[Dave at Pelican Parts]

The beauty of the OBD cars--the Check Engine Light (CEL) will tell you if the problem has been cleared or not. So replace the t-stat, bleed the coolant system and such, and clear the codes. Then drive around for a while (several days, I would think!) and see if the CEL comes on. If not, stop in for your inspection. If it comes back on, replace the O2 sensor. Or have the codes read again and see what they are.

--DD