P2303: Ignition Coil B Primary Control Circuit Low

What it means

The ECU reads a low primary control voltage on ignition coil B. Misfire is typically observed on the cylinder this coil serves.

At a glance

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

How it works

Ignition coil B charges through its primary control circuit and then fires the spark plug on the cylinder it serves. The engine control module monitors coil B's primary signal and sets this code when the circuit level is too low, commonly from a short to ground or an internally shorted coil winding.

Common causes

Most likely first.

1. Internal coil short
2. Primary wire shorted to ground
3. Connector pins corroded - high contact resistance

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 review live misfire counts to identify when the cylinder served by coil B misfires.
2. Inspect coil B's connector and primary wiring for corrosion, damaged insulation, loose terminals, or a wire touching ground.
3. Check that coil B's connector is seated and its terminals are clean and secure.
4. Use the wiring diagram to test coil B's primary control wire for a short to ground and test the winding to the manufacturer's specification.
5. If the coil and wiring pass, have the ECU driver signal checked using the manufacturer's diagnostic procedure.

Common fixes

• Clean and secure corroded or loose coil B connector terminals.
• Repair primary wiring shorted to ground.
• Replace coil B if its primary winding fails testing.
• Repair the ECU driver circuit if testing confirms it is faulty.

Can I keep driving?

A flashing check-engine light or a pronounced misfire means stop driving as soon as it is safe, since unburned fuel can damage the catalytic converter. If the light is steady and the engine runs smoothly, limit driving and avoid hard acceleration until diagnosed.