I have a Lennox G20Q3E-75-1 furnace. The ignitor lights the pilot light but not the burner. The ignitor never shuts off. The fan blower will come on after about a minute or so but the burner never lignts. Any ideas??

Most likely, the ignition module has failed. Once pilot flame is proven, the module powers the main gas control AND initiates the blower-on delay. If the blower operates, the gas control should too.

Do you know what the voltage should be to the PV, MV, and PV/MV terminals should be when trying to start?

After blower starts, check for 24-27v between MV & PV/MV. If present, the gas control is bad. If not, the ignition module is bad.

When starting up there was 25vAC at terminal PV. The was 0vAC athterminal MV and MV/PV. However, I don't think the fan was running when I checked the voltages.

When I check the voltagewhen fan is in operation, do I check terminal MV to ground and terminal MV/PV to ground? Or do I use one of the terminals as ground?

Quote:

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Originally Posted by njk53

I don't think the fan was running when I checked the voltages.

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If the pilot lights, then NOTHING happens, try carefully removing & cleaning the flame sensor mounted to the pilot assembly. This is a much different scenario than what you previously described (pilot + blower, no main burner operation).

Sensor is brand new.

P.S. Use PV/MV as ground.

When checking the PV/MV terminal should I use MV as ground?

Quote:

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Originally Posted by njk53

Sensor is brand new.

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O.K. don't bother trying to clean it then. Turn up thermostat to demand heating, wait for blower to start (if it does), THEN check voltage between MV & PV/MV.

Quote:

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Originally Posted by KC13

P.S. Use PV/MV as ground.

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Re-read thread.

I will log on tomorrow and let you know the results. Thank You, you have been a big help.

KC13 and Builder Bob, Thank you very much for the help. Your advise was spot on and helped me to diagnose the problem. The root cause of the problem was the ignitor box. I ordered one online for $118. Put it in today. The old Lennox is now purring like a kitten.

Thanks Again
NJK

Just check for 24 volts at main gas valve if you have voltage and no gas the valve is bad. Check and make sure the gas valve is in the on position.If no voltage check all safetys and switchs before valve for voltage opr condanuity.