My 15-year-old GMP100-4 gas furnace recently stops or restarts randomly after the burners ignite and the blower starts. Sometimes it will restart the ignition process immediately one or more times and succeeds after that; sometimes it won't even try to restart at all. If it does not restart, it seems to go into lockout mode. During the whole process, I have never seen the red status light blinking at all (light always on ). I already cleaned the flame sensor though it looked clean without any buildup even before cleaning. The manual says if the flame sensor does not detect flame, it will retry for 4 times and then go into a one-hour lockup mode. Then I turned the power off/on and restart the thermostat, the exhaust blower tries to start, but stops immediately even before it gets to full-speed. Eventually it will come on again by itself randomly after a lot more than one hour. However, sometimes, it starts and completes a full heating cycle normally. My questions are:
1. It looks like a flame sensor failure, but why did it burn for so long that even the blower started before the flame sensor reported "no flame"? Why does it retry less than 4 times (as the manual says), or even no retry at all, before it stops?
2. Why does it not get out of lockout mode after resetting power and t-stat and no flashing light, though the exhaust fan moved a bit? Why is the "one-hour" lockout period not in effect and it takes randomly longer time to come back up?