On 11/12/2012 7:57 PM, SMS wrote:
On 11/12/2012 10:45 AM, wrote:

Alternatively, if you suspect the gas valve is bad and
the coil goes open after it gets warmed up after a bit,
here is what I'd do. Close the gas shut-off valve to the
furnace so no gas can flow. Then jumper 24V direct to
the gas valve for 15 mins. Then remove the jumper, turn
the gas back on and try to start the furnace.

I did this (had to run to the surplus store for a transformer). They
only had a 42V CT, so I got 21V or so, but that was enough for the 24V
solenoid, and if it's a heat problem in the coil then the lower voltage
means higher current so it should heat up even more. The valve opens and
closes just fine. Even after 1/2 hour if I turn on the gas it flows
through the valve. I connected the valve back to the control board and
started the furnace and it started. It ran for about the same amount of
time before the valve closed. Then the same symptom, the 24V goes onto
the terminals of the valve but the it doesn't open.

Perhaps it's a heat related problem.

Oh, and even though maybe I shouldn't have done this, after the control
board would no longer open the gas valve I turned on the furnace with
the gas control valve connected to the external transformer, and when
the ignitor came on I plugged it in to open the valve and ignite the
burners. It didn't open and the burners didn't ignite.