Set t stat to heat auto sm fan turns on then unit shuts down tries after about 2 min again same result

I think what you are describing is a fan that runs for a few minutes to initiate a draft for your exhaust. Is there a small black tube nearby that forms a loop? Remove that ttube and blow it out of all condensation, try again for iginition

I did clean out tube and small fitting to fan retried but same results small fan turns on a few seconds then shuts off