I have a Bryant 373LAV/376CAV (not sure exactly which one) mounted horizontally in the attic that is <5 years old and has been functioning perfectly since installed. However, because Houston doesn't get very cold winters and this furnace serves the second floor, which we rarely use, I am not sure how long the heater has not worked.

This evening when we had guests over I noticed that the upstairs was cold and heater was not coming on. Initially, there was a 31 error code flashing and when I power off the unit and power up, I get the normal 90 second main blower operation (and the '12' error code indicating 90 second power-on blower), followed immediately by a shutdown and nothing else. I tried to eliminate the thermostat from being the problem by removing the red and white wires from the furnace, to no avail.

Now, the diagnostic light is completely off and nothing comes on when the unit is powered off/on. Any thoughts on what I can try next?

Thanks...

Sounds like you blew the low voltage fuse working on it. See https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html

Once you fix the fuse, check the inducer and its pressure switch. If the pressure switch doesn't signal the inducer blower is going, it shuts down.

Check the door safety switch. The access panel may be out of position not allowing the system to operate. Follow labman's recommendations.

Thank you for the prompt responses! I checked the door safety switch and it looks to be working properly. I replaced the fuse and diagnostic light is back on. 31 is still flashing. Checked pressure switch but didn't see anything out of the ordinary. Will replace pressure switch and let you know outcome. Please advise if I should try something else in the menatime. Thanks again!

I've replaced the pressure switch. Fan comes on for 90 seconds then it sounds like the burners come on for about 10-15 seconds then furnace shuts down. Diagnostic code is now 21. Did I mis-wire the switch?

Thanks...

Thank you for the prompt responses! I checked the door safety switch and it looks to be working properly. I replaced the fuse and diagnostic light is back on. 31 is still flashing. Checked pressure switch but didn't see anything out of the ordinary. Will replace pressure switch and let you know outcome. Please advise if I should try something else in the menatime. Thanks again!

I have the same unit... with perhaps the same problem. Where is the fuse in this unit? Is it the "fusable link" picture in the referenced link? Thanks.

On my furnace, 3 Amp fuse is on the mother board, to the left of the diagnostic light. Good luck...

I was able to clear error code 21 by turning the gas valve off for 5 minutes and turning it back on. Error code 31 is now on display again. Pressure switch has been replaced. Any thoughts on what I should do next? Thanks...

If the burners come on and go off in a few seconds, usually it is a dirty flame sensor. Pictures of them in the sticky. A few minutes with fine sand paper and the furnace should be running. Note, there should be a decal giving the codes.

The inducer motor has to be running to close the pressure switch. Not sure about code 32 but code 23 is the pressure switch did not open on shut down. Blow and suck lightly on the tubes going to the pressure switch, you should hear it click. Make sure the collector box is not obstructed with rust at opposite end of tube. TURN POWER OFF FIRST. Code 12 is the thermostat was calling for heat when you turn power back on to the system the blower runs for 90 sec.

OK. Here's the latest on my saga... Initial problem was error code 31 on my Briant 373LA horizontal furnace. After replacing pressure switch, burners fan would run for ~90 sec. the inducer, or what I think is the inducer, would start spinning, the burners would come on and then shutdown after 5 seconds or so. Diagnostic code now reads 21. Sanded the flame sensor, to no avail; same results as presviously.
The definition I found for error 21 is: "Gas Heating Lockout - Control will NOT auto reset. Check for stuck gas valve relay on control or miswire to valve circuit." The only rewiring I've done was disconnecting and reconnecting to the pressure switch.
What do you suggest I try next? How do I check for stuck gas valve relay (if that's really the problem)? Could I have the pressure switch wires reversed? Don't think it'd be working this far it that were the problem. Thanks for your patience with my problem.

Make sure gas valve is in the on position and that the gas is on going to the furnace. Make sure you did'nt pull a wire loose when messing with the pressure switch. The gas valve has a ground wire on the common side of the coil it has to be grounded to the equipment to work(green wire with yellow tracer).

Thanks... Made sure gas valve is in the ON position on gas line and furnace; all wires, including ground, are connected properly. Same symptoms; solid diagnostic light initially, fan comes on for 90 secs or so, inducer starts spinning, orange flame comes on behind burners, leading me to believe burners light up, for 5 seconds then furnace shuts down and error code 21, listed above, starts flashing. Please advise...

