could not find my book to much of the in-laws junk packed in the garage, I think the code is a P0117, check the connectors that hook to the thermostat housing make sure they are tight and free from grease etc. Kelly ? when you put in the thermastat did you make sure the little jiggle valve on the thermastat faced forward.

2001 5sp. K&N stock filter 215/70/15
If your check engine light is on at least you know the light still works