[kwoodwar]

Of course, just as a cold snap is about to hit, our furnace quits working. Having fixed it successfully the last couple of times it quit - once due to the intermittent pilot ignition control module packing it in and once when a small spring had broken which was supposed to push on a micro-switch that proved the damper was open - I figured I would tackle it again. I'm pretty sure it's the intermittent pilot ignition control module again, but I'm not 100% sure and those things are expensive (~$300) to replace, and once you walk out of the store with it, you own it - no returns are allowed - so I'm loathe to by one on the off chance that's what I need. I want to be sure, so I'm hoping someone with more knowledge than I have can point me in the right direction.

Furnace: Lennox G20, 10 years old.
Intermittent pilot ignition control module (IPICM): Johnson Controls G776RGD-14C, five years old.

Symptoms:

When thermostat calls for heat, igniter clicks and pilot lights, but main gas valve does not open so burner does not light. Green LED on IPICM glows - no flashing.

I cleaned the flame sensor, checked its insulator for cracks, and cleaned the pilot burner.

I've checked:

• Continuity from flame sensor to IPICM - 100% (i.e. No resistance)
• Continuity from pilot burner to ground - 100% (i.e. No resistance)
• Current through flame sensor - couldn't detect any when pilot was lit. My multimeter will measure microamps. The manual for the IPICM says there should be a minimum of 1.5 microamps through the flame. The furnace manual says 0.7-1.2 microamps. I'm not sure if my multimeter is sensitive enough for either measurement, though.
• Voltage from thermostat to THS terminal on IPICM = 28V AC
• Voltage to pilot valve solenoid = 26V AC
• Voltage to main valve solenoid = 11.5V AC

My guess is the IPICM is no good (again). Various DIY forums suggest that the Johnson Controls G776RGD units are prone to failure. If ours is faulty, this will be the second one that's failed us in 5 years.

Am I correct in assuming there should be at least 24V sent to the main valve solenoid to open it, and 11.5V is not enough? And, since the IPICM is supposed to send that current, it is at fault? I imagine the transformer is OK because the pilot valve is getting the correct voltage.

Am I also correct in assuming that, since the IPICM is sending some voltage to the main valve solenoid, just not enough to open it, the flame is being detected (so the flame sensor is OK), but since the voltage to the voltage to the main valve solenoid is too low the IPICM is faulty?

Any ideas?

Various posters on DIY forums recommended getting a Honeywell 30W33 replacement unit, by the way.