I've got a Goodman Model #GCH90904CXAB It is 2 yrrs old and heated fine all last season.This season it has started to run 20-30 minutes and will shut down and flash 4 times .Which says high limit switch on the door.My techs have replaced the roll out switch and the high limit switch.It still does the same thing.About a month ago we had a bird in the 3" exhaust pvc,we removed pipe and induction blower fan,cleaned everything up and have had this high limit ever since.We have cleaned the A-coil and the whole return system.Any Ideas? Thanks,Tom

Check gas input & temperature rise. If input is correct but rise is high, try increasing blower speed until rise is correct. P.S. what type of air filter are you using? Same type as last year?