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Q: Disconnected 2ga wire from battery; rehooked, but warning light on

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After fixing the alternator, I was checking the voltage near the battery, from within the trunk. By accident, I unhooked a wire that goes from the battery to the distribution box. The car turned off. The wire that I had accidentally unhooked was a 2ga. I hooked it back in and the car started again. After that, there was an “engine malfunction reduced power” warning light on the dashboard. I disconnected the battery for 20 minutes, but the light is still staying on continually. What should I do to get the warning light to go off?

A: Hi, thanks for writing in about your 2006 B...

Hi, thanks for writing in about your 2006 BMW 740i. Disconnecting battery cables while the engine is running causes a voltage spike in the electrical system. Electronic control modules typically do not take this well! Sometimes this will damage some electronics, sometimes not. Sometimes the fix is reprogramming (reflashing) the engine control module. What you’ll need to do is have an experienced technician, such as one from YourMechanic, diagnose your BMW’s warning lights by using a compatible scan tool. Checking for fault codes will indicate what systems have been affected and provide a starting point for how it can be fixed.

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