I have a Trane TUD080C936J0 gas furnace/ac. The furnace starts up via a hot surface ignitor.

The ignitor that was in it went bad, and I bought a replacement. When I plugged it in, it worked fine - once. The next time it called for heat there was a pop and a tinkling sound and the ignitor was in pieces - it looked like thermal shock or something.

I took that back and got an exchange, but the one they gave me was pre-cracked, and as neither of us checked it out at the store they wouldn't give me an exchange.

A couple of weeks later, a new one came in from an eBay seller. This one has about 40-50 ohms resistance, but the furnace still flashes "check hot surface ignitor. I've plugged my multimeter into the plug that goes to the ignitor and when the thermostat calls for heat it spikes for a moment (I typically see 500/1500/200v flash) and then it goes back to zero. I even plugged my multimeter in series with the ignitor and got the same behavior, but still no dice. Is it possible I just got another faulty ignitor? Is there a way that I can test the ignitor, like just plugging it into a wall socket or something? Presumably not while actually holding it, of course.

The furnace did flash one of the other trouble codes at first, but that switch was apparently just a little stuck - pulling it out and warming it up let it reset, and it doesn't flash that code anymore.

Any ideas?

  • Those things are very fragile, and if you don't know easily contaminated (never touch with fingers) the first one I replaced many years ago only made it a few cycles because I handled it with bare fingers. I have found the name brand ones are packaged better but I will order 2 or 3 online at the cost of 1 name brand element. I have had a bad one several times but never 2 on the same order – Ed Beal Jan 12 '18 at 21:43

First, never get anything on the igniter like finger oil. Depending on the circuit board it may be a constant voltage or a variable voltage ignition system. If it is constant voltage you should get a constant 120 volts +/- . If it is a variable voltage system then it shouldn’t be much above 90 volts. The variable voltage system was supposed to extend the life of the igniter by only using the bare minimum voltage. It would try about 60 volts and if it didn’t ignite it would up the volts until it did. The only problem with the system was that when the igniter did fry it tended to take the board with it. Either you aren’t taking a proper meter reading or the board is bad. With either board the most you should be reading is line voltage and it should stay on for at least 10 seconds or so.

  • well it's definitely not staying on for 10s – Wayne Werner Jan 13 '18 at 8:42

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