Burner ignites - goes out after a few seconds
- low gas pressure
- intake or exhaust vents obstructed
- dirty burner
- electrode(s) out of adjustment, dirty, or defective
- bad control board
- micro switch with an arm that extends into the blower air stream which closes when the blower reaches approximately 75% of its rated speed. Also called an air prover switch.
- safety switch, a normally closed switch that opens if it gets to hot, opening turns off power to the gas valve and ignitor board
- a normally open switch that closes at a preset temperature, causes the furnace to run for a short time after the thermostat opens, allowing the furnace to cool down. time delay relay
- same function as fan switch, only has its own heater, also used instead of a thermostat relay in newer models.
- similar to a spark plug, 2 versions, 3 probe (remote sense) or 2 probe (local sense.)
ignition control board
- when powered, initiates gas valve opening and spark sequence which lasts approximately 7 seconds. Older models are 1 try, thermostat must be cycled off for at least 10 seconds before another ignition cycle is attempted. Newer boards are 3 try, will attempt to ignite 3 times at approximately 60 second intervals.
Some furnaces now come with a fan control ignition board. Fan control boards will shut the blower down if ignition does not occur after 3 attempts. Some models will shut down the blower and wait an hour and recycle, if ignition does not occur the furnace is then shut down. Older DSI furnaces, with some exceptions can be upgraded to fan control. See Fan 50 Plus
- LP gas pressure must be 11" of water column (6.25 oz per sq. in.), checking and adjusting requires a manometer.
Marksrv.com rv furnace troubleshooting