U0409
Invalid Data Received from Vehicle Security Control Module
The vehicle security / alarm controller is sending implausible status data (e.g. armed/unarmed jitter). Alarm and immobilizer behaviour can become unpredictable.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: hard
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
Ordered by how often each turns out to be the cause.
- Security module internally failedPossible
- Gateway module routing CAN messages incorrectlyLess likely
- CAN terminating resistor (120 Ω) drifted out of specLess likely
Parts involved
- Vehicle Security Module
U0409 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.