A bulb or connection is loose. A road bounce and loose part can trigger it. Or even rust. My X5 had an intermittent plate light. One bulb was so rusty, it would work on and off. The light cover was missing. I replaced both lights with LED inserts. The lights are super bright at night and no more light warnings. Plus, I cleaned off the contact points.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)