I apologize profusely. In my haste, I gave wrong info in the previous
post.

I just spent the last hour with the panel off the furnace, looking for
obvious problems (burnt wires etc) and re-checking what's happening.

The blower motor is NOT coming on. The vent (exhaust) motor IS coming
on.

When I dial up the thermostat to request heat, all that happens is that
the vent motor turns on. It stays that way for as long as I leave the
thermostat turned up. The igniter never glows, the gas does not turn
on (no gas smell), and there is no fire.

When I dial the thermostat back down, the vent motor shuts off after
about 30 seconds.

This unit has a White-Rodgers model 50A50-206 control with the
diagnostic LED. The LED does not indicate any problem (no flashes).

Just to confirm that the control was not dead, I removed one wire from
the vacuum sensor that monitors the vent motor's suction. Then I got 3
flashes on the control's diagnostic LED, indicating that the sensor was
stuck open. So the control was healthy enough to detect that.

What are the most likely culprits here? Does a failed igniter fit
these symptoms? Is the igniter easy to test? How? I'm assuming that
the White-Rodgers control does not monitor the igniter, because there
is no error code listed for it. The error codes from the
White-Rodgers are pretty limited. It's not like a modern auto computer
which monitors just about everything.

I'm not smelling any gas *at all*, so the control is apparently not
opening the valve. Is it supposed to, even if the igniter is faulty?
In other words, does the control monitor the health of the igniter,
and refuse to open the gas valve if it's not glowing? Or does it just
wait a short time, then open the gas valve, and if the temperature
doesn't rise rapidly it turns the gas back off?