2001 Monte Carlo engine temp gage problem

I have a 2001 Monte Carlo LS, very recently the engine temperature gage does not respond to a temperature increase, the cooling fans come on as soon as the engine starts, and the engine check light illuminates and has remained on. I replaced the engine coolant temp sending unit, and there was no change. The car only has 41K miles on it. What other sensors are involved in the circuit. Any suggestions. Also the airconditioner compressor will not engage since this problem started.

The problem with the temperature gage was corrected with the coolant temp sending unit. After disconnecting the battery to reset the engine check light, the system has returned to normal. The gage is in normal operating range after allowing time to warm up, the a/c compressor comes on and the check light is out. A simple problem with a simple fix. Hopefully in the future there will be as simple fixes to problems.