A0B4

What the Code Means

A0B4 flags a fault in the starter motor signal circuit. The CAS (Car Access System) module handles authorization and sends the start signal to the starter motor. If CAS can't form or deliver that signal correctly, the starter motor won't engage — the engine doesn't crank and won't start. The problem can sit in the CAS module itself, the starter relay, the wiring, or the actuator circuit.

Symptoms

• Engine doesn't crank when you try to start
• Engine won't start
• Starter motor doesn't engage (no cranking)
• Clicking sound from the starter motor when you try to start

Diagnosis and Repair

1. Check the starter relay and the start circuit wiring: corroded contacts, breaks, and loose connections in the connectors are a common cause of a missing start signal.
2. Check the CAS module output signal to the starter motor while cranking. If the signal isn't formed or doesn't reach the starter motor with the wiring verified good, the module is the problem.
3. Check the starter motor itself: clicking when you try to start can point to a bad starter solenoid or starter motor, not CAS.
4. If the fault is confirmed, replace the CAS module (OEM part number: 61356993373).

A common diagnostic mistake is replacing the starter motor without checking the control signal from CAS and the relay. Swap the starter on a hunch and you've burned the money and still have a no-crank. First confirm the start command reaches the actuator circuit.

Related Issues

• The starter motor can fail from repeated start attempts: back-to-back engagement overheats the windings and the starter solenoid.
• The CAS module can fail completely, which also affects other access and start functions on the car.