When it comes to a high temperature limit fault, we must look at a couple of things to diagnose the issue properly. First of all if the system is truly overheating, we have our obvious checks, gas pressure and airflow. 

But what if the system is not overheating and we are locking out for no reason?

First, verify the correct temperature of the switch and make sure its the one that the manufacture specifies.

Sometimes a faulty thermostat can cause incorrect faults, so I recommend to bypass the thermostat (R-W jumper) during testing.

Next we verify the circuit. Keep in mind we are dealing with a switch, power in-power out. There is no need for a common or ground for a switch to work. But, You'll need to reference ground when testing with your meter, so clamping to a screw with your black test lead or touching metal on the cabinet works. Test the circuit for 24vac incoming to the switch, if present (coming from the control board typically). If not, we need to check the wire from the board to the switch. If we move to the other side of the switch and we should see 24vac leaving  (going back to the board). If no voltage leaving the switch, we can determine that its open or not working correctly. 

So what if we have voltage coming into, and leaving the switch, and still have a fault? Start by checking the wiring and make sure its not broken. Perform an Ohm test on the switch and see how it compares to a working one (this should be relatively low, 10 ohms or less depending on ambient temperature). Replace as needed.  Check to see if there is another switch in line that could be malfunctioning. Determine the signal is making its way back to  the control board. If we have the signal making its way back to the board, we can determine at this point that the control board has failed, and is "ignoring" the signal.

Good luck and happy testing!

Carlos