2021 Haval H9 temperature gauge sometimes doesn't work

You either have a problem with the temperature sensor or the gauge in the dashboard itself. Suspicions would be that it’s the sensor at fault which is not sending the correct signal to the gauge to make it read the actual temperature of the engine. If that’s the case, the sensor would also be sending false information to the car’s engine-management computer, and that’s why you have the check-engine light illuminating. The computer is getting confused at the discrepancy it’s seeing, and firing the engine light to warn you that something’s not right.

Either way, this is a warranty issue, so make it Haval’s problem.