I have an American Standard(Trane) Freedom 80 2 stage downflow furnace mounted on an AC unit. When it started to get cold, I turned it on a month and a half ago the furnace would go on for 6 to 10 minutes, then cut out after three ignition retries. So I emory clothed the flame sensor, and everything worked ok until a few days ago.
Now I get the same thing again. When the thermostat sends a call for heat, the furnace will come on for 6 to 10 minutes, then the flame will go out. Then the igniter will retry 3 times to light, but no gas gets turned on, then everything just shuts down, and the red flashing light on the computer flashes quickly 2 times repeatedly. That's a system lockout according to the computer codes. I've reset the furnace several times and wire brushed the flame sensor, same thing happens every time. I checked the voltage in the thermostat batteries
, and they're fine.
I had a company come over for a diagnosis. For $96, his "guess" was the computer.
The tech checked the hi & low pressure switches by shorting them one at a time and then together. Then he shorted the TCO high temperature limit switch that's in the Burner compartment. He didn't even look at the wiring diagram on the back of the panel to see that there was another high temp limit switch in the blower compartment. I checked that TCO only by looking at the wiring diagram and finding the thick yellow wires, and checking that there was indeed a short circuit across them. I wasn't able to get inside the blower compartment. Every time the same result. Turn up the thermostat, exhaust circulating fan comes on, then ignition, then it stays on from 6 to 10 minutes, then shuts off. System retries ignition 3 times, the shuts off with 2 blink error code.
Oh, and even if it was the control module(40A51-405), I can't find the it anywhere.