Gmt090-4 furnace the igniter and gas valve relays both kick out when the gas valve tries to kick in

For this answer, we need a model and serial number.
Generally, be sure the power is on, does the heat moving device run? (Fan for air, or pumps for water)

ignition failure. insure air pressure switch closes, hot surface igniter is getting 120volts. check continuity on hot surface igniter, make sure all gas shut off valves are open and gas valve is turned on. make sure there is no blockage in the exhaust

This happened to me where the glow plug was working but couldn't hear the gas valve click on.
I tapped on the relay w channel locks and the gas came on, Flame on!

power to gas valve ok .. Replace gas valve after checking gas valve coil for continuity/resistance... with valve removed ...and power to valve ... try blowing thru valve from supply side ... stuck valve.... replace...

Try cycling your disconnects a few times. Turn off the propane at the tank. Turn on the vent fan. Then turn on the furnace. Yes' I said turn off the gas and turn on the furnace. As the furnace tries to ignite "you will hear the spark" SLOWLY open the gas valve. I say slowly because the tank valve has a safety feature that stops the flow of gas if it is going to fast. By opening the valve slowly the gas will pressurize the line and the valve will stay open.
Here is the reasoning behind this. If your refrigerator, water heater or anything else gas was turned on while the gas valve was off the pressure left the line. When you turned the gas back on there was a rush that let the over sensitive safety valve kick out.

Some models have an electric safety valve connected after the regulator and it is controlled by the gas leak detector. If that detector is set off by anything..hairspray, Frebreeze, bug spray, silicone spray etc... it will turn off the gas at that electric solenoid. Clear the offending spray and reset the detector.

Try this----turn furnace off at stat and unplug furnace for 10 minutes. Plug it back in, the combustion and blower motor may both kick on for 2 minutes or so or not. Turn t-stat to heat and turn up stat to highest setting. Hurry out to furnace and you should hear the combustion motor(little motor piped to flue) kick on, then wait for a "click" next you should see a glow from the igniton element, then another click, next will be the gas valve opening. If no ignition, it will try 3 times and then lock ur *** out, and just turn on the blower just as a precaution. If you dont see the glow plug glowing after the first click, replace the $20 glow plug and you should be good to go. The glow plugs crack, and this is what breaks the connection. If you gas valve has a knob, and says pilot----turn the valve to pilot, push down on the knob, light the pilot and hold down knob for 2 minutes to heat up flame sensor. Make sure the t-stat is all the way down or off. After 2 minutes of holding pilot knob down, let go and your pilot should be still lit, if it is, then turn gas valve to ON and then turn t-stat up to kick on furnace. If ur pilot doesn't stay lit, get a new thermal couple and replace it-$12 bucks. Good luck and let me know how it went. [email protected]

the gas valve could be sticking or you have a loose connection I would look no line for a fault finding a chart in there manual before you buy any parts.

Hi,

Check your manual for your t-stat to be sure that it is set to your type of heating system, this is usually the cause. In the manual for the t-stat, it has settings for your type of system. You state furnace, that means it blows warm air, a boiler circulates hot water. This is most likely the cause of your heating system not operating.

I am assuming that this is a gas system. It sounds as if you are getting ignition, which is good, but not any fuel. I ask that if you are not familiar with gas systems to call a professional, but you can hear if the gas valve opens by starting it up and if you hear a series of clicks and the sound of air coming through, then the valve is opening. It can be a number of things and you will need a multimeter to check the operation of all the connections and relays.

Hope this helps.
Paul

Check your voltage going into the gas valve. It should be 24-27 volts. If it is to low the gas valve will chatter. If the voltage is strong enough you have a bad gas valve.

replace the gas valve....test for 24volt at the gas valve input.  if you have this then it's a def gas valve relay not working.