Furnace is about 13 years old... we just replaced the ignighter in it last night because it was running through all the steps to run except ignighting. It fired right up and ran for about 7-8 hours... now it won't ignite at all (it is running through all the steps it needs to except for ignighting). The L.E.D. indicator light is flashing 4 times which is says that there is a problem with the open high temperature limit switch. What and where is this and how can I, if at all, fix this myself??

There is a wiring diagram on the back side of the blower door that may help.