Unit's been on the fritz for awhile now. I had an HVAC guy out multiple times for the same problem but always a different result. They replaced the ignitor module, ignitor and control board and it still runs intermittently. Monday I grabbed the harness and gave it a tug an noticed a clicking sound which I traced back to the pressure switch with the small hose going into the box. The switch was completely disintegrated so I ordered one online and replaced it as soon as it came in. Furnace lit up no problem and ran for 3 hours straight, the time it took to bring the house temp up from 52 to 65, when it got up to operating temp it shut off like normal. When the thermostat called for heat everything sounded normal, the ignitor lit the gas released and it fired back up immediately... next time it took a little longer before it lit then longer then ignitor would turn off and not ignite at all. It stays lit for a reasonable amount of time but its almost like the timings off or sequenced incorrectly, like the gas is hitting it after its started to cool down.
Any advice?
Mike

Clean the flame sensor with fine sand paper or steel wool.

Clean the flame sensor with fine sand paper or steel wool.

Will do... hopefully its something simple like that. I'll let you know, and thanks for the advice.
Mike

Nope didn't work. The heater actually does nothing now... well the blower runs non stop but the ignite circuit doesn't even think about starting. The cpu reads 1 light for ignition lockout. I replaced the pressure switch exactly how it came out but is it possible it was wrong to begin with?

If the blower is running nonstop as you say then check out the safety limit controls since if one of them is open it will cause the blower to run all the time since it thinks the unit is over heated.

Before you cleaned the flame senser the unit was working somewhat? Are you sure you cleaned the correct part. I hope you didn't clean the glow plug, if you did, you will need a new glow plug.

before you cleaned the flame senser the unit was working somewhat? Are you sure you cleaned the correct part. I hope you didnt clean the glow plug, if you did, you will need a new glow plug.

By glow plug do you mean ignitor? Then no I didn't tough that

A restriction in the vent will keep the pressure switch from closing. Or if it's a 90+ furnace a restriction in the drain line will do the same.

That raises a good question. The drain condensation "catch" can, I don't know what its actually called but the pvc tube can that has 2 spade terminals on it. From all the reading Ive done on this site the overflow will cause the furnace to kick off. I pulled the board cover and the schematics have wires for bdts (trip sensor) but there is no wiring connecting to it... should there be?

It is a 90 + unit