my fan for clearing residual gas comes on and continues to run without starting the furance. The circuit board is flashing 4 times which means bad high limit. I replaced with a new one, ran for a few minutes then went back to 4 flashes and no heat. I check the new switch again for continuity and it checks good.
Make sure you have a clean non restrictive filter. Make sure all ducts are open supply and returns and are not blocked by anything. Make sure if you have a high efficiency furnace that exhaust and intake are clear. The high limit switch is because your furnace is getting too hot not because of failure of the switch.
Also clean your flame sensor if gas starts to flow and then turns off fairly fast.