A103

What the Code Means

The CAS module logs a fault on the immobilizer bus — the dedicated communication line between the CAS, DME, and key reader (antenna ring). Normally these three components exchange data on this line every time the key is polled: the CAS recognizes the key through the reader and sends confirmation to the DME, which then enables engine start.

When the line is damaged or one of the modules fails, the data exchange chain breaks and the control unit doesn't get confirmation that the key is legitimate. Causes can include wiring problems, a faulty key reader, or a defective CAS or DME module.

Symptoms

• Engine won't start.
• Key isn't recognized by the system.
• Immobilizer indicator is on.
• Intermittent (unstable) engine starting.

Diagnosis and Repair

Step 1. Check the Wiring

Inspect the wiring between the CAS and DME that carries the immobilizer bus: look for opens, shorts, damaged insulation, and connector condition.

Step 2. Check the Key Reader Coil

Measure the resistance of the key reader coil. The normal range is 10–20 ohms.

Step 3. Replace the Reader

If resistance falls outside that range, replace the key reader.

Step 4. Replace the CAS or DME

If the wiring and reader are good, replace the CAS or DME module and program it afterward.

Step 5. Clear and Verify

Clear the fault codes and confirm that A103 doesn't return.

Secondary Damage

Continuing to drive with this fault can lead to the key being fully locked out by the immobilizer, which requires replacing both modules — CAS and DME — along with programming.