I have noticed my furnace will cut the exhaust fan before lighting or sometimes start heating for a few minutes then stop. A few minutes later it tries again and will stay on until reaching the correct temperature.
Twice now I have woken up to 63 degrees in the house which I am guessing it tried enough to lock itself out. Turning the power off then on it would tried 2 or 3 times then stay on for over an hour to reach the correct temperature.
I have tried replacing the filter, cleaning the flame sensor, securing the pressure switch tube, and checking the connections to the board.
I had taken the panel off a few times to see the error code and every time it has been 3 blinks followed by a pause then repeat. That is for the case where it was on for a min then stops as well as were the exhaust fan starts for not even 5 seconds and it kicks off.
The manual says that would indicate the pressure switch is open.
Looking at the vent from the ground I don't see anything standing out which would be an issue .. not saying there isn't a problem in the vent somewhere but why would it work for hours at a time and only have the issue when starting up after being off for a few hours.
The furnace is only 3 years old and is a Lennox ml180df 80% eff because we have a metal exhaust vent.
Best Answer
Well, I know this post is old but a simple way to test your pressure switch is to turn off your furnace first and close the gas valve for safety, then gently unhook the air tubing from your draft inducer and gently suck on it using just the suction your mouth creates, I wouldn't inhale it, you should hear the pressure switch click and feel a restriction while sucking on it. If you do not hear a click, can keep pulling air, or cannot pull any air at all your pressure switch is probably bad. If your pressure switch tests good, then try cleaning out the inducer connector it could have junk in it. If it does have junk in it, it may be a good idea to change it out because it is probably rust. That is what was wrong with mine. I had a faulty pressure switch and s rusted out inducer fan. The inducer motor itself ran fine. After changing out the pressure switch it still would not fire up. So i pulled off inducer motor and the fan was completely rusted out.