How Car Door Locks Work

The basic idea

Every car door has a latch that grabs a metal post on the door frame (the striker) when you close it. Locking the door doesn't hold it shut. It disconnects the outside handle from the latch, so pulling the handle does nothing.

The parts

• Latch: the hook inside the door that grabs the striker and holds the door closed.
• Lock cylinder: the keyhole. Many car locks use small flat wafers instead of the pins in a house lock. The right key lines all the wafers up so the cylinder can turn.
• Rods and cables: connect the handles, the lock knob and the keyhole to the latch.
• Power lock actuator: a small electric motor with gears that moves the lock rod for you.
• Body control module: the computer that hears the lock button or fob and tells each door's actuator to lock or unlock.

Power locks, step by step

• 1. You press lock on the fob or the door switch.
• 2. The car's body computer gets the signal.
• 3. It sends power to the actuator in each door.
• 4. Each actuator's little motor moves the lock rod, the same rod your lock knob moves by hand.

Why newer cars are harder to get into

Older cars had simple rods and open space inside the door. Many newer cars use shielded latches and cables, and have wiring and side airbag parts inside the door. Going in with a coat hanger or slim jim can damage wiring, weather stripping or the lock itself. That's why we use professional lockout tools.

When a lock isn't working

• One door won't lock with the button, but the rest do: usually that door's actuator, or a broken wire where the wiring passes into the door. You might hear it buzz, click or do nothing.
• The fob doesn't work, but the door switch does: likely the fob. Start with its battery.
• Nothing works, fob or switch: check the power lock fuse.
• Key is hard to turn or won't go in: a worn key, dirt in the lock, or ice. Don't force it, because a snapped key turns a small problem into a bigger one.
• Frozen lock: use a lock de-icer spray. Never pour hot water on it. It refreezes and can crack cold glass.
• Doors lock by themselves: many cars lock when you start driving, or relock if you unlock and don't open a door. A failing actuator or switch can also cause random locking.

Related: Locked out in winter? What to do · How car keys and fobs work