My Payne gas furnace (PG8UAA 42091) automatically shuts down after it lights and burns for two minutes. It then gives a 33 fault code. I've done some research and some testing and have narrowed the problem down to the limit switch, but I don't think the switch is bad. Here's the sequence:
- Furnace gets call for heat.
- The induction motor begins running.
- The blower motor also begins running, but at low rpm.
- The ignitor glows.
- The gas kicks on and is lit by the ignitor.
- The furnace operates for two minutes or so before it shuts down.
- The gas is shut off and the induction motor stops running while the blower motor continues to run at low rpms.
I've checked all the common reasons for this sequence (dirty filter, blocked vents, bad limit switch, etc.), none of which apply. I've tested the limit switch. It has 27 volts at either of the two connectors when the second test lead is grounded to the furnace and 0 volts across the two connectors. Until the furnace shuts down, and then there are 27 volts across the two limit switch connectors. Which I interpret to mean the switch is working as intended.
I have seen no discussion of this anywhere, but I think my problem is related to the blower motor. I don't think the blower motor is running at a high enough rpm to supply enough cold air to the plenum. As a result, the heat in the plenum builds until it triggers the limit switch and shuts the system down. What I don't understand and, therefore, don't know how to fix, is why the blower motor runs, but doesn't kick up to a higher rpm when the gas ignites.
In the past, when the furnace was working properly, you could hear the blower kick up to a much higher and more forceful rpm to distribute the heat around the house. Can you tell me why my blower runs at lower rpm (smoothly and quietly), but does not kick up to the higher rpm when it's supposed to? What should I look for? What do I need to test?