The furnace in my vacant rental property does not light. The blower runs but the igniter never glows and the gas valve does not open. The LED blinks once which means the burner limit circuit is open.

How do I replace the burner limit circuit?

Inside the furnace cover I found the electronic gas valve, the main controller, igniter, burner, combustion air blower motor, pneumatic blower switch, etc, but could not find a "burner limit switch". What am I looking for exactly?

I can't afford an expensive visit from a repair guy as I am doing this on a shoestring budget.

The furnace is in very good shape (6 years old) and clean, sounds like a single faulty switch. Thanks!

What is the brand of the furnace?? That would help to know. Most limits are up on the firewall. It should have two wires going to it, some have four. It can be almost flat or it can be a box that has a slip off cover. I am going to tell you about the flat one that is on a Carrier furnace, one of the two wires will have a small inline fuse (it looks like a tiny firecracker.) This is for burner rollout protection.
Two ways to check this out first use a length of wire and short across the little fuse, short from one side to the other. If that is the problem the furnace should start, that is if you have the heat turned up and the bottom door is on depressing the door switch. If it does not start then short across the two poles of the limit itself, then same thing, it should start if that is the problem. DO NOT USE THE FURNACE WITHOUT THE FUSE LINK INSTALLED.

Actually I just figured it out. I went through the wiring diagram in the furnace manual and chased the wires to both the primary and secondary limit switches. The primary switch is a thin, flat, brown, 1.5" square of material that is mounted flush against the main furnace housing.

The secondary is small stainless disk that looks somewhat like a watch battery. It had a tiny reset pin on the outside of it and as soon as I pushed the reset button, the furnace fired up!

I did notice some coking on the 1st burner along with an orange flame being emitted. The other 3 burners were perfect blue. I will take it apart tomorrow and clean the black soot off the burner to see if I can get the blue flame pattern back on #1, else probably a new manifold.

Thanks!