My 7 year old Lennox G51 furnace has an intermittent failure where if it runs for more than 1 hour continuously the flames go out, it begins blowing cold air and the status LEDs indicate that it has entered a Watchguard state. This failure is repeatable and always happens after 1 hour.

When I turn the thermostat off and then on again, the furnace lights first time and continues without any problems... for an hour until it starts blowing cold air again.

If I do nothing and let the blower run continuously for another hour then the furnace will restart and work fine for another hour until it goes out again.

The watchguard mode indicates that either the

burners failed to ignite (This isn't likely since I've never seen the furnace fail to light first time)
limit open longer than 3 minutes
flame sense lost >4 times


I've looked everywhere on the internet and I am absolutely baffled by this behavior! Does anyone have any idea what the problem might be? Or even how to describe the failure mode I'm experiencing?

Thanks!

We had a similar problem with ours, except the flame would continue to go out at much shorter intervals - seconds, sometimes minutes. I bet your problem is one of the following, which are easy fixes / tests:

1. The flame sensor is dirty. It's just a little metal rod in the burner chamber - easy to clean, but hard to get to. Google "How to clean a flame sensor on a furnace" with emory cloth / steel wool. If your thing runs for an hour, it is unlikely this is the problem though, as it would go off much sooner than that if it were the flame sensor.

2. The G51 "pressure switches" are super sensitive (there are two of them), and also seem to just plain go bad. That was the problem with ours - one of them was just bad. We simply replaced it, and the problems disappeared. I found it cheapest at furnacepartsoutlet.com for around $27. Try this. Open your top panel. You will see two black round things with two wires going into each. Now get a piece of short wire lying around the house somewhere, expose the two ends of it. (Do a Google search on testing a furnace pressure switch.) It is simply doing this for each pressure switch, one at a time: inserting the two ends of your wire into the two wire connectors of the switch (you are effectively bypassing the pressure switch and telling the furnace main circuitboard that "everything is fine"). Now try the furnace. If it works fine, either the pressure switch is faulty, or it works fine, but there really is a pressure problem.

You can also try switching the two pressure switches, they are the same part # (they were on ours, check and make sure on yours though). Our furnace problem went away when we just switched the two switches - which was another way we knew one of them was bad, and that it was not the case that we really had a pressure problem.

3. It could be your exhaust or intake lines are dirty / spiderwebs / bugs, wasps, etc. Like I said, the pressure switches are really really sensitive. Clean the lines somehow - a 50' plumbers snake (Harbor Freight), or by some other method.