My temp gauge goes all the way up to H but it doesn't overheat. What's wrong?

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This happened after the battery and ECU were disconnected and reconnected. I had an auxiliary LED lamp installed and had to take out the battery and ECU to have access to the firewall where the wiring went in. That night, it was the first time I noticed the radiator fan on when normally it won't (A/C was off). The next day, the gauge went all the way up but no signs of overheating. No engine warning light and it runs normal. I plugged in an OBD sensor but no fault codes.