I have a 4 yr old Lennox G51MP Furnace I bought used at a Habitat for Humanity ReStore. It is about 4 years old, and I've hooked it up and it runs fine, the problem is that after about 20 minutes of heat, the burners shut off, and don't re-light. The fan keeps blowing, and the two green flashing indicator lights flash slow in an alternating pattern. I need to shut the furnace off and wait an hour to re-start it, and go through the same cycle. This setting is called Watchguard, however the manual doesn't help diagnose it at all. I've read that a likely cause is the flame sensor needs cleaning, but the book doesn't show where this is, or how to safely do this. Does this sound familiar to anyone/make sense? Is there any diagnosing and testing I can do to try and fix it before going for a major repair?

Check the return air for

Dirty air filter
To small of a return air duct
Return air or supply air ducts shut off not allowing full flow.

It is a good possibility your furnace is overheating from the above causes. This in turn will cause it to shut down and not restart till you cycle the electric on and off.

There are other possibilitys but with this kind of problem there is also a lot of guess work due to the description.

Check the return air for

Dirty air filter
To small of a return air duct
Return air or supply air ducts shut off not allowing full flow.

It is a good possibility your furnace is overheating from the above causes. This in turn will cause it to shut down and not restart till you cycle the electric on and off.

There are other possibilitys but with this kind of problem there is also a lot of guess work due to the description.

Thanks for the suggestion, the return air was a little small for the furnace, so I opened it up more (now 16x25) as recommended. It stopped a slight squeal I had from the motor, but the furnace will still go into watchguard mode about 20 minutes after start-up. The fan continues to blow like before, but no flame or heat. I also lightly cleaned the flame sensor with sandpaper to take off any corrosion.

Something is causing it to overheat. Check the gas valve pressure and make sure it is at or a bit below the name plate rate.