Make sure the flame sensor is not touching any metal(if ceramic around flame sensor is cracked it can go to ground and not work), check to see if flame sensor wire is not grounded,damaged or loose. If the burners light up its not a bad gas valve. Are you using L.P gas or natural gas?

Thanks T-Top. Will check sensor. Am using natural gas. This is one of two furnaces in the house and the other is working fine...

Well, I checked sensor and it's free from any contact; however, I did find that the wire connected to it was loose, so I tightened it (must have come loose when I removed sensor to sand it down). Anyway, the error code now displayed is 31 again, and the burners will not come on and the inducer motor will not spin. Before, when error 21 was being displayed, the burners would come on for a few seconds. I have to believe that the inducer motor works fine because it was spinning when the burners would light up. I'm at a loss! I've been asked to check the inducer before to try to correct error 31; what specifically am I looking for? Do I need to test the capacitor? If so, how? Thanks again.

If the inducer runs, the capacitor is fine. Check the hose and wires to the pressure switch.

I've checked the two wires on the pressure switch and connections look good. The hose seems to be OK as well. It's at eye level with pressure switch and there are no kinks in it. 31 still flashes. Anything else I should try? I'm no expert, as evidenced by this log of entries, but it's surprising that any one part is deffective when they randomly work, switching from error 21 to 31. Anyway, please let me know if there's any part you think needs replacement.

The pressure switch diagnostic simply tells you the pressure switch is not opening/closing properly. Most often, that means something is *preventing* it's operation-not a bad switch!

Check the intake to the burner compartment for leaves-most common problem. Disconnect the PVC pipe going to the burner compartment, move it aside, and check inside with a flashlight. Remove any leaves, debris from intake (clean it up GOOD) and reinstall. If you found a bunch of leaves-this was undoubtedly the problem.

If you've cleaned the intake and still have the error, then start looking at the condensate drains. A clogged condensate discharge trap (small white box with rubber hoses connected to it-it's in the blower compartment of most) will also prevent the pressure switch from opening sometimes. Blow through ALL of the ports on the trap to clear it, put it back together. You should be in operation by this time.

Hope this helps.

It only takes a very small restriction in the vent pipe to keep the pressure switch from closing.

Thank you. I think there might have been some issues there because when I disconnected the vent from roof connection and blew on it using a blower, nothing happened at first, but later the motor started spinning. Error 31 cleared. However, error 21 came up. I'm convinced I've erroneously described the situation before when trying to explain what I was seeing. Let me tell you what's happening now: Blower comes on, inducer motor runs, and then surface ignitor lights for 3 seconds or so. Burners never come on. Could it be that the gas valve relay is bad or something? What can I check for? The gas line to the furnace is open and the relay knob is set to on. Thanks...

Thank you. I think there might have been some issues there because when I disconnected the vent from roof connection and blew on it using a blower, nothing happened at first, but later the motor started spinning. Error 31 cleared. However, error 21 came up. I'm convinced i've erroneously described the situation before when trying to explain what I was seeing. Let me tell you what's happening now: Blower comes on, inducer motor runs, and then surface ignitor lights for 3 seconds or so. Burners never come on. Could it be that the gas valve relay is bad or something? What can i check for? The gas line to the furnace is open and the relay knob is set to on. Thanks...

Hmmm... limits are good, or no ignition would exist. Thermostat-ok-calling for heat as it shoud. Problem appears to be in igniton. Is gas turned on? At house? At appliance? If so, is there 24VAC across the MV and C terminals of the gas valve (or whatever the two pins are marked). If so, is there gas coming through? 24v and no gas tells you gas valve is shot. If gas is coming through,are the burners clear and clean? Is the flash trail across the burner assembly clear for proper ignition? Is the flame sputtering and dying? If so-clean the flame sensor-replace it if you can as it's probably only about 25USD for a new one or less.

That sounds like quite a bit of work (it's actually not), but that's the trail you'll need to follow to solve this.

Gas is turned on; shares same line as water heater and another furnace. Checked for 24v on the MV and C connections; nothing (checked it before, during, and after inducer motor was running). Checked for 24v on panel on circuit/mother board, where thermostat connects, and good there. Here's something else that's puzzling me; after a couple of attempts to get furnace going, error 31 came back on and inducer motor would not run, then after two or more times, motor would run and error 21 would come on. Anyway, your help is greatly appreciated!

Thank you all for contributing to the resolution of this problem and educating me along the way. The furnace is operational now. The final resolution to the problem was the replacement of the circuit board. I'm not sure how often these go bad, or what caused it, but that solved it. Again, thank you for your assistance...