Picture this: you’re driving along like you would any normal day. All of a sudden, your dashboard lights up like a Christmas tree. Your car still drives, but it barely has any power.
What just happened, you ask? Your car has probably entered limp mode.
What is Limp Mode?
Limp mode (also called limp home mode) is a very conservative calibration or map used by your engine control unit (ECU) or transmission control unit (TCU) when a potentially harmful fault is detected in one of the powertrain components.
You can think of limp home mode as a safe mode for your car that is designed to prevent or mitigate engine damage. Instead of shutting off the engine completely and leaving you stranded, this calibration allows the driver to carefully “limp home”, or drive to the mechanic for a diagnosis and repair.
When a vehicle is running in limp mode, it can do several things that bring the vehicle’s available performance down to a safer level.
Firstly, most engine calibrations tend to run rich, because a rich air fuel mixture is often much safer than a lean air fuel ratio. An ECU in limp mode may also limit the maximum engine speed (RPM), pull ignition timing (delay when each cylinder’s spark occurs), or alter valve timing (change when the intake and/or exhaust valves open).
If your vehicle is turbocharged, the map will likely run with the turbo’s wastegate fully open, which will limit boost pressure to its mechanical minimum. This boost pressure is likely an order of magnitude lower than the maximum boost pressure you would normally see.
If the fault was detected in the transmission, your transmission may shift into second or third and stay there. This will cause sluggish acceleration from a stop and a much higher engine speed on the highway.
What Causes Limp Mode?
There are several causes of limp mode. Some causes are vehicle specific, since each manufacturer’s ECU and TCU implementation is a bit different.
#1 – Faulty Sensor(s)
Sometimes a vehicle will enter limp mode when the ECU gets confused, either due to a faulty sensor, a missing sensor signal, or a sensor reading that is out of spec.
If the engine has no way to accurately read the air density entering the combustion chamber, it will not know how much fuel to inject. Injecting the wrong amount of fuel for the given situation could cause engine damage, especially as RPM and engine load increase.
#2 – Overboost
On a turbocharged vehicle, the boost controller controls wastegate operation. A wastegate normally allows excess exhaust to flow around the turbocharger (rather than through the turbo’s exhaust turbine). This is done to maintain target boost pressures.
Overboost is a condition when the actual boost pressure is much higher than the ECU’s target boost pressure for a sustained period of time. If a turbocharged vehicle enters an overboost condition due a failure of the boost controller or wastegate solenoid, it may trigger limp mode to avoid catastrophic engine damage, such as spun bearings, broken piston rings, or bent connecting rods.
Overboost due to a wastegate that simply cannot flow a high enough volume of exhaust is called boost creep, and will sometimes manifest at higher RPMs on modified vehicles. Boost creep is most likely to appear on cold days near sea level when the air is very dense. This is a mechanical problem that cannot be fixed by recalibrating the ECU.
#3 – Knock or Misfires
Consistent engine misfires or knock may eventually put a vehicle into limp mode. One or two misfires probably won’t trigger limp mode, and you’re unlikely to have issues with intermittent knock that occurs while cruising down the highway.
Nearly all engines knock at some point, but in mild cases the ECU will often just pull ignition timing for a little while until things return to normal.
#4 – Vacuum or Boost Leak
A significant vacuum or boost leak often introduces a large volume of unmetered air into the combustion chamber, which drastically alters the air fuel ratio. If the actual air fuel ratio differs significantly from the expected air fuel ratio, a vehicle may enter limp mode.
#5 – Low Fluid
Some ECUs and TCUs monitor fluid levels. If the engine oil or transmission fluid level is very low, it may decrease oil pressure which could trigger limp mode in some vehicles.
#6 – Missing Emissions Equipment
Modifying a vehicle to remove emissions equipment will often trigger limp mode due to a missing sensor or other component (additionally, this practice is also illegal in many places).
#7 – Overheating
If your vehicle is overheating, the ECU may cut fuel to some of the cylinders to allow cool air to reduce engine temperatures.
How to Bypass Limp Mode
Bypassing limp mode is generally not recommended unless you have a very good reason for doing so and understand the potential consequences. Something wasn’t right with the vehicle when limp mode was triggered, and if you attempt to keep driving the vehicle using the normal engine or transmission parameters, you run the risk of causing damage to the powertrain.
The following tips are very temporary fixes that can be used to get you home or to the shop, but will not address the root cause of the issue. Please bring your vehicle to a mechanic for a diagnosis and repair of the actual problem as soon as possible to avoid the very real possibility of engine or transmission damage.
If any of the following tips work, you can expect limp mode to return fairly quickly if the fault continues. Try to drive with this in mind, placing yourself in a lane with a shoulder or taking less traveled roads.
#1 – Restart the Car
Sometimes simply turning the car off and back on again will exit limp mode.
#2 – Clear the Code
You may be able to clear any codes you find using an OBD2 scanner. It’s best to simply buy a good OBD2 scanner since it can pay for itself after one use but your local auto parts store will likely have scanners you can borrow. Unfortunately, they may not be allowed to clear the code on your behalf for liability reasons.
#3 – Disconnect and Reconnect the Battery
Many vehicles will forget a check engine light that was thrown after the battery is disconnected, then reconnected. Simply disconnect the negative terminal of the battery and reconnect it after a few seconds.
You may want to hold down the brake pedal for a moment while the battery is disconnected to make sure any remaining electricity in the system has dissipated.