Hey all, I've got a ~20 year old furnace that has been running without any issues for the last few years right up until today. Here's my issue. When I set my thermostat to 'heat' and select a temperature, nothing happens. The unit is a standing pilot system, the pilot is lit. I turned everything off and relit the pilot, no change. I can set the fan to 'on' at the thermostat and it fires right up. I have measured voltages at the fan limit switch and here's what I've got. The white line that goes from the transformer -> limit switch -> TH (gas valve) is at 20.8V
The red line on the limit switch is at 120V before turning the fan on, after turning the fan on it drops to 9V and the red line on the other side of the switch then becomes 170V (not sure about this). TR (gas valve) stays at ~1.5V (0V in reference to C on the transformer). It seems to me like I've got a few things happening.
1) The ground between the 120V supply and the thermostat don't seem to be connected. This would explain why TR measures differently when I reference it to different points, is that normal?

2) The gas isn't turning on, that makes me think the gas valve isn't working.

Being that the fan turns on manually, and that the pilot lights and stays lit, I'm inclined to think it's a gas valve problem. Also, I've only owned the house for three years but I noticed today another gas valve sitting next to the furnace. I'm betting its an old one that may have had problems at some other point. Any ideas? Thanks in advance!

The gas valve needs 24 volts to work. Back track the low voltage circuit till you find where the 24 volts stop. That is usually the component that is defective since it will not allow the 24 volts to pass. Attach one probe of the meter to the C= Common on the terminal strip and use the other probe to track down the defective item causing the trouble. Leave the thermostat calling for heat while you are testing so the thermostat circuit is complete. If you want to test the thermostat heat circuit jump W=White to R=Red on the terminal strip. This jumps the heat circuit in the thermostat.

I jumped the R -> terminals, no ignition. Let me make sure I understand here; the gas valve should have a 24V potential across it (meaning 24V on TR and 0V on TH), or should the gas valve be at 24V on each terminal? I've only got one terminal (TH) at 24V, if there both supposed to be at 24V then that would be where my problem is. Thanks in advance!

You should have 24 volts together not on each terminal.

Okay, so then that tells me that everything is fine electrically, the gas valve must be malfunctioning. All right well thanks for the help everyone. Effingham eh? They've got an Effing good burger over there.

Oh boy, been through here I assume?

Do you hear the gas valve click? Are you sure you have gas to the furnace?

Well, the pilot light is on so that tells me there must be gas coming to the furnace. No I cannot hear the gas valve click.

If it is a GUI series then this may be of help.

Does your unit have this for the spark box. It is grey in color

Control : Goodman Janitrol B13707-24 control module

"Goodman Janitrol control module number B1370724 or B13707-24 , 90 second lockout, 3 retries, fits GUN, GUI, GUP, GUPI series."

I

Post back with answer

Well I pulled the trigger and got the gas valve, just installed it and am glad to say my house is back up to a comfortable temperature... or at least my girlfriend says so. Thank you all for the help you provided, its an old furnace but I need to make it last just a bit longer. Thanks again!

Glad you got it going.