My furnace is a Sears Kenmore 90 (Propane)
The info on the placard inside the cabinet is:
MFG.: HQCD (I believe it to be a TempStar)
Model: NULK125DK03867
Mfr. No.:NULK125DK03
The problem:
Thermostat calls for heat
The ID fan starts and I've inspected the vent and there is air being blown outside.
The blower motor starts
The hot surface igniter glows cherry red.
After about 20 seconds, the igniter shuts off.
The burners never produce any flame.
The system cycles 3 more times and then the LED glows solid red or Lockout.
It appears as if everything is OK except for opening the gas valve.
The propane tank is at 80%
The flame sensor was replaced last heating season.
Any way to test the controller to see why the gas valve doesn't open?
Or??

You have one of the 5 or all: dirty burners, no gas(shut off some where), low gas pressure, no 24 volts going to gas valve or bad gas valve. You should hear click at the gas valve about 10 to 15 sec after the hot surface igniter starts to glow.

I agree. Remove the burners, check for rust around the front of the burners not allowing the gas to pass through.