I have a 10 year old Carrier 58CLR oil fired hot air furnace. Starting the end of last week it started behaving strangely. We'll have the thermostat call for heat and the furnace will start normally (Burner pre-purge, ignition, burn beyond flame sensor shutdown/lockout). Shortly after the burner starts the fan to circulate the air across the plenum/heat exchanger starts. I know that under normal operation when the thermostat stops calling for heat the burner will shut down, but the circulating blower will continue for a few minutes to cool the plenum/heat exchanger down. The weirdness is that now the blower stays running for much longer. I thought it was controlled by a timer, but now I'm wondering if there is a plenum temperature sensor that is going bad and reporting a higher plenum temperature causing the fan to stay on. I've also found that at this point if the thermostat calls for heat again while the air circulator is still running the burner doesn't turn on. I know the burner could be in lockout if the flame blew out, however when I hard reset the furnace (turning off the main power and back on) the burner ignites normally, I've also cleaned the flame sensor to make sure that it is working, and if I turn on the furnace without fuel turned on the system shuts down because there is no flame, so that appears to be working properly. My only hypothesis is if there IS a temperature sensor it is causing the system to think there is a hot plenum so keep the blower blowing and don't bother turning on the burner. Does anyone know if there is a temperature/limit sensor on the Carrier 58CLR low-boy furnace, and where it might be or how to access/test/change it? I haven't been able to find a service manual for it, and the installation manual I found doesn't specifically address this point. Or if I'm completely barking up the wrong tree I'm willing to be told so and any suggestions for where I should be looking would be gratefully accepted. Thanks, Metalmangler

I have been nursing this along for a couple of months now, and just trying to determine just what will fix it. I have read another old post about this same issue. The furnace will start up calling for heat, blow cold air, the Diagnostic LED will flash twice (system lockout - No Flame). I have cleaned the igniter, flame sensor, and removed and cleaned the burners. I also monitored the voltage at the gas valve to try and determine just when the valve would open for "normal" operation. What I have noticed is that opening the panel, removing the plug at the board that connects to the igniter as well as other components, seems to always make it return to working. Switching power off and on does not always seem to work. When I was monitoring the voltage at the gas valve and watching the igniter glow, I did not see the voltage change at least twice, and of course the gas never came on. I have only noticed once that I heard a "click" when the gas should have come on, and it did not, I thought the click came from the board tho, not the valve itself. What I am wondering is what tells the system to turn on the gas? Is it just a timing thing, or is something monitored to tell the system it is ready to turn on. Thanks!