Hi,
I have a 1996 York Diamond 80 furnace. Everything starts up OK (thermostat fine, flue fan kicks on fine, then pilot orange glow, then click, then gas burners fire up, then main blower fan starts). This all happens in about a minute or so, but then the main fan only runs for 22 seconds, then it goes off for 61 seconds, then on for 22, then off, etc etc. The thermostat is set at 69, the house temp is 66, and it is 32 degrees outside (not extremes of any temps). What is causing the short cycles, and how can I fix this?? It hasn't happened before, usually the main fan runs for 15-20 minutes before it kicks off, and then its off for 30-40 minutes. Note - there are no "LED lights" in this system (only a red indicator near the main blower, but that is not lit); and I have a new filter (also tried with filter removed - no change); and I looked at the inside of the unit - it is pretty clean. Also after a while, the burners kick off, the flue fan runs continuously, the main fan continues its "20 seconds on, 60 seconds off" thing, and eventually the burners light up again after a short time being off.
Help! Thanks!

The flame sensor might have dust build up on it. The flame sensor makes sure that the system has ignited before it starts the main blower. If it ignite and burns for 15-20 seconds then goes out that is most likely your problem. Identify a probe that is in the direct path of the flame(usually in the last burner tube from the gas source), it will have one wire coming off it and usually has one screw holding it in place. As time goes by the sensor will get a build up on it and stop functioning. Turn the power to your furnace off, then remove the screw and then gently pull the flame sensor out. Using a MILD abrasive like panty hose of paper towel (DO NOT USE sand paper) clean the sensor and then put it back into place. Tighten the screw and then turn your furnace back on. That should fix your problem.

Thank you john 0204 I am absolutely amazed that your fix worked I know very little about hvac and started to take apart various small parts of the heater to buy new and replace, just hoping my problem was not a circuit board or even something where I would need a real hvac guy. Then by chance saw your fix online and it worked. Also I am extremely ticked off at big business such as the manufacturer of this heater for just about deliberately or maybe stupidly designing this unit as well as probably many others in such a way that this sensor just happens to be located directly below the condensate outlet for the a-coil. This is where water (condensation) inevitably drips onto this sensor creating a minor build up to prevent the unit from working costing consumers endless dollars in repairs. I'm sure in many cases replacing parts unnecessarily until you get to this simple solution. That is, if you have a repair guy smart enough to firgure it out or one not wanting to rip you off. Thank you again

I too tried this same fix and it worked wonderfully from that point on. BTW I had very similar simptoms that lead me to this fix via Google. Thanks again!

It seems more than one person has the same IP address. It kind of makes you wonder who is talking to themselves. LOL

I had the same prob.. mine would come on, light, go out, light, go out, light go out, then shut down...

I cleaned both sensors the one on the right first then the one on the left..

After cleaning the one on the left the single rod with the purple wire attached my furnace was back to working properly.

Thanks

I had the same problem tonight, getting down below zero. Thanks for the info, would have cost me one chilly night, a cantankerous bird dog and no telling how much for the service call.

Im having trouble with my heater as well. Everything will kick on right, but the flames will only stay on for a min at the most. Cold air is blowing out of the vents. I don't see any led lights on? Please help

I wish I would have found this post a day ago. Now, one HVAC repairman visit, two new thermostats and 300 dollars later, fixed the problem with this simple, yet ingenious recommendation. Thank You!

Thank you, John! A 20 second fix!