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Trane VX95 Short Cycle Error Fixed by Cleaning Flame Rod

For all its faults, the Internet has definitely made life better in a lot of ways. Since YouTube was founded 10 years ago, I have benefitted many times from people who have taken the time to record, edit, and post instructional videos. The money I have been able to save by fixing stuff myself rather than paying a serviceman can be measured in the thousands. Subjects range from car, truck, and tractor maintenance to plumbing, drywall, and now (again) a furnace.

Trane VX95 Gas Furnace - RF Cafe

Trane VX95 Gas Furnace

In 2008 we had a new Trane VX95 gas furnace (95% efficient) installed in our home here in Erie, Pennsylvania. It has performed flawlessly thanks in part, I assume, to regular cleaning. There was an issue with AM radio interference when it was first installed due to the variable speed motor controller, but an RFI mitigation kit was installed by Trane to fix it (no charge). The programmable thermostat's touch screen failed after about a year, but that was a Honeywell product, so I don't blame Trane.

This morning the indoor temperature was at 70 degrees even though the thermostat was set at 73 degrees. After a little troubleshooting, I noticed the flame would fire for a few seconds, turn off, then fire again for a few seconds, and then shut off for good. The fan motor would then turn on and run continuously. A diagnostic LED on the motherboard flashed 8 times, which is the error code for "Low flame sense." A quick Google search turned up a video by grayfurnaceman, who demonstrated the exact symptom my furnace was exhibiting. He referred to the issue as a "short cycle error," likely cause by a contaminated flame sensor rod.

Trane VX95 Gas Furnace Error Code List - RF Cafe

Trane VX95 Gas Furnace Error Code List

The suggested solution was to remove and clean the flame rod and reinstall it. After turning off the AC power at the circuit breaker panel and turning off the gas valve, I removed the flame sensor rod and sure enough, it had a slight coating of residue on it. A few seconds of scrubbing it with a piece of ScotchBrite pad gave it a nice shiny look. It was put back in place, power and gas turned back on, and the furnace fired and functioned normally again.

Trane VX95 Flame Sensor Rod Location - RF Cafe

Trane VX95 Flame Sensor Rod Location

Trane VX95 Flame Sensor Rod Before and After Cleaning - RF Cafe

Trane VX95 Flame Sensor Rod Before and After Cleaning

Many thanks to grayfurnaceman!!! His

"Why does the gas furnace short cycle?" YouTube video, by grayfurnaceman

Hopefully, by posting this information other people will be able to remedy similar problems.

BTW, when I first went to turn off the local switch for the furnace, it did not work; the furnace kept running. Jiggling the lever made it work, but I replaced it out of an abundance of caution. This house was built in 1956, and the Slater 4200-SP appears to be the original. "Lifetime Switches" is molded into the bottom of the case, so I guess the 'lifetime' definition is up for interpretation (kind of like what the meaning of 'is' is). I was hoping it would be a mercury switch as was common with the early 'silent' type, since it initially did not make the familiar snapping sound when changing positions. It turns out the lack of a snap was a symptom of the switch not working at all. After exercising it a few times, the snap returned, but I will not bother re-installing it.

News Flash: I needed to do the same type flame sensor repair to my A.O. Smith GCV 40 100 gas hot water heater in June.



Posted December 16, 2015

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    Kirt Blattenberger,

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

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