Resetting your car´s ECU

Normally, car owners don´t have to worry about the ECU on their cars, as long as they take their cars to the dealer for regular maintenance. There will be occasions though when the ECU has to be reset, and it is the purpose of this article to provide you with a basic idea of what is and how to perform it.

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All ECUs in cars nowadays have an adaptive circuit which monitors engine performance, environment factors and driver habits. Combined with inputs from other sensors, the ECU stores minute adjustments to the maps stored in its memory. For example, the octane that is readily available in your area may be different from the octane 50 kilometers away. Repeatedly using the octane in your area will cause ignition timing parameters to be stored in your ECU´s memory, which may be different from the default maps programmed by the manufacturer. So, despite a deviation from the ideal, your car´s engine will still be able to achieve the best combustion efficiency based on the fuel it has been regularly receiving. But suppose you move away or sell your car and the gas station in the car´s new operating area is selling fuel with a higher octane. At this point, because the default settings are being overridden by the learned settings, the car´s engine will not be operating at peak efficiency. You are losing out on potential performance and subsequent fuel savings. But, if you know how to reset the ECU for your car, the engine will then default to the optimized settings from the factory and normal use with the higher octane will teach the ECU the best fuel and ignition settings for the new gas being used.

Other reasons to reset the ECU include a change of driving habits (because of a new driver), installation of aftermarket engine parts or the replacement of a faulty sensor.

Resetting the ECU involves clearing the learned values in the ECUs memory by one of two methods. The simplest method is to disconnect the battery for about 15 minutes. The loss of power to the car´s components, including the ECU, will cause the learned values stored in the ECU´s memory to be erased. This is much like a computer´s memory where you lose your work with a sudden power outage. The factory maps are stored in ROM (read-only memory) so removing power will not affect them. The values stored in these maps are what will be used when the engine is started after a reset. ECUs with backup batteries will need much longer times in order to drain the battery, or there will be a specific procedure to override the backup power.

The second method applies generally to newer cars, particularly those with on-board diagnostics (OBD). Removing the battery or power from the ECU does not work because it will not erase the stored values. Instead, a model-specific cable is attached to the diagnostic port and a laptop, PC, or specialized module is used to issue a command from a program, which will reset the ECU. Not all diagnostic scanners on the market have this feature but there are open-source programs online that have this feature and even allow you to modify the maps and other operating parameters stored in the ECU.

Specific reset procedures for different car models can be found online, and they can be as simple as the first procedure described above, or involve a 10-step procedure where even the windows and sunroof have to be fully closed, steering wheel straight, etc., like those involving some Mercedes-Benz models.

After a reset, the engine must be started and allowed to idle for several minutes before being driven normally. After one or two hundred kilometers, the ECU will have relearned its new operating parameters and the owner can forget about the ECU until the next major operating or environment change. Since a reset can involve removing power from the radio, make sure you have your stereo´s anti-theft code, or else you´ll be without your favorite tunes until you can get your dealer or stereo manufacturer to give you the proper code.