Furnace ignites then stops

Hey all just wondering if you might have any suggestions. I have a stage 1 furnace that gets ignition then stops. At first I was getting a pressure switch stuck closed code. I replaced pressure switch and reducer fan a couple weeks ago and all was good. Now it's doing the same thing but the board isn't flashing any codes. I replaced the flame sensor and still no go.
Thanks much

 

Inspect your flue for obstructions, bird nest, a bird, whatever. Test the switch if you can. Try to watch a cycle and see if it actually ignites and then turns off or see if your getting flame rollout

 

Flue is clear and no flame rollout and checked the safety switches. I do get flame for a few seconds then shuts off.

 

Sounds like sensing rod. I know you said you changed it already though.

 

Try cleaning or replacing the thermo coupler. My old furnace I had to take some fine grit emery cloth to the thermos coupler every couple of years and clean off the white build up.

 

Last year around this time we had a similar problem. $836 later it was a control board and a thermostat.

 

I just went through this almost exact same problem. Thought it was the flame rod, turned out to be a loose connection on the thermostat terminal strip.

 

What brand?
Model and serial# ?

 

Thermocoupler. This is a very cheap easy replacement. Most likely this.

 

Also if you’re mechanical room (floor) is dusty enough your furnace won’t act properly as the air intake is on the bottom. Try blowing it all out with compressed air.

 

I had the same problem with my furnace about 5 years ago. It slowly went from kicking out, then relighting after a couple minutes of running to lighting and almost instantly kicking off.

I cleaned the flame sensor, added another ground wire from beside the flame sensor to the control board, no improvement.

I called an HVAC tech, had him service /troubleshoot, seemed like a good idea, right? He called the control board since he couldn’t find anything else, but wanted way too much to replace it with no guarantee of success. They really pushed for a full replacement, which would be an expensive nightmare in my house.

I ordered a board and a vacuum switch for the inducer fan just in case, replaced both and still the issue remained.

Finally I took the vacuum sense hose off the switch, let it run it’s light-up cycle with the clear hose stuck in a cup of water and found the vacuum from the inducer fan was < 1” of water on a .8” of water rated switch. Teeing the line back to the switch, the actual vacuum dropped even lower with the burner lit up, tripping the vacuum switch, shutting the burner down before relighting.

That’s when I pulled the inducer fan to find nothing wrong. Next step was flue inspection, from above it looked great. When I pulled the short, nearly horizontal pipe just above the furnace over to the main flue, I found it half-full of fine ash accumulated over many years.

After cleaning that out, the inducer vacuum was nearly 3” of water, and problem solved.

 

Man. I just realized I don’t know crap about furnaces.

 

We had an old Carrier that was doing this. Turned out to be a control board but I wanted to know what would cause the control board to fail. After poking around, I found our heat exchanger was cracked anyway. Had to either replace the exchanger and the control board or spend about $900 more to get a new furnace with warranty. The union was offering a $400 rebate at the time making the price difference around $500 more for a new unit.

I’m glad I didn’t just purchase and replace the control board on a failed unit.