Fraser johnson intermittingly kicking high limit switch,code flashes 4 times on board,it never has any other codes,replaced limit switch,still same problem

Something is causing the unit to overheat. Check the air filter, replace if clogged. Is the blower running correctly? If the system has air conditioning, is the evaporator clogged? Is the gas input too high? These are the most likely culprits. Chances are, the old limit wasn't bad, just doing its job.