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Unfortunately this is a fairly common fault.
The board is flashing ignition failure because it thinks it has sent power to the ignitor and then to the valve, but there is no flame.
It ia always one of 2 items....it is the board thinking it sent 24v to gas valve but actually did not...or the board did send 24v to valve but the valve is bad.
It is more commonly the valve on this unit but the 24v test tells for sure.
Valve gets 24v but doesnt open = valve
No 24v =bad board
do you have the TUD or TDD?
the model number...the XL is just a series...there is a model num ber on a tag inside the furnace
Thank you...then 2 of the 3 wires to valve are red and black?
Ok so your unit is older than 5yrs then?
Ok lets back track a little...do you have 2 pressure switches?
I would like to do a test if you game for a hands on test?
ok turn power off and take the wires off the pressure switch. Shoild be 2 browns on one side and a orange on the other..correct?
this will move quicker now that I know you have a 2 stage..
we want to take the wires off
And AFTER the exhaust fan starts connect the wires direct together...a short wire shaped in a U works well for sticking in the wire end connectors.
Then tell me code..
thank you...now put those wires back...and do the same thing to the other pressure switch
understood....at any time has the gas valve clicked?
Well we took the entire pressure circuit out of the picture...which puts us back to the valve issue.
You can unplug the 3 wire plug and put meter lead black to a ground and meter lead red to the black during ignition...you will only see voltage for 3 seconds...then test the gray and brown the same way...during the glow it will be energized for 3 seconds..
no 24 on any wires = board
24 on any wire = bad valve
we want ac volts~
thank you..you just confirmed bad gas valve.
I have wedged tooth picks...tapped with hammer...depends if the there is a connection bad, poor solenoid etc.