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i. If 24 VAC is not present, check the fuses. If they are good, the probable causes are
failed ignition module(s) or a failed interface board. Replace the questionable
ignition module with one known to be good to isolate the cause.
ii. If 24 VAC is present, the probable cause is a failed interface board.
• 24 VAC is
present on interface board J3 pin 9 (LED 5 (GV)) and, on dual units, on J1 pin 9
(LED 1 (GV)).
1. If 24 VAC is not present across the gas valve main coil (MV terminals), probable causes are
an open high-limit thermostat or a failed wire between the interface board and gas valve. Be
sure to check both valves on dual units. It may also be caused by a failed drain safety switch.
Check continuity of high-limit thermostat and drain safety switch. If both are zero, problem
is in wiring.
2. If 24 VAC is present across the gas valve main coil (MV terminals), the 24 VAC circuit is
okay and the problem may be with the gas valve. Be sure to check both valves on dual units.