Gas and Oil Home Heating Furnaces - lennox elite series furnace (8 years old)

hi im getting a flash code of primary/secondary limit switch open.
does anyone know what this means and how to fix problem.
1) furnace will run but cut off before temp reaches what thermostat is set at.
2)furnace will not restart always with out turning to off the thermostat, and restarting thermostat to on. still temp will not reach setting on thermostat.
3) or i could turn off the blower motor and turn back on and then furnace will start,but with same problems.

i cleaned the flame sensor with steelwool and that did not solve problem. its the coldest day of the year tonight -11 degrees.

please help if possible....

Other than a faulty limit switch (which is rare) this usually happens if there isn't enough circulating air. Make sure the air filters are clean and nothing is restricting the return air register (like a carpet, or furniture, or a St Bernard dog.)

If there is a restriction in your duct work a quick, safe, temporary fix is to kick up the blower speed. Note what color wire goes to the "Heat" terminal from your blower motor and hook up a higher speed wire. This will probably require swapping the wires on the Heat and Cool terminals. Don't forget to switch them back before cooling season.

Motor wire color code:


Black ---- High Speed
Blue ---- Med High Speed
Yellow ---- Med Low Speed
Red ---- Low Speed
White ---- Common or neutral (don't move this one)
Brown, Brown with black stripe ---- Capacator wires

What causes that limit to open is the furnace gets too hot.
The normal causes are airflow issues, such as a dirty air filter, registers closed, returns blocked, evaporator (a/c) coil dirty or volume dampers closed. You want to make sure these things are in order first.

Hey xpogi---

Have you actually found a St. Bernard asleep across the air return? Could be an explanation for a filter full of dog hair too, I suppose.

thanks guys problem solved. i had forgot about my electric air filter system that was very dirty...
i did how ever change the disposable filter regularly:)
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