Ignitor not working - Control board? Lennox G61mpv

[Bigrockk]

My Lennox G61MPV-36B-070-12 furnace died on me this morning. It was flashing the 4+7 code (Ignitor circuit fault - failed ignitor or triggering circuitry)

Ignitor is 34 ohms. I'm getting 0 volts to the ignitor during start up. On the initial call for heat the inducer runs for about 20-30 seconds (no glow from ignitor) then shuts down and flashes the 4+7 code.

Just to be sure I disconnected the ignitor and powered it up for a couple of seconds, it immediately started to glow.

Tested for voltage again during startup right where the ignitor wires connect to the control board, 0 volts.

Checked the safeties that I could see, low flame pressure switches changed state, and checked three temp switches (one high limit, two flame rollout switches I believe) all good.

So is there anything I'm missing here? I'm thinking control board is bad but she's a spendy one so just want to be as certain as possible before I replace it.

Thanks in advance  
 

[Admin]

That WatchGuard error code usually follows another code after several attempts.  Have you verified the pressure switch closes?

[Bigrockk]

Hey thank you for the reply, I just went and double checked and yes the two "low fire" pressure switches closed after the inducer started up.

I did not check the two high fire pressure switches as the furnace is set to fire on low flame only (2nd stage is disabled at the thermostat) I will check them and see what they do.

I'm not an HVAC person but a retired electrician, I have just been following what I can figure out from the sequence of operation and the wiring diagram from furnace manual.

Update: Just checked the two "high fire" pressure switches and they both close as well after the inducer starts up.
Also after power cycling, the furnace only makes one attempt to start and then locks out with the 4+7 code.

[Admin]

That model can have 2 or 4 pressure switches.

If the low pressure switch closes, and the inducer starts running it's 15 second pre purge, it's possible you do have a faulty control board that does not power the ignitor.

Have you made sure the drain line is clear?  It's usually easy to remove the burner box cover and disconnect the exhaust vent temporarily to see if it starts.
If it does that would indicate a blockage in either the intake or exhaust vent.  Obviously it's not safe to run the furnace without the exhaust vent connected though.

See page 56, Heating Sequence of Operation, in the manual,

https://hvactechgroup.com/files/G61MPV.pdf

[Bigrockk]

To answer your questions, it does have four pressure switches, two low fire, normally open, wired in series and two high fire, normally open and wired in series. When the inducer runs they all change state to "closed" and provide two control voltage feeds to the board. So that all seems normal.

Drain is clear, pulled plugs, drained and re-applied plugs

My intake air comes from inside the house and the exhaust vents outside, so if one pipe was blocked it would most likely be the exhaust.

The burner box cover is easy to remove so I'll give that a go, but if it was blocked wouldn't that cause one of the pressure switches to not change state from open to closed when the inducer is running? All four on my furnace do change from open to closed with the inducer running?

[Admin]

I think your diagnosis is good and I would order control board part number 63W27.

Removing the burner box cover won't matter since you have a nondirect vent installation.

Are you hearing a click or the gas valve opening after the inducer motor pre-purge? It should happen about 20 seconds after the igniter would normally energize.  If you don't hear a click that could also indicate an issue with the control board.

I like to jumper the pressure switch wires after the inducer starts and then keep my audible ohm tester on the pressure switch itself to make sure it's remaining closed and not opening and closing quickly.

[Bigrockk]

No can't hear a "click" from the gas valve, I timed the inducer, it runs for about 32 seconds after the call for heat, no glow from the ignitor and no click from the gas valve. It just locks out on the 4+7 code after the one attempt.
I did bypass the pressure switches right after the inducer started and still got the same result. I'll replace the board and post back the results, thank you again!
 

[Bigrockk]

Replacement board showed up today. Installed and the furnace is working great and the house is warm.
Thank you much for the input!