Exhaust blower keeps cycling on and off after furnace reaches set point and blower shuts off

Is it just when it reaches temp and shuts down or it does this consistently then operates normal while heating the home?

Yes it does it all the time but works fine when heating, I changed the t stat but it made no difference

Remove the stat and see if it stops... let me know, thank you

Well since my last post it has been working fine, as soon as it acts up I will disconnect the t stat and let you know. Thanks for your help.

My Tempstar DC90 furnace does not light up and the exhaust blower runs all the time. I have tried cleaning the flame sensor - it looks like it as the flame sensor and ignitor on the same mounting. I have also tried jumpering the high heat sensor and disconnecting the pressure switch. What should I do next?