Sorry for the delay in responding. Unit starting working again yesterday morning but then quit again yesterday afternoon. It is not working this morning. When the unit was working yesterday morning, it had an exhaust-like smell so I took the vent cap off and inspected. I didn't find any obstructions so I put the exhaust cap back on. Now . . . the answer to your question. I checked the unit this morning and when there is a call for heat, the vent motor does start and continue running. I watched the green light on the ic board and it flashes 3 times then there is a 1 second delay and then it repeats. Any ideas? I'm wondering if there is a clog in the natural gas line?

Tom -

Sorry for the slow reply again. I had to work late yesterday and didn't get a chance to look at unit until this morning. Answers to your questions about the unit. I bought the house back in August so that's as far back as I know. House was built in 2005 and looks like the unit was new when the house was built. Unit has worked okay until recently (Sept - Present). I just checked the limit control and flame rollout switches with an ohm meter and both checked at 0.1 ohms. I noticed the thermal wrap on the flame rollout switch near the heat exchanger had slid off and was exposing the copper wire. I slide the thermal wrap over the exp0sed copper conductor and reinstalled the flame rollout switch. When I powered the unit up, it started working. Also, I visually inspected the heat exchanger and didn't find any cracks or rust . . . but quite a bit of dust which might be what I was smelling last time the unit was working. Anyway, I'd like to give the unit a day to see if it keeps operating. Can the limit switch work intermittently or do they usually fail once and quite working permanently?

Tom -

Heater is working fine now. Problem must have been bad electrical connnection to limit switches. Thanks for your help!