Background: I have a Lennox G20 WhisperHeat furnace (on natural gas) with an intermittent pilot light. The unit is around 25 years old, and has functioned flawlessly until recently.
The problem: Intermediate Pilot light won't light.
Detailed description: I am familiar with the steps of how an intermittent pilot light furnace works. My furnace is failing to light itself. Here is what is happening with my furnace: the thermostat calls for heat, the damper opens, the pilot valve opens, and the igniter fires for a second or two. Then it stops and nothing more happens. I can hear gas coming out, and spark is present, but the flame does not light. The sparking stops after 1-2 seconds but stops before a flame is present. However, I discovered if I supply 24v to the terminal on the gas valve to prompt the ignition sequence, and hold it for about 2-3 seconds, then the intermittent pilot light will light itself (and in turn light the burners). It seems that the pilot ignition works but the control board gives up before it successfully lights. If I jump the PV for 2-3 seconds to help it light, the furnace will fire up properly and run a heat cycle all on its own. The only problem is that it is giving up before it successfully lights the pilot light.
What could be my issue? I would assume control board, except for the fact that it works 100% once I help it start. I have already cleaned the flame sensor and pilot jet tip. Thanks in advance!