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.

  • Hello, and welcome to Home Improvement. What do you mean by "the primary may be only one moving off of 0ohms or 0volts"? And, props for taking our tour before posting; few newcomers do. – Daniel Griscom Oct 6 '19 at 1:50
  • I tried to keep my DMM leads on each of the switches during failure. Being they r normally closed, they all read 0v or 0ohm in circuit. The Primary switch is the only one that seems to budge from 0. I am measuring in circuit, so it is possible that I am seeing some resistance looking back into the control board when the primary switch opens. I have not pulled the leads from it immediately after the failure. Seems to be overheating, then primary switch opens. That is why I pulled the filter to ensure it wasn’t I take air flow. Just seems like blower fan isn’t blowing air as hard as it should – MN_Nice Oct 7 '19 at 2:19

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