Expert: Norval Trimborn - 11/2/2009

Question
QUESTION: Norval Trimborn,

I have a Trane XR 80 forced air furnace about 7 years old.  The problem with it is, and for now it's an intermittent problem, the flame does not light.
The vent fan motor starts, the glow plug lights up (like it should), then it skips lighting the burner, and the fan starts blowing and the glow plug goes out.
If I turn off the power to the furnace for a few minutes, then back on, it resets.  Sometimes it fixes the problem, other times it does not light again so I have to reset it again, and sometimes it works and runs properly a few more time then back to the no lighting of the burner.
Pressure switch, gas valve or WHAT DO YOU THINK???

Thank You,

Rick

ANSWER: There has been a problem with some of the gas valves in Trane furnaces at the point where the wiring harness plug inserts into the gas valve. Try moving the plug slightly when the furnace does fire up. If the flame drops out, replace the gas valve.

---------- FOLLOW-UP ----------

QUESTION: Norval Trimborn,

On the front of the heater the is little a red flashing light for diagnostic purposes.  When it's not lighting, the light flashes 4 times and looking through my furnace papers it says 4 flashes equals "Open Temperature Limit Circuit".
It couldn't be the high temp limit if it's not even lighting, correct?
Is there another limit switch that would keep it from lighting?
If so, where about would it be?  Is it the high limit input or high limit output?

Thank You,

Rick

Answer
An open high limit can be from a previous cycle of the furnace. This can be due to overheating from a restriction of the airflow( dirty filter,blower wheel,evap. coil or closed supply registers). The limit will eventually fail due to repeated cycling and need to be replaced.