Hello everyone. I have a Lennox G60UHV-36B-090-10 furnace that will not start-up. It flashed red led error code 3-1 which the panel says is the primary/secondary limit switch open. The furnace is just over a year old. Can someone please tell me how to test this switch and also whether something else could trigger the same error code? Also are there two switches I should test? I have recently replaced the thermostat for a Nest unit could this have done something to the furnace? The furnace is getting a start-up command when I send it from the thermostat and I can see the combustion air fan start to spin up behind the top panel but it stops spinning after 3-5 seconds and the furnace does not fire up. The furnace main fan continues running indefinitely but the furnace will not restart. I have replaced the filter and can't see any other obstructions. Thanks in advance.

Yesterday I made some line repairs and installed another shutoff. I used pipe thread compound with teflon in it on some of those fittings. This morning I discover my fuel pump is shot (webster M34DJ-3). So my question is what does teflon do to fuel pumps and will it cause further damage if I just replace it without pulling those fittings. I have a beckett model AF-15 burning kerosine. Thanks