My '99 S420 has been giving me problems with the climate control system. The ventilation system defaults to defrost when I'm driving, and won't let air come out of the two center vents. I replaced a check valve in the gray plastic line that is on the passenger side of the engine compartment before it enters through the firewall. This seemed to help as my old check valve was bad, but every now and then the car still defaults to the defroster vents. I was just wondering if anyone else has experienced a similar problem.

Thanks in advance.
Dave