I have a tempstar (ICP) furnace put in when the house builder built the house. Model is N9MPD075F12A2. Installed late 2009. This problem only happens with the furnace not the AC. When calling for heat, the furnace will run 10 - 15 minutes then give me a 4 blink error which is (limit or roll out switch open). flames turn off, inducer finishes it run along with the blower motor. Then the furnace resets and turns on right away again. This may happen 1 - 3 times to reach the desired temperate especially when wanting to raise the temp by 2 or more degrees. I have removed thermostat and hardwired w and r wires with same condition. I have a non pleated filter at the furnace, and a 20x20x4 return filter on the first floor right above the furnace. Honeywell model FC40R1003. These have been replaced and the issue is no better. Two ceiling return grills on the second floor also. I have removed the honeywell return filter and this has still happened.

I have cleaned the flame sensor, no help. There is no rollout of flames out of the heat ex-changer that I can see. The flames stay inside the exchanger when they first light and also when the blower motor turns on. This is a 9o or higher efficiency furnace so I have intake and outtake pipes 2" ID with three 45's and 4' going up to ceiling and 10' across to outside.

Looking for solutions and things to check. Thanks

  • Do you lose power to the thermostat when the 4 blink error happens? That'll tell us if the flame rollout/high limit switches are actually opening, or if this is the control board's fault... Jan 2, 2020 at 1:33

2 Answers 2


Thanks guys, I had a pro check it out. The furnace is working fine. He found too much air restriction on the return side, and corrected it. There was too much of a temperature difference between the cold return and the hot side. The heat exchanger was fine.

  • "and corrected it" - by doing what? Removing pleated filers and putting crappy ones in?
    – Mazura
    Jan 9, 2020 at 2:54

Great question and thank you for asking, but you clearly don't know much more about furnaces than I do and you really MUST call-in a Technician.

Air-flow, filters nor flame dectection have anything to do with your problem and you could be risking the entire furnace with continued use. This/these sensors are vitally important and may simply need replacement or they may actually be detecting a serious problem.

Sure, it could be that your burners need to be cleaned, that exhaust draft has reduced or that your gas pressure is set too high. But please, spend 1 or 2 hundred dollars to have someone knowledgeable check the situation with proper gauges and meters to address anything that's actually wrong.

I'm sorry to give such a vague answer, but once manufacturers began relying upon circuit boards to run the show everything changed. In most cases it turns out, ironically, to be replacement of the circuit board that's actually the only problem.

It's a backwards world, but a cheap visit by a technician is a whole lot better than thousands for a whole new furnace.

  • lggy, I totally agree, and have a tech coming tomorrow to look at the situation. Three phaseEel, I am not sure if I lose power to the thermostat. I will check that. The thermostat is a lux ts9000s with batteries. no com wire is hooked to it, but the thermostat wire does have an extra wire run that is not hooked up to anything. (could be used for com if I got a thermostat requiring that). I will do my best to update when the real problem is found.
    – Matt
    Jan 2, 2020 at 5:06
  • Actually low air flow can cause a limit issue dirty filters and vane switches do cause this issue, but I agree with a pro checking it out.
    – Ed Beal
    Jan 2, 2020 at 15:18

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