My 1985 Lennox furnace would start, shut off and did that several times. Now won't even start. Any ideas?

SOURCE: Lennox Gas Furnace that turns on and shuts off in minutes

After troubleshooting like mad, I found that the flame sensor (rod that sits within the flame) was fouled with carbon on the surface. I believe this shields the rod from proper heat detection and the sensor reports that there is no flame and to shut off the gas valve to avoid explosion. Once I pulled out the rod and cleaned it with a wire brush, I have no problem whatsoever--fixed.

SOURCE: lennox intermitent flame problem

Yes the flame sensor is not allowing the ignitor to start, you will need to remove the flame sensor, and clean the black carbon build up from the stem of the flame sensor, replace the flame sensor and turn the furnance back on, this will correct the problem.

SOURCE: lennox furnace 17 yeanrs old. flame comes on and

I have a Lennox G40UH furnace. The burners fail to ignite about every 5th time. The furnace will try to ignite the burners then shut down. Once in a while the temp will drop down to 62 then fire up. I have only gotten a Diagnostic code once "Watchguard - burners fail to ignite" once I shut the power down and it started.
The furnace is clean, 5 years old, the flue is not clogged.

SOURCE: I have Lennox pulse 21 control module light goes out

If this is a high eff model, check to make sure the intake vent is not plugged.

SOURCE: Lennox gas furnace G21Q4/5-100-1 sometimes won't kick on

Hi check out the flame senser is clean also jump ou tthe pressure switch see if it lights when you do this , igntion modul is just a box with some push on wires just get the right one the info is all on the module