Furnace Time, Again

Yes, it's that time again. My furnace is acting up, showing signs that there's something amiss in the high limit circuit.

My suspicion now has been narrowed down to the main 12-pin harness connector. Prodding at it seems to change the furnace's behaviour when it's in its 4-flash error mode, so my thought is that perhaps the connector pins aren't making good contact?

This is a molex connector, where this circular female pins fit over top of a slightly smaller round male pins.

The pins are held into the connector body via a set of spring tabs around the end, and luckily there's a tool designed specifically for removing them.

The key to using this tool (pun probably intended) is to push on the wire from the other side of the connector before sliding the tool all the way over the pin. The tabs tend to dig into the plastic of the connector, and so pushing it forwards helps keep the tabs from jamming up and bending the tip of the tool, which is made out of a pretty thin steel tube.

Anyway, with the two pins for the high limit circuit removed from the connector, we can see that...

They kind of look fine? I guess? I sort of expected to see a bit more wear and tear here, maybe the seam split wide open or something, but no. It just looks like a regular molex pin.

I did go ahead and flex the male pins on the controller board to make sure that they haven't gotten crushed. They're a split pin design that's intended to spring out against the inside of the female pin so as to make reliable contact, so maybe that'll help.

If these connections are still misbehaving, I did buy a set of replacement pins (in fact, a whole replacement connector kit) so that'll be the next step if the furnace continues to misbehave.

One other thing I wanted to address while I was poking around in here is this peculiar wire nut in the high limit circuit.

It's always seemed a little suspect to me, despite the fact that it's on there pretty good. One side of it connects to the high temp switch on the fan, and the other end goes down to the thermal fuses next to the burners, if I recall correctly. Anyway, it was easy enough to replace with an insulated spade connector pair.

If nothing else, it looks much cleaner, I suppose.

Anyway, the furnace still works, though only time will tell if it'll continue to misbehave or if it'll finally calm itself down and just produce heat properly.