I've got a problem with my 15 year old Goodman furnace. It starts w/o problem, inductor begins working, hot surface ignitor lights up and ignites the gas. Then the main blower starts working and the whole thing stays on for 2-3 minutes. Then the flames go off before the thermostat reaches the target temperature. Though it continues to recycle the air. It will restart 20-30 min later with the same effect. Sometimes the flames don't appear at all (I guess gas valve stays off) even though everything else stays on (including HSI).
I checked the termocouple. It looks clean but may be I still should replace it? Could it be a faulty air pressure switch?