P2305: Ignition Coil B Secondary Circuit

What it means

The secondary side of ignition coil B is faulty. Spark energy at the corresponding spark plug is insufficient, producing misfires.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The secondary side of ignition coil B sends high voltage through its boot or lead to the spark plug it serves. The engine control module detects a problem with coil B's secondary ignition performance and sets this code when spark energy is missing or too weak for reliable combustion.

Common causes

Most likely first.

1. Worn spark plug
2. Coil secondary failure

Symptoms

• Check-engine light on, possibly flashing
• Engine misfire or rough idle
• Hesitation under acceleration
• Reduced power

How to diagnose it

1. Record freeze-frame data and check live misfire counts to confirm whether the cylinder served by coil B misfires under particular conditions.
2. With the engine off, inspect coil B's boot or lead and plug well for cracks, carbon tracking, oil, or moisture.
3. Remove the spark plug served by coil B and check for fouling, wear, or an excessive gap against the manufacturer's specification.
4. Test coil B's secondary winding and insulation using the manufacturer's procedure.
5. If the plug and coil pass, check coil B's primary supply and control circuit before considering an ECU fault.

Common fixes

• Replace a worn or fouled spark plug and correct its gap to specification.
• Clean and dry the plug well and replace a cracked or carbon-tracked coil boot.
• Replace coil B if its secondary winding or insulation fails testing.
• Repair a confirmed fault in the coil B supply or control wiring.

Can I keep driving?

If the check-engine light flashes or the engine shakes from a misfire, stop driving when safe because continued misfire can damage the catalytic converter. If the light is steady and the engine runs smoothly, avoid heavy loads and arrange diagnosis soon.