On Tue, 10 Dec 2013 23:08:39 -0700, Tony Hwang wrote:

That is located inside the flue. You took a pic. of it.
Sqare bakelite lo0king thing with two wires mounted
on the flue.

I see it. Thanks. This is the backside of the thing:
http://farm8.staticflickr.com/7323/1...56224304_o.gif

So that makes four safety switches to debug next time:

1. The fusible link (also visible in that picture above)
2. The vent safety shutoff switch (also in that picture above)
3. The lockout timer (also in that picture above)
4. The limit switch (visible in the picture below)
http://farm3.staticflickr.com/2814/1...0de1c8bd_o.gif

The only one of those four that I don't understand is the
lockout timer. Googling for the part number 990-141-1, I find
it's a $200 24VAC 60HZ SPST switch:
http://technicalhotandcoldparts.com/...r-control.html

This PDF purports to explain what it does:
http://www.lennoxproducts.com/wp-con.../HH72CH003.pdf

It seems to have a "pickup time" of 7 seconds, and a "dropout time"
of 5 minutes.

Since it's hooked in series with the pilot light and spark igniter,
it has something do do with stopping that circuitry after about
five minutes. But, I'm not sure what controls it, unless it's a
pure timer.

Is that what it is? A timer to interrupt current to the pilot
switch after five minutes?