Atwood 8535-iv furnace won't ignite

[koppx003]

New member to the forum. I have been chasing down an ignition failure on my Atwood furnace 8535-iv. Initial failure code was flame sensor fault. That led to replacement of the flame sensor/igniter unit. New part sparked only intermittently then locked out. Replaced the circuit board and electrically tested fine, but still no ignition. Replaced gas valve with solenoids and all functions well, i.e. blower kicks on, gas valve clicks, gas smell present, spark clicking, cycles three times with no ignition then locks out. Any ideas on what is failing? I have heard that the exhaust unit must be connected, and it is, though not screwed in tightly. My LP seems to be functioning fine as other appliances are working. Any help is appreciated.

[Old-Biscuit]

Spark GAP.....1/8"

Ground wires not clean/tight

[VolDawg]

Did you check to ensure a clean exhaust? I rebuilt my furnace and found a lot of old mud dobber nests in the exhaust. It had to of been nearly 40% restricted from flow.

How about the sail switch. It detects air flow before sending a signal to turn on gas. I'm not sure if it can intermittently fail. Mine worked but I replaced it anyways since they can cause so much trouble and are inexpensive to replace.

[A1RVTraveler]

Originally Posted by VolDawg

Did you check to ensure a clean exhaust? I rebuilt my furnace and found a lot of old mud dobber nests in the exhaust. It had to of been nearly 40% restricted from flow.



How about the sail switch. It detects air flow before sending a signal to turn on gas. I'm not sure if it can intermittently fail. Mine worked but I replaced it anyways since they can cause so much trouble and are inexpensive to replace.

As a RV repairman, i would look closely at the propane system. I hook up a manometer to the incoming line to the furnace. I test supply pressure under a full system load using a bleed orifice. You would be surprised have many propane regulators drop to very low pressure after 3 seconds and the furnace won't run. Also inspect and clean the furnace LP orifice. I find lots of bugs and spiders stuck behind the orifice in Atwood 8535 series furnaces.

[153stars]

Spiders love to nest in gas air mix tube like mentioned. Read a flea collar or insect repellant cattle/sheep/pig ear tags can keep bugs out as well for outdoor and garage/ building electrical panels and outlet boxes. Farmer showed boxes filled with bugs/big ants chewing insulation in outlet boxes. Then none next year with ear tag in them. Hard to get small quantity, I thought about buying tags offer in classified just send stamped envolope.