How to Diagnostic Codes

Picture1_NEWCommon furnace problems happen. Sometimes it takes an expert to diagnose the problem, and sometimes it is as simple as seeing what the computer is telling us. Most major brands these days include a diagnostic feature on their control board which makes it super easy to pinpoint where the problem is. Usually problems arise for a reason and is not typically due to a simple component going bad, but rather a whole range of problems. Look into the sightglass of your heater to begin; you’re looking for a flashing LED. Somewhere on the door or on the unit is a sticker which tells diagnostic codes to reference. They are as follows:


Description: Control has 24 VAC Power

This is your control voltage and is normal operation. If you have a heartbeat, you have a happy system.

2 Flashes

Description: Pressure switch did not open, check for obstructed pressure tubing.

Could be a bad pressure switch or debris is in the vacuum line. Take it off and make sure it is clean before you test the pressure switch.

3 Flashes

Description: Pressure switch did not close, or re-opened- Check for: excessive wind, proper vent size, defective inducer motor, low inducer voltage, defective pressure switch, inadequate combustion air supply, disconnected or obstructive pressure tubing, low inlet gas pressure, restricted vent, condensate clogged.

This diagnostic is typically pretty simple. Check to see that the inducer is running, sounds clean. Inspect the vacuum line for any cracks or breaks. Rarely, does a pressure switch go bad. Sometimes all you have to do is check the connections on the pressure switch and adjust its angle. Also make sure there isn’t a blockage in the venting. Sometimes venting can condensate and the angle of the pipe affects proper condensation drainage (high-efficiency systems).

4 Flashes

Description: Limit circuit fault-a limit or flame rollout is open. Check for: restricted vent, proper vent sizing, loose blower wheel, excessive wind, dirty filter, defective blower.

The limit switch chain is all in place to prevent excessive temperatures on the unit. What most likely happened is you were goofing around with vacuuming or wiping the unit and you depressed the underside of a limit switch. They usually have a small button on top that you press to reset them. Just watch it run and verify proper operation. See Furnace Problems – Diagnosing a Furnace.

5 Flashes

Description: Abnormal flame proving signal-Flame is provided while gas valve is de-energized. Check for: leaky gas valve, stuck open gas valve

This one is screwey. Look for leakage on the gas valve or look for abnormal operation. This is usually one that is easily fixed by cleaning the flame sensing rod.

6 Flashes

Description: Failure to ignite or flame sense lost while running. Check for: Flame sensor must not be grounded, low inlet gas pressure, oxide buildup on flame sensor, Manual valve shut off, proper flame sense microamps, gas valve defective or gas valve turned off, defective hot surface igniter, control ground continuity, inadequate flame carryover or rough ignition, green/yellow wire MUST be connected to furnace sheet metal.

Clean the flame sensing rod, make sure that the flame sensing rod is connected to the board. Ensure that the green/yellow wire is attached to the cabinet. If this still continues, replace the flame sensing rod.

7 Flashes

Description: Limit circuit lockout – Lockout occurs if the limit or flame rollout is open longer than 3 minutes. Control will auto reset after 3 hours. Refer to #4

Now you’re in trouble you made it mad. The blower is probably not blowing. Make sure the blower is blowing, make sure the filter isn’t a shag carpet made out of pet fur, and make sure there isn’t a damper in place. Also, make sure there is nothing in the vent flue. I had this problem once where the customer wired a thermostat wrong which failed to energize the blower relay. Have fun waiting 3 hours.

8 Flashes

Description: Gas heating lockout-control will NOT auto reset. Check for mis-wired gas valve, defective control valve.

Did you try doing a homebrew repair? Unplug the unit and call a technician now before you break it more.

10 Flashes

Description: Polarity-line voltage, polarity reversed. If twinned, refer to twinning kit instructions.

  • Billy bob install. Take your ohm meter ( and test out the continuity on your pigtail. Make sure hot goes to black. If you don’t know what I’m talking about, call a pro.


Description: Blower on after power up- Blower runs for 90 seconds, if unit is powered up during a call for heat (R-W closed) or (R-W) opens during blower on delay.

Simple. The blower is on when you energized the equipment immediately. You need to turn the thermostat to the “off” position, then plug it in. This problem should stop.


Description: Ignition lockout – Control will auto-reset after 3 hours. Refer to 6.

Stop messing with it and call a professional. Or clean your flame sensing rod. Choice is yours.


Description: Secondary voltage fuse is open – Check for door switch not closed, 115VAC at L1 and neutral, 24VAC RED and COM BLUE, 24V Fuse 3A, short circuit.

You most likely need to tape off the door switch. If the door switch is not visible to you, think what you did to cause this. Did you recently do some home renovations? Were you wondering where “that wire” goes? You probably caused a short circuit. Even a small nick in the wire can cause the fuse to blow. Refer to Furnace Problems – Diagnosing a Furnace.


Description: Control circuitry lockout – Auto reset after one hour.


Description: Gas valve relay stuck open-Flame sense circuit failure, software check error, reset power to clear lockout, replace control if status code repeats.

You might be looking at a new board buddy. Call a pro or order your specific board.

That’s it. This should fill you in on what each code means. If you need help, feel free to email me, other wise happy guessing!

Also, if you need replacement parts, you can message me on the website messaging board with your model number and serial number. I can ship parts anywhere in the country. I do mark up my parts, but you get a whole better deal with me than with your local HVAC contractor. I also do facetime to help you install the component and return the unit back to working order for a small charge. Message me if you need help, that’s what I’m here for.