My furnace is short-cycling. I've tried the following:

  • replaced the flame sensor
  • Shorted the thermostat heat demand signal
  • Checked the blower capacitor
  • Checked the air intake filter
  • Checked the exaust vent

It seems to only works after I cut the power to it for 8+ hours

When it starts working it will work for about 48 hours and then return short cycling.

It has only begun short cycling at night (maybe temperature related).

What else should I trouble shoot.

  • Does it have a control circuit board? If so, couldn't it be something there? Years ago our now 27-year-old Carrier NG-fired forced-air furnace required a new board to restore function (done by our repair service). Jan 24, 2019 at 10:02
  • 1
    Hello, and welcome to Home Improvement. What do you mean by "short-cycling'; how long on, how long off? Jan 24, 2019 at 12:12
  • It may be working just fine. Good thermostats learn how the system performs and they may use one or two short cycles to avoid over shooting the desired set point, conserve energy and keep you more comfortable. Some thermostats allow you to set the differential (2 degrees means ON at 68 degrees, OFF at 70 degrees). You could check that. Oct 27, 2020 at 15:26

1 Answer 1


I would start by trying to find a service manual for your furnace, and see if it has a troubleshooting flow chart.

I am thinking either:

  • Something is not functioning correctly, and the control board is shutting it down to prevent damage
  • A sensor is reading incorrectly
  • The control board is having issues and not applying logic correctly.

If the control board is detecting something wrong, it might throw an error code. Previous systems I've used had an LED on the board that would blink out the error code.

You might be able to check the output of the various sensors by back probing the connectors. You can do your 8 hour reset to determine the 'good' values, then test it when it is short cycling to see if these are different enough to cause the control board to think something is wrong.

Look over the control board for signs of damage - blown capacitors, burn marks.

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