On certain vehicles, there could be a problem with the ventilation control system (HVAC). As a result, certain controls may not work, including the windshield defrost function.
Safety Risk
A windshield that cannot be defrosted/defogged can reduce visibility and could increase the risk of a crash.
Corrective Actions
PACCAR will notify owners by mail and instruct you to take your vehicle to a dealer to inspect the date code of the HVAC control head and replace it, if necessary.