Not a lot to go wrong on TD's. you checked heater, As long as not short/open cct ok. next thermostat, should read short cct when cold, open cct when at temp. other than that control cct, varies by m/c relay or triac. both can be replaced, But is it relay or triac, or control cct. can only be found by tests. Repost if needed.
SD
February 2012