Hi, new here... but will definitely be bookmarking this for future use...

I just got done with school in HAC&R but I am just now beginning my job hunting in my field and I really haven't had a chance to apply the knowledge I've learned. I work on a farm and am basically the property caretaker. As luck may have it, the other day I noticed our furnace in one of our main barns wasn't running. The thermostat was set for 55 and the temperature was about 45 and the thermostat was calling for heat. You could hear the flue fan running on full blast, but the furnace wouldn't fire.

I looked at the LED codes... code 23.

Pressure switch not opening. I fiddled around with it for a while checking various voltages and suddenly it started working. Cool! I fixed it!. or not.

A little while later I got code 13.

Flame rollout sensor or limit switch open.

I traced the schematic back... both the limit switch and the flame rollout switch were both closed according to my multimeter. Just in case, I pushed the manual reset switch on the flame rollout sensor, but it didn't change anything.

It ran with it like that but shut off on high limits after running for about 5 minutes or so, so I left it for the night... I didn't want to risk the pipes freezing in the barn since we had a cold front moving in that night. The next day when I got there, the barn was once again too cold and when I looked at the furnace, it was once again throwing Code 23 for the pressure switch.

I decided to start trying my hand at things so I started trying all the troubleshooting possibilities.

1.) Plugged tubing from the switches... tubing was fine from what I could tell.
2.) Bad switch - definitely possible
3.) Clogged air filter... we don't have an air filter... well not a fancy one. Just a cheap "pre-filter" style of filter... I cleaned it up just in case.
4.) One of the flex ducts had been folded over (this furnace is in a small attic and somebody must have set something on it) so I straightened it back out to make sure that duct had good air flow again.

I also read up on how the switches work... when the furnace kicks on, the flue fan starts up, then the low heat pressure switch closes, then after 15 seconds, the burners light, blower kicks on, etc...

When all is properly wired up, the furthest it gets is the flue fan kicks on and the low heat pressure switch never closes (which is why code 23 is odd... because it says it's not opening? It's open from the get-go like it's supposed to be).

I decided to go a little further though... and I set the thermostat for a call to heat, followed by making a mad dash into the attic (sucks doing this with one person... lol), and then I jumped the switch wires together on the low heat pressure switch. When I do that, the furnace starts normally. I can't recall if it shut off on high limit with only that switch jumpered... I'm thinking it may have. I can check this tomorrow to be sure. However, if I jumper both the low heat pressure switch AND the high heat pressure switch together (each switch jumpered... not both switches jumpered all together), the furnace runs absolutely fine... no codes whatsoever even after running for nearly an hour straight.

I'm thinking it's just a bad low heat pressure switch or is it possibly both.. How can I check to make sure... or what else may it be? Also, what's a good online site to order the parts from? Any advice would be greatly appreciated... I graduated with an A+ overall, but we didn't really get too deep into the all the switches/sensors in the furnaces so this is a little new to me. Most stuff I can figure out, but this ones got me stumped... mainly due to throwing multiple codes which may or may not be related.

13 is a lockout. It's going to happen.

Aren't intermittant's fun?

Loose connections are a possibility. So is corrosion.

This is where you have to learn something. By knowing what the circuit looks like around the switch or a contact, you can use voltage across the contact to determine whether a switch is open or closed.

Pressure switch means look for flue obstructions, even dead critters in the pipe or too much condensate in the trap. Check there.

Thanks, will check those out tomorrow... I'll be compiling a list if multiple people start tossing out suggestions. I was kind of wondering if it could be due to a flue obstruction or something along those lines... I figured out how the switches work and noticed that when the flue fan kicks on, it's not pulling the switch in very far (not enough to close it)... but if I pull the tube off and blow air lightly into it, I can watch it move and hear it click closed. Definitely could be a possibility.

Defective low-heat pressure switches are fairly common on the 352. The original switch design is a metal canister with an external microswitch. Oxidation of the switch contacts is the enemy. Try this: nudge the wires on the lps while the unit is firing. Main burner drop-out indicates a bad switch. Try to get the plastic version with the switches sealed inside the canisters. Congrats on your trade school achievement, and good luck going forward in this field.