Model PBNU-LD2DN100D, 86k BTU output, external air intake, with White Rogers ignition control model 5OE47.

Have had problems in the past with partly blocked combustion air intakes or exhaust outlets, with
Blocked pressure sensor tubes at inducer fan, with slow inducer fan, with water drain blocked,
With bad igniter, bad limit switch, and bad gas safety valve.

Have corrected all (new safety valve, new limit switch, new inducer fan, new control board, new igniter within last couple years, all seem to be working, also new blower motor) and checked all intake & exhaust & drains & sensor tubes which are not blocked at this time. Have 28v coming off last pressure sensor switch (3 wired in series) & going through fusible link & through limit switch to control board. Igniter glows, runs for the stated 45 sec's for igniter warmup, then safety valve clicks but apparently no gas being released so shuts down after the prescribed 4 second time limit with no ignition happening.

When power applied to system, control board LED gives off 1long flash - does this signal a problem or is
It just advising that it is getting power and functioning? No codes listed on wiring schematic on door.

I have 28v going to the gas safety valve from the control (black wire) but the other, yellow, wire between
The control & the valve never shows any voltage, even during the 4 second attempt at ignition.

Should this wire ever show voltage or is it just a "neutral"? Can't tell by the wiring schematic.

The schematic says 24v for all low voltage circuits but I show 28v on my digital multi meter.
Problem?

Being I have all the req'd voltages going to the control from all pressure & other switches, &
Voltage going to the safety valve after the igniter warmup period, is maybe the safety valve bad
Again? It has been several years since I replaced that. I have a used spare I will try next.

This seems to happen every winter early in the heating season. This year I've checked everything
And can't find a problem other than gas not being released by the safety valve. Tried tapping on it
During the 4 second time it is supposed to release gas to the burner, no effect.

Appreciate any help!

Model PBNU-LD2DN100D, 86k BTU output, external air intake, with White Rogers ignition control model 5OE47.

Have had problems in the past with partly blocked combustion air intakes or exhaust outlets, with
Blocked pressure sensor tubes at inducer fan, with slow inducer fan, with water drain blocked,
With bad igniter, bad limit switch, and bad gas safety valve.

Have corrected all (new safety valve, new limit switch, new inducer fan, new control board, new igniter within last couple years, all seem to be working, also new blower motor) and checked all intake & exhaust & drains & sensor tubes which are not blocked at this time. Have 28v coming off last pressure sensor switch (3 wired in series) & going through fusible link & through limit switch to control board. Igniter glows, runs for the stated 45 sec's for igniter warmup, then safety valve clicks but apparently no gas being released so shuts down after the prescribed 4 second time limit with no ignition happening.

When power applied to system, control board LED gives off 1long flash - does this signal a problem or is
It just advising that it is getting power and functioning? No codes listed on wiring schematic on door.

I have 28v going to the gas safety valve from the control (black wire) but the other, yellow, wire between
The control & the valve never shows any voltage, even during the 4 second attempt at ignition.

Should this wire ever show voltage or is it just a "neutral"? Can't tell by the wiring schematic.

The schematic says 24v for all low voltage circuits but I show 28v on my digital multi meter.
Problem?

Being I have all the req'd voltages going to the control from all pressure & other switches, &
Voltage going to the safety valve after the igniter warmup period, is maybe the safety valve bad
Again? It has been several years since I replaced that. I have a used spare I will try next.

This seems to happen every winter early in the heating season. This year I've checked everything
And can't find a problem other than gas not being released by the safety valve. Tried tapping on it
During the 4 second time it is supposed to release gas to the burner, no effect.

Appreciate any help!

Gas valve should give an audible "click" when it receives power. If not, measure with one meter lead on each of the two gas valve terminals. Should show 22-28 volts AC during 4 second "trial for ignition". If voltage is there but valve don't click or open, the valve is probably bad. You can also check the resistance of the valve coil. With furnace power turned off, remove one wire from the valve, put your meter on "ohms", and connect one meter lead to each of the two terminals on the valve. I'm not sure of the reading for your specific valve, but probably more than 10 ohms, but less than 200 ohms should be ballpark. A reading of a fraction of an ohm or no reading (same as with meter leads not connected) would indicate a bad valve.

Also don't overlook the obvious, no gas at the valve for one reason or another. Gas turned off at a external valve, or on the safety valve itself!

I believe that ignition module gives one long flash at power-up, saying "I'm OK", and the only codes it has is a steady light for module failure, and a flashing light for ignition lockout, which occurs after 3 failed tries for ignition in a row. Code should be listed on the module itself.

Thanks for that info on the safety valve, will save for potential future problems. Never knew how to test them before.

Did find the problem. I work away from home for extended periods and when something goes wrong I try to talk my
Wife through things over the phone, sometimes successfully sometimes not. When not, only recourse is a repair tech.
The brand dealer left town some time ago so the local repair outfit isn't as familiar with this brand.

One time 2 years ago we had to have the service tech out and he did get it going, but I discovered a couple days ago
That what he did was causing our problems now.

Our safety valve was clicking at the right time but I didn't know for sure just what that signified. I had my wife turn on
The furnace while I checked outside for airflow in & out. This was after I had the burner airbox cover off and removed a
Wasp's nest that had been built in the air intake pipe & got sucked into the airbox inlet and was blocking airflow somewhat.
When I saw that I figured that would be it as we'd had similar problems before. But this time it didn't do it.

I'm hard of hearing so got down close to the inlet and then the exhaust while the furnace was trying to start (inducer fan was
Running) and I did notice a mild gas smell out of the exhaust, so knew it was getting some gas while it was trying to light.
While trying to figure out the wiring schematic and what did what, I pulled the airbox cover off again to see what the fusible
Link looked like that was in the airbox and was wired in series with the pressure switches. That's when I noticed something.

The air intakes for the burners have an airflow adjustment, and the one nearest the igniter was choked down maybe 5% to 10%
Compared to the others. Apparently the service tech had done that on his last visit a couple years ago and it got it running
At the time. I readjusted that one to match the others and it's been working fine ever since. Never fails to fire right up.

Not your typical troubleshooting situation.