Hello everyone,
First I want to say thanks, I've already solved one problem with this furnace this weekend. I had a burners failed to ignite code so I cleaned the flame sensor but now I get the primary or secondary limit switch open.

First lets start with the sequence of events for my natural gas lennox 80UHG:
Thermostat calls for heat
Combustion air inducer starts up (motor sounds good)
Ignitor starts to glow
Gas ignites
Furnace stays lit for a few min
Gas is cut off
Diag lights flash code for primary/secondary limit switch open.

Now what I have checked the primary limit switch with my multi-meter and it is indeed opening (I think correctly as its getting hot). I have also tried supplying power directly to the blower motor and with the blower motor running the furnace runs fine. My questions is when in the start up is the blower supposed to come on? Is the control board not telling the blower to come on? Or is there some other problem?

Thanks in advance for your help.

If anyone needs any more information let me know

Thanks
Brad