the furnace is tripping on high limit , not rollout. It will initiate a call for heat. The flue fan will run the air switch will make and the ignitor will glow. The MV comes on and the furnace lights. It continues to run for appr. 2 minutes and the high limit will open. It is not a variable high limit and it is an auto reset. The air flow is good, and the filter is clean. Could it be that the high limit is just bad? Or something else I am not seeing. The unit is about 13 yrs old. Thanks

How did you determine that the airflow is good? Check the supply air temperature if possible - preferably at the plenum.