The problem we have with our furnace is occasionally we smell gas when furnace starts and that lasts a while. It's not always very obvious but happens quite often. We have an HVAC technician in today to diagnose the problem, here is what he found (it's quite amazing we got an email with diagnose report after the inspection, I didn't know HVAC companies do that nowadays!) I copied result below:

"upon arrival found that furnace wasnt cycling correctly. control board on furnace has water damage from previous water leak. 1st stage functions but 2nd stage is intermittent. Bad relay on board. Client had other service company out here who attempted repairs - couldnt find out who or what but found door switch disconnected and power wired directly to board."

The company suggested to have control board replaced and ask for $900 for it.

Is " door switch disconnected and power wired directly to board" really bad? Will the founding the technician reported cause the problem (bad smell) we experience? We do get enough heat (in Dec, midwest weather).

I recall we had some AC problem before and I changed thermostat, could that be something related?

Should we replace the control board or furnace all together? furnace is maybe 10 year old.

Thanks.