High Beams fail only when driving

Here’s one the dealership said they haven’t heard of - 2003 Buick Century, with automatic sensor that turns on headlights in the dark. When driving with the high beams on, after a minute or so, all headlights go out. Switching to dims turns the dims on, and switching back turns on the brights, but they go out again.

Attempting to replicate this problem while sitting in the driveway does not result in the lights going out. They stay on. Additionally, the other night we noted that driving just before dusk, with the lights turned on manually, with high beams on, they did not go out that way either. We have tried manually turning on the lights, instead of letting the sensor do it, but the result is the same - they go out.

So this is weird… They only fail when it is both dark and the car is moving. Suggestions?

It sounds like something is overheating. You should be able to simulate driving conditions by revving the engine up while in park or neutral. This may raise the electrical system voltage slightly. You could also simulate darkness by covering the light sensor (somewhere on the dash top I assume).