Arrived at location on 6/14/22 - Customer explained unit showing 909f and open flashing. Called tech support and verified thermocouple is bad. No other recommendations at this time. Customer wants part ordered. Returned to location on 6/15/22 - Removed failed left thermocouple, installed new thermocouple, ran wires to terminal block, secured wiring, closed up unit, turned on and let heat, heated up to set point of 476F, burners cycling, confirmed unit working correctly at this time.