Trane XE 80 furnace doesn't stay lit

Old 01-24-13, 01:47 PM
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Trane XE 80 furnace doesn't stay lit

For several days I have been aware that our furnace sometimes blows cold air when it is supposed to be heating. Today I finally looked into it, and discovered the flame is sometimes going out. After reading some older posts, the first thing I did was to clean the flame sensor (which wasn't terribly dirty), which did not solve the problem.

When the request for heat first comes, the burner fan starts up, and shortly after that the igniter starts to glow. When the igniter is hot, then there is a click at the gas valve, and one of several things happens: sometimes the burner lights for a few seconds and then goes out (what I might expect from a faulty flame sensor?); sometimes the relay chatters and the flame never really lights; sometimes the burner lights and burns for a minute or two, and then goes out (followed by a sequence of attempts to relight); and sometimes it stays lit until the thermostat reaches the proper temperature.

The chattering relay makes me suspect the small circuit board on top of the gas valve. I removed the plastic housing and looked at the underside, the solder joints all looked good. There was some corrosion on the outside of the wire lugs, I pulled them off and pushed them back on, the tangs on the circuit board and the insides of the lugs looked good.

I am considering replacing the flame sensor, simply because it is cheap. (I also found a trouble shooting guide that gave the expected resistance.) I haven't yet gone over all the ground connections, because they look pretty clean. I'm not sure how old the furnace is, but it is at least 10 years old because that is how long I have owned my house.

Any advice will be greatly appreciated!
Old 01-24-13, 04:42 PM
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Typically the chattering you speak of is caused by the pressure switch. This is usually do to a partially plugged draft motor vacuum port. Follow the hose on the pressure switch to the other side and remove the hose; make sure the port is clear by inserting a paper clip or small drill bit deep into the port. If it is clear and you still have this problem your draft, pressure switch, and heat exchanger need to be check by a professional.
Old 01-25-13, 02:04 PM
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Are you getting diagnostic lights flashing on the circuit board? If so, what are they?

The explanation for the codes is usually found on the inside of the fan compartment door --- please post the explanation for any codes you observe.

Don't waste your time and money replacing the flame sensor. Concentrate on identifying the cause of the problem.

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