check engine light

What is your question?

I replaced the rear O-2 senser at the cat. converter and I replaced the t-stat with a 195 failsafe and also the temp senser. started it up and the check engine light was on and did not go off. It it possable I have a bad senser or the failsafe t-stat which was not stock but the 195 is the recommender temp. or does the computer just need to be cleared. Your advise is greatly appreciated.