We have an American Standard high efficiency gas condenser furnace with electronic ignition model # CUC100A948A5 and an electric air filter, installed in 2003. (We bought the house last year.) We've been investigating this problem for a week now, by ourselves, with my partner's mechanically-inclined father, and with a furnace tech. None of us can recreate the problem on command, and no one is sure what it is. After starting up and running normally for several hours (usually around 8 hours or more), when the thermostat calls for heat, the inducer motor will start to run then stop repeatedly, struggling to get started. As this happens, the pressure switch clicks, the gas valve clicks, and sometimes the unit will glow and then the burners ignite and it runs normally until the next cycle, sometimes it just glows and nothing ignites, sometimes the inducer just keeps trying to get started the unit shuts itself down. If we leave it off for awhile, it runs fine until the whole problem repeats. While it's faulting, the indicator light blinks three times, which is the code for a pressure switch issue. We tested the voltage of the pressure switch and fan--everything appears normal. The pressure tubes are intact and connected. Blowing on them shows that the switch is working. The condenser drainage tubes appear to be draining properly. The PVC exhaust is venting with enough pressure that it doesn't seem like it's blocked. The intake and exhaust lines are level, so water isn't building up in them, and the fan makes no water-swishing noises at any point. The tech replaced the board, but that didn't fix it. He came back and put the old board back in (and didn't charge us for it). He also tested the voltage near the burners and flame sensor and a bunch of other places--everything seemed to be correct. Then he noticed the ground was disconnected or loose--not sure whether we or he may have knocked it loose--but it's connected now and the unit is still doing the same thing. We replaced the pressure switch before the tech came back, but it's doing the same thing, still with the 3 blink code when it faults. Last year the blower had to be replaced. The tech said it was due to the filter not being cleaned for so long. We regularly slean the filter and have had no problems until now, including when running AC sometimes in the summer. Here are some of my theories, after reading as much as I can: 1. Is it an issue with the limit switch? Could the switch be getting stuck on the high limit after the unit heats up and preventing the unit from starting as a safety? (But wouldn't that make the indicator flash for a limit switch problem?) 2. Could the heat HSI need replaced? 3. Could the flame sensor be the problem? 4. Is it the inducer fan, even though it appears to be getting the right voltage? 5. We've shut the unit off to work on it a bunch of times, but haven't shut the circuit breaker on and off for that part of the house. Does that matter? There was a power surge in our neighborhood the other day, but the problem had started before that. I really appreciate any help anyone can offer!

Hello I have had a York Diamond 90 furnace for 11 years and had no issues until 3 weeks ago. I have this weird situation where the surface ignitor would not come on unless I disconnect the end of the rubber hose from the burner box. There are two rubber hoses (one to the combustion blower and one to the burner box) connected to the pressure switch. If I take the end of the rubber hose that connects into the burner box, everything works normally. I called in an HVAC tech who spent hours troubleshooting, cleaning intakes, exhaust, blowing through hoses, disassembling pieces, found no blockage that i know of, even mentioned that he pretty much did a complete tuneup of the furnace, and could not find anything. He suggested to replace the pressure switch which I did myself with a new one but no luck. Any advice would be greatly appreciated. Thanks