Ignition Coil Boot
A protective rubber sleeve that insulates the connection between the ignition coil and the spark plug, preventing voltage leaks.
An ignition coil boot (or spark plug boot) is a flexible silicone or rubber sleeve that connects the output end of a coil-on-plug ignition coil to the spark plug. Inside the boot is a metal spring or resistor that carries the high-voltage electrical current to the plug terminal.
The boot's primary job is insulation. It prevents the high-voltage current (up to 45,000 volts) from leaking out and grounding against the metal cylinder head before it can reach the spark plug gap. The boot also seals the spark plug well, keeping out moisture, dirt, and engine oil that would cause electrical shorts.
Over time, the rubber boot degrades from engine heat, becoming hard, brittle, or cracked. Small micro-cracks allow the voltage to arc (short) to the cylinder head, causing engine misfires, rough acceleration, and check engine lights. Replacing the coil boots when replacing spark plugs is a cost-effective way to prevent ignition issues.