If you drive a vehicle that carries a CA number, this applies to you!
13 CCR 1232 Motor Carriers shall ensure all
vehicles and all required accessories are
regular and systematically inspected,
maintained, and lubricated to ensure safe and
proper operating condition.
The above link will take you to a .pdf from the California Highway Patrol titled "Understanding A Terminal Inspection." where the code excerpt was taken from.
Have you seen the handy little dash tray that some trucks come with?
It sits right in center on the top of the dash board. It's so convenient for pens, change and other miscellaneous little items. Little items that can fall right through the defroster vents and cost the owner of the vehicle quite a bit of money in repairs.
This is where those "little items" end up. In the heater control box blend door area. The "little items" jammed the blend door and prevented it from opening up the defrosters. Eventually the gear drive broke from multiple attempts to open. The clicking noise the driver heard was the broken gear trying to catch.
Don't leave items on your dashboard! Even if there is a special tray just for it!