B2798

Code

Generic B — Body

Immobilizer/Key Transponder Communication Error

Brand: Generic

Type: B — Body

Views: UK: 5 EN: 16 RU: 8

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Damaged or unprogrammed key transponder (chip)
Faulty antenna coil or immobilizer reader ring at the ignition or start area
Weak/flat battery in a proximity or smart key (reduces transponder signal)
Open/shorted or corroded wiring or connectors between antenna, immobilizer module and BCM/ECM
Faulty immobilizer control module, body control module (BCM) or engine control module (ECM)
Loss of CAN/LIN network communication or intermittent bus errors

Symptoms

Immobilizer/security warning lamp illuminated or flashing
Engine cranks but will not start (fuel/ignition disabled) or starter disabled in some vehicles
“Key not recognized” or “Immobilizer active” message on dash/infotainment
Intermittent starting (works sometimes after cycling key)
No key ID shown or missing key ID when reading with a diagnostic tool

What to check

Read and record freeze frame and full DTC list with a capable scan tool
Check whether the scan tool can read the transponder/key ID and immobilizer module status
Verify key battery voltage (for proximity/smart keys) and try a known-good or spare programmed key
Visually inspect antenna/reader, connectors and wiring for damage, corrosion or water ingress
Check power and ground at immobilizer/antenna module (12 V and chassis ground)
Check continuity and resistance of antenna coil and associated wiring harness

Signal parameters

Antenna coil resistance: typical hundreds to thousands of ohms (manufacturer-specific) — compare to spec
Antenna supply voltage: battery/ignition-switched 12 V presence, 5 V reference where applicable
Transponder response: presence of valid key ID string when read by scan tool (ID shown/blank)
CAN/LIN: message frequency and error counters; loss of expected immobilizer/BCM messages
Wake/poll signals from key fob (for proximity) — RF beacon activity, key battery voltage (typically ~3 V coin cell)

Diagnostic algorithm

Connect a factory-level or full-feature scan tool, read DTCs and freeze frame, and record related immobilizