1994 Saturn SL service engine light after running for a while

Hi I have a SL and it runs fine except on very hot muggy days. If i run it for 30 minutes or more the temp goes up to 3/4 before cooling down. I know it needs a radiator and have used stop leak for aluminum engines with good success. Any idea why the light comes on?

Temp should read 1/2 from the gauge for normal operation.

Replace your ECTS (2-wire connector) with a brass one.

Get the code :)