Troubleshooting a Furnace Pilot Light Problem

How To Lite the Pilot Light

1. Be sure the gas to the furnace is on.
2. Turn the knob on the gas valve to pilot.
3. Press the knob down and hold (or slide and hold on some furnaces). Insert a long match inside the furnace and light pilot.
4. Once the pilot is lit, continue holding down the button for at least 30 seconds.
5. After 30 seconds, let the button up. If the pilot remains lit, turn the knob to ‘on’.
6. If the pilot goes out, wait 5 minutes and repeat steps 1 thru 3.

If the pilot does not light, you may have air in the line. Trying several more times should eventually purge the air and allow the pilot to light.

It is a good idea to wait five minutes before trying to relight a pilot light to let all the left over gas to dissipate.

If the pilot still doesn’t light, read on.

If the thermostat is set correctly and your pilot still won’t light, you could have a problem with the thermocouple or a dirty pilot light. Check that the thermocouple hasn’t slipped or shifted and not getting the full flame from the pilot light. If that’s not the problem, then replace the thermocouple and check that the pilot light is clear of any grime or dirt.

If you get the pilot lit, here’s one way to test that you have a strong thermocouple:
a…. With the gas valve on and a watch in hand, blow out the pilot light.
b…. Listen to the gas valve and count the seconds until you hear a click telling you that the gas valve has closed.
c…. If you hear a click in 25 seconds or less, replace the thermocouple and clean the pilot light.
d…. If you hear a click in 25-60 seconds, your thermocouple is probably fine.
Wait 5 minutes before relighting pilot.

If the thermocouple is good, it will take it longer to cool down, so the long wait time is a sign of a good thermocouple.
Thermocouples are cheap and are available at many stores, even online on Amazon or eBay. Thermocouples come in different lengths, but length is not important; the important thing is that it reaches the gas valve.

The gas valve needs 24 volts to operate, so a transformer is used to step down 120 volts to 24 volts. A multi-meter is needed to test the transformer. One side will say a line, and the other side will say load.

The side that says the line will be 120 volts
The side that says the load should be 24 volts.

Not all transformers will have the words load or line marked on them but tracing the wires or simply seeing if both sides are getting the right voltage will tell you if a transformer is working.

Gas valves are always the last thing to replace since they are the most expensive part of the gas assembly. A new thermocouple should always be installed when a new gas valve is installed.