2004 Impala SS 3.8L Supercharged P0128 code
 

Just replaced the thermostat. How do you clear the code or how long do I have to wait to see if the code clears or still is a problem?

The ECT sensor is next on the list but I wanted to check it one at a time.

Thanks in advance.

Unplugging the battery for a few hours should clear the codes.

Or go to an auto parts store that offers free code scanning. They should be able to reset it.

What thermostat did you go with? I've got a 180 since I'm in South Florida.