wiring and connectors can cause limp mode.
For engine; overheating, low engine oil, wrong timing, sensor faults, actuator faults, misfires, turbo faults and so on.
How power supply problems cause limp mode
Sensors and actuators of a car work with electrical voltages and signals. For example, the solenoids of an automatic transmission is powered by the battery and alternator. The solenoids seizes to function properly when it does not get required power and this throws the transmission into limp home mode.
Below are the different ways power supply issue can cause limp mode:
1. Low voltage supply
Solenoids and actuators have minimum threshold voltage that they can work with. When the voltage goes below this threshold, they stop functioning.
2. Intermittent power supply
An intermittent supply due to bad wiring or connector causes malfunction of solenoids and actuarors. This leads to limp mode.
3. Faulty battery or alternator
The battery and alternator are the main power sources in a car. If any is faulty, it can cause limp mode. If for instance it is the alternator that is bad, everything continues to function until the battery voltage becomes low.
Symptoms of limp mode
1. Presence of check engine light.
2. Reduced engine rotational speed (rpm).
3. Reduced engine power.
4. Transmission stuck in one gear or cannot exceed a particular gear.
Is limp mode a fault?
Limp mode itself is not a fault but it is a safety feature that prevents engine or transmission from more damage whenever there is a fault.
How to fix limp mode
The eliminate limp mode, the root cause will have to be fixed. To do this, the fault should be diagnosed first using a computerised scan tool. After the root cause is fixed, the error code can then be cleared using the scan tool.