Secondary furnace is driving me nuts.

The unit is an ICP LP Furnace Model #t9mpd050f12a3.
I noticed about a month ago that when the unit was on, the blue flame would pulse every second from normal to somewhat weaker flame but not close to going out, just slightly weaker. I didn't think too much about it but the furnace has since stopped working. It's displaying 6 led pulses.

I have another unit that is not having any issues and I've been able to pull parts to test with and so far have tested:

Gas valve (got a new one)
Pressure switch - swapped with the one working from the other furnace with no change. Tested the original with a multimeter and it's working properly.
Igniter and flame sensor - swapped with other unit with no change in result
Removed filter
Cleaned small wasp nest from air intake line

Runs through the standard startup process but it seems like the gas valve never opens. Fortunately, this is the 2nd floor unit and heat rises from the first floor but on the really cold nights, it's needed to keep the air temp consisent (and if we want to close the doors to the bedrooms)

Not sure what else to test other than a new circuit board.

Any thoughts or suggestions?

thanks,
Jim

 

If the igniter is heating....... the pressure switch is ok. You'll need to check the voltage to the gas valve. If the voltage is missing when the valve should be opened..... that would indicate a control board issue.


T9MPD manual (pdf)

 

Quick update. I decided to go ahead and replace the circuit board. It was a relatively low expense and the last major component that could have malfunctioned. In any case, the new board is in and the issue still remains. Checking for blockage in the gas line and the nozzles next. Any thoughts?
ReplyForward

 

All lines are free of obstruction. Also checked the flame rollout switches and both tested good. At a loss. May have to bring in a furnace guy.

 

The furnace follows a logical startup. The last step before the burner lights is the igniter heats up.
If the igniter IS heating up..... all safeties in the furnace are ok. They must all be ok or the igniter will not heat up.

After the igniter is heating for 30 seconds or so..... the gas valve receives 24vAC to open. The flame needs to light in 7 seconds. If there is no flame.... the 24vAC is removed and the igniter turns off. System will retry.

You need to check and see if the valve is getting that 7 seconds of 24vAC.
If it does...... you have a gas supply problem...... since you've changed the valve.

 

Issue resolved - all components were functioning as they should. Turns out the gas line to the igniter was clogged and that stopped the ignition cycle. Thanks for the replies!