It's a high efficiency condensing furnace that draws in outside air for combustion, its isolated from the air in the house. When the thermostat calls for heat, a small fan starts up to draw in outside air and pressurize the combustion chamber. A pressure switch detects that and ignites the burners after 3 seconds. If the switch doesn't open, the system assumes the fan is not running (even if it is) and there's insufficient airflow for combustion, sets an error code in a status LED, and shuts down. That's what happened all three times the furnace failed to start, and the error code indicated the switch was closed when it should be open. The service guys paid no attention to the error code, did not test the pressure switch, they just noted that the main control board was sending 5 volts instead of 120 volts to the fan, assumed the board had failed, and replaced it. Twice.
They missed the real problem. Because the furnace draws in cold outside air for combustion, there's a lot of condensation in the combustion chamber, and there's a set of drain hoses and a drip pan to take care of that. But I noted that the air line from the pressure switch enters the combustion chamber at exactly the same level as the drain hoses, so if the drains are blocked there'll be water in that air line, the switch will not open, and the furnace will shut down. 48 hours ago I took my little air compressor and blew out all those air and water drain lines, and the furnace has worked perfectly since.
And here's what pisses me off. The furnace service guys have a fixed set of troubleshooting procedures and they can't think outside of them, they don't actually understand how the machine works, they've just memorized the testing procedures. The second time, on Thursday morning, I challenged them on that: why are you testing the voltages on the control board when the error code indicates a problem with the pressure switch? I was told that the error codes often don't point to the real problem and the board is the most common point of failure, so that's what they test first. At the time I lacked the knowledge to make the now obvious point: if the pressure switch doesn't open the system shuts down, it's dumb to expect 120 volts at the fan leads, it's all been turned off, so look at the pressure switch and its connections.
I had a car once that suffered something similar. The fuel pump failed at 10,000 km, and it was replaced under warranty. It failed again around 20,000 km, and was replaced again. It failed a third time around 30,000 km, and the dealer replaced it again. It never occurred to anybody but me that this is profoundly abnormal behaviour, fuel pumps failing like that has nothing to do with defective fuel pumps, there's some other problem causing the pumps to fail. I eventually took the car to a private mechanic, who is now for obvious reasons my favourite mechanic and gets all my car maintenance business, and he figured it out in 10 minutes. It was a manufacturing error, the fuel line was crimped at a certain point, was only half the diameter it should have been, which created strong back pressure on the fuel pumps and wore them out prematurely.
So, I guess my question is, how common is this? How often have you called a service company to repair a fridge, stove, washer, drier, air conditioner, furnace, or some other major appliance, and had them fail to fix it? I should probably also tell you that I bought the furnace from Sears, and since then Sears has moved its service call centre from Canada to India. I had to call twice for warranty service before I heard from a local service contractor, the people I got on the phone at that Indian service centre spoke English so badly it just sounded like a string of random consonants to me. I had no idea what they were saying, and I simply could not impress upon them the idea that this was an emergency. It was -35 degrees outside, 17 degrees in the house, my furnace wasn't working, and I could not wait two business days to hear from a service company.
Bit of a long post, but I'm deeply pissed off and needed to vent a bit.