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My furnace is tripping the high limit. It is a Downflow Payne 90+ have replaced the limit switch. I pulled the blower door off and checked all vents to ensure they are open and it still short cycles. I also changed the speed on the blower and checked gas pressure. I'm stumped! Does anyone have any ideas as to what's going on?

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You checked the exhaust vent for blockages? –  BMitch Nov 5 '13 at 16:04
    
Have you changed the filter, and checked for inflow restrictions? Are you sure it's the high limit? –  Tester101 Nov 5 '13 at 16:18
    
Yes like i said had blower door off. if exhaust vent was blocked would b press switch right –  Dallas Nov 5 '13 at 16:36
    
99.9% of the time high limit trips are airflow problems. Check for dirty filter, clogged/dirty blower wheel, closed dampers, blocked vents (supply and return), etc. If you have an inducer motor, exhaust flow should trip the pressure sensor not the limit. However, in the unlikely event that the pressure switch has failed closed, that would be a problem. –  Tester101 Nov 5 '13 at 16:45
    
What is the model number of your furnace? Do you have the owners manual? –  Tester101 Nov 5 '13 at 16:46
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