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Q: Headlights dim when revs drop while driving

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The zap in Corolla. Whenever the revs drop during driving, coasting for example, the headlights dim to the parking light level. Should I have a battery replacement? I'm planning to do that. Is there something else to look into?

A: Check the battery and have the alternator l...

Check the battery and have the alternator load tested. A lot of times it is as simple as you needing a new battery or an alternator. You can get the car load tested or try or test the alternator yourself. They will tell you which one it is. A lot of times, it is more of a bad alternator than a battery because the alternator runs when the car starts. Unless your battery is really bad, then it is usually the bad alternator. If the battery is bad, it wouldn’t start at all. If you need help having this problem resolved, a certified mechanic, such as one from YourMechanic, can check your headlight issue to verify what the actual problem is.

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