I have Lennox G51MP-48c-090-06 ~14yrs old, starting up for first time this season. It is throwing error code: Primary or secondary limit switch open.
Unit shows normal operation when powered up and when heat is demanded. Inducer starts, pilot lights, flames start for a few minutes, then shuts down, with code listed above.
I started troubleshooting: - New filter ~1 month ago - All switches seem closed as normal, the primary may be only one moving off of 0ohms or 0volts. - Indoor blower does turn on, but seems quieter than normal - Blower spins freely by hand, capacitor measured good with resistance test, and blower runs when provided line voltage - Blower seems quieter than normal and doesn't seem to be pushing air out of registers as I recall, but it has been a summer since I have heard & felt it. - I did test with no filter & filter door open to see if it was intake airflow, but it still fails the same. -viewing flames through port on burner box, they seem fine, all blue flame shooting into the four tubes, but a small amount of blue flame going left-right prior to the heat exchanger tubes. - The rollout switches on this unit look more link flat thermostats on both the L and R sides of burner box, they remain closed through failure.
Not sure if it is related to this, but late this summer we had a frozen evaporator coil above the heat exchanger.