B2799

Code

Generic B — Body

Immobilizer system communication fault

Brand: Generic

Type: B — Body

Views: UK: 38 EN: 40 RU: 44

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Faulty immobilizer (anti‑theft) control module
Intermittent or lost CAN/LIN bus communication between immobilizer and other modules
Damaged, corroded or loose wiring/connectors at immobilizer module, antenna coil, BCM or PCM
Failed/transponder key or key fob (bad transponder chip or battery)
Failed antenna/reader coil (key detection loop)
Low battery or poor battery connections/grounds causing resets or communication errors

Symptoms

Engine cranks but does not start or no crank/no start depending on immobilizer behavior
Immobilizer/KEY/SECURITY warning lamp illuminated or flashing on dash
Intermittent starting problems or requires multiple key insertions/push attempts
Key not recognized, key‑not‑detected messages on cluster or infotainment
Other modules reporting lost communication or multiple related DTCs

What to check

Verify battery state of charge and clean battery terminals; ensure battery voltage ≥ 12.2 V during tests
Read and record all DTCs from all modules (scan tool capable of immobilizer/BODY/BCM/PCM diagnostics) and freeze frame data
Inspect fuses and relays for immobilizer, BCM and instrument cluster circuits
Visually inspect connectors and wiring at immobilizer module, antenna coil, BCM and PCM for corrosion, damage, or poor pins
Check ground connections for engine/BCM/immobilizer — wiggle to reproduce fault
Use scan tool to check module presence and message counts on CAN/LIN network

Signal parameters

Module supply voltage: typically ~11–14 V (verify at immobilizer power pin)
Ground continuity: low milliohms expected between module ground and chassis ground
CAN bus resting voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (nominal — verify with scope/scan tool)
CAN dominant state: CAN_H rises toward ~3.5–4.5 V and CAN_L falls toward ~1.0 V during transmission (use oscilloscope)
Transponder/antenna systems: low‑frequency (≈125 kHz) or high‑frequency (~13.56 MHz) coupling depending on design — check for expected signal or modulation with appropriate tester
Antenna coil resistance and continuity: typically tens to hundreds of ohms depending on design — compare to manufacturer spec

Diagnostic algorithm

Confirm battery health and stable supply voltage; retest after battery service if needed
Connect a capable scan tool and read all modules; note related communication U‑ and B‑codes
Clear codes, attempt to reproduce issue and note when B2799 returns and any freeze frame data
Verify immobilizer module power, ignition switched power, and ground pins for correct voltages and continuity
Inspect and secure connectors at immobilizer module, BCM, PCM, antenna coil and steering column; repair any damaged wiring or pins
Test spare key(s) and replace/verify key fob battery; if available, verify transponder response with key tester or manufacturer tool
Measure antenna/reader coil continuity and compare to specifications; if coil signal absent, probe antenna signal with oscilloscope or dedicated reader
Monitor CAN/LIN traffic for missing messages, error frames or excessive bus errors; repair open/shorts/terminators as required
If wiring and external components check good, consider immobilizer module fault or BCM/PCM other module fault — consult manufacturer documentation for reprogramming/replacement and follow security procedures
After repairs, perform any required key relearn, immobilizer module programming, and clear codes; verify normal communications and absence of B2799 on road/test cycle

Likely causes

Corroded/loose connector at immobilizer or BCM (common, easy to inspect)
Failed key transponder or weak key fob battery (common cause of no‑start/recognition)
Damaged antenna/reader coil under steering column or ignition bezel (common wear point)
Faulty CAN bus wiring or short to battery/ground causing intermittent loss (less common)
Immobilizer module failure or need for reprogramming (requires dealer/tool)

Fault status

⚠️ Status

Immobilizer control module communication fault detected — immobilizer module is not communicating correctly with vehicle control network or related modules. May inhibit engine start or key recognition.

🔴 Repair difficulty: Hard

⏱️ Diagnostic time: 1.0-3.0 hours