On 11/11/2012 6:49 PM, Tony Hwang wrote:

Hi,
Doee the furnace have a flame sensor? Then that is prime suspect. Clean
it and try again. Furnace usually goes into lock up mode when
it fails to stay running after ignition(safety issue). It usually stay
locked up an hour or two. If you want to reset it turn the power on/off.
No blinking LED indicator to spell out the trouble code?

As I wrote above,

The flashing trouble code LED just indicates "fail to ignite," and
indicates that the trouble could be the gas valve, the gas pressure from
the supply, or the flame sensor.

A bad flame sensor would not stop the burners from igniting, it would
just shut them down when the controller looked at the flame sensor and
didn't see an indication that the flame was present (I've had that
happen on a water heater).

As I understand the operation, the controller checks the flame sensor
maybe 5-10 seconds _after_ the gas valve opens and the burners light (to
give some time for the flame to heat up the sensor). But in this case
the gas valve doesn't open to start the flame. So yes, it's almost
certainly the flame sensor that's the reason the system shuts down, but
the reason there's no flame is because the gas valve isn't opening. I do
see the controller trying to open the gas valve (24V on the gas valve
terminals) but there is no gas.

Things I tried:

Ignite the flame manually in case the glow plug ignitor wasn't hot
enough. No effect because I don't think the gas is on.

Checked that the gas valve is getting 24V shortly after the glow plug
ignitor is ready. The valve is getting 24V. The main solenoid coil is
energized (can feel the magnetic force when it gets 24V). I can't feel
anything on the secondary solenoid.

Checked that none of the flame-out sensors or pressure switch sensor was
tripped (but this is obviously the case since the controller would not
even try to energize the gas valve if any sensor was tripped).

I'm pretty sure that it's a flaky gas valve. One HVAC tech wrote: "Make
sure you have a 24 volt signal to the gas valve terminals. If you do and
it doesn't light, the gas valve is bad." This is exactly what is happening.