We have a Carrier 58STA/STX Furnace that is only short cycling in Heat and not A/C. When it cuts off it is throwing Code 33 which is pointing to the following issues:

  • Dirty Filter or Restricted Duct System - Replaced Filters & Using a MERV 2, still trips
  • Loose Blower Wheel - Removed today and was not loose/excessive play
  • Defective Switch or Connections
  • Defective Blower Motor or Capacitor - Capacitor was at 50% of nominal, replaced and still occurs, have not done anything with blower motor.
  • Inadequate Combustion Air Supply (Flame Rollout Switch Open) - Flame Rollout Switch stays closed
  • Restricted Vent Sizing - Spec'd to House
  • Excessive Wind - Happens on Windy or Calm Days

If I either remove the filter or remove the blower access cover it does not throw the code.

  • 1
    Thank you for informing us of your situation. Is there a question you want to attach to your post.
    – Alaska Man
    Jun 29, 2020 at 21:46
  • Have you had the static pressure of your duct system measured? Has it always done this, or was there some event that prompted it to start doing this? Jun 30, 2020 at 0:34
  • Was your system designed with a merv2 filter ? Your blower gains may be full of dust it is really wild that that slight curve can get a thick enough layer of dust / dirt to drop your flow / pressure.
    – Ed Beal
    Jul 1, 2020 at 21:01
  • We purchased the home in September of last year so this year was the first Winter with it. I don't recall it happening immediately but I also may have not noticed. I had used high MERV 4 - 7 and started dropping it as I noticed more issues.
    – Pat
    Jul 9, 2020 at 19:21

1 Answer 1


I was able to figure out it was the Draft Safeguard that was tripping. After digging around online some more there were a couple recommendations:

Right when we purchased the home we redid the flooring immediately from linoleum that had a threshold at the closet to engineered hardwood and we did not put a threshold at the closet (effectively increasing the gap at the bottom). I both verified the intake was clear and also placed towels at the door gap and let the furnace run in which it finally did not short cycle.

  • Glad you were able to lick the issue! Jul 15, 2020 at 15:52

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