It all started a couple weeks ago when i got a no heat call. the furnace is a bryant 398aaz series a (plus 90) (nat gas). on my first visit i made the mistake of opening the blower door to start my troubleshooting and of course the controls reset and all components started and ran normally.I cycled the furnace on/off several times to try and re-create the situation that caused the lockout to no avail. the spark igniton will light the pilot the first time, every time.
A few days later I got the callback right before i needed to leave for my class at HVAC school. I had a limited time to diagnose this and the condensate hose appeared to be backed up, so i thought i would clean out the hoses and trap and check the air hose to the pressure switch. the furnace again started up and ran just fine.
Today it locked out again. talked to the local carrier dealer and got schematics for the furnace (not readable on furnace) and found that there was no voltage leaving the ignition lockout control. I replaced the lockout control and furnace is running again, but with 2 call backs on this job already i am concerned there may be another issue i am missing. Does anyone have experience on these nightmare furnaces? is this lockout control known to fail? why is this happening intermittently?