My home has a gas furnace with electronic ignition (intermittent pilot). Sometimes it doesn't want to start up, even with the thermostat turned up, and it indicating heat should be on. The fan-only mode works fine, as does the A/C.
Yesterday I shut off power to they system and opened the side panel where the gas valve, burner and ignition system live. I noticed these three sensors all wired in series (see photo below). The "upward" facing sensor had a lot of dust on it, so I brushed it off. I then turned on power to the system and it started right up. I thought that was the culprit until this morning when again the furnace would not come on. So I repeated the sensor cleaning, but this time I used a bit of isopropyl alcohol to gently clean the face of each sensor, and again the furnace kicked on as soon as I restored power. But I'm still not convinced that the sensor cleaning was the trick; maybe it was just having it powered off for 5-10 minutes did some sort of system reset, and this problem may come back.
So, can anyone identify these sensors, and what is their purpose? Maybe one of them is faulty? And better yet, can anyone give me any other troubleshooting ideas if the problem does indeed come back.
Update: Thanks to all who have contributed. As Ed Beal, JPhi1618 and others mentioned, the sensors are snap sensors, designed to "snap" when the temperature reaches a certain point, which opens up a safety circuit and likely shuts off the system. I was able to push one in, then use the little red tab to push it back out (reset). Must be some sort of bi-metallic thing.
And as I suspected (and others have eluded to), these sensors are not the problem. The non-start-up problem has happened again, but I wasn't home so I instructed my wife to cycle power so she could get the house warm. Next time it happens though I will hopefully be home so I can look for a blinking LED on the control board (which I found last night in an adjacent box). I will also try to clean off the flame sensors too since they are reasonably accessible. If/when I solve this I will report back so others can learn, and accept an answer if applicable.
Update 2: Woke up this morning to the heater not starting up again. The access panel has a small plastic window which allowed me to see the control board, and the flashing LED. A troubleshooting sticker on the panel told me the pattern of the flashing LED indicated "pressure switch closed" (it must be opened in order to startup, therefore likely got stuck in the closed pos after the previous cycle). Googling shows this to be a common issue, and solved by either cleaning out the pneumatic lines that attach to the sensor, cleaning out the sensor itself (gently!), or else replacing the sensor since it is likely bad. I'm just happy it is not the control board or something like that. Thanks again to all who replied, and I hope this update helps others (also, here's a good video of diagnosing this problem)