What Does It Mean When a Car Goes Into Limp Mode?by Craig Woodman
In the early 1980s, computerized vehicle control started with the emissions system, but soon computer use in vehicles became more prevalant. Today, computers control the heating and air-conditioning systems, and even control most of the other convenience features in the vehicle, such as lighting and power windows. When the vehicle systems do not work correctly, the computers can institute default programming, called limp mode.
When a car enters limp mode, one of the vehicle control modules, or computers, has detected a problem with its logic. A reading from a sensor or other input is not correct, and the computer knows that the readings are outside of the normal ranges. The module cannot process the correct outputs based on the data that it has to work with. When this happens, the vehicle's computers will set a trouble code, and turn on a check engine or other malfunction indicator light.
Making it Home
If the faulty data is in a major vehicle system, such as the fuel delivery or ignition system, or other engine or body controls, the computer will often enter its limp mode. This is designed to allow you to at least get the vehicle to safety, such as off a busy street to a nearby parking lot. In some cases, you may be able to drive the vehicle home or to a repair facility, although the vehicle may drive slower than normal or behave erratically.
Transmission Limp Mode
Modern vehicle transmissions consist of mechanical and electronic systems, with the electronic systems responsible for much of its operation. When a vehicle enters limp mode due to a transmission concern, it will behave differently depending on the failure. The vehicle may only operate in one or two gears, which will substantially limit the acceleration and top speed performance. In some cases, the transmission control module will direct maximum fluid pressure to clutch packs and other components. This will result in extremely hard shifts. While this may feel like it is damaging the transmission, it is intended to protect the transmission from failures due to low fluid pressure.
Correcting the Condition
If your vehicle is in limp mode, when you get to safety you may be able to shut the vehicle off and wait a few minutes, and then start the vehicle again. Sometimes, this will reset the electronic controls, restoring valid data. You should still have your vehicle checked as soon as possible, because the original fault may still be present. If the vehicle does not reset, you will need to take the vehicle to a repair facility for proper diagnosis and repair.
When Not to Drive
A check engine light that is on steady means that you need to have the vehicle repaired soon, but it is still okay to drive. If the check engine light is flashing, you should not drive the vehicle. The flashing check engine light means that a misfire is present in the ignition system, and your vehicle exhaust could be pushing unburned fuel into the exhaust. This can cause a problem with the vehicle's catalytic converters, which is a very expensive repair. If the anti-lock brake system warning light or air bag warning light is on, be aware that these safety systems are inactive and will not work in an emergency.
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