Having some trouble with a 441 . Serviced it last summer , cleaning , burner mat , blower bearings . Burner ring very slightly warped , reused . Unit worked good for several weeks then flashing lights . Started right up for another few weeks , then flashing lights . It is now down to a day or two before shut down . Filter on tank and unit changed/cleaned after first shutdown . Is there any testing method for any of the shut down sensors ? Thanks for any help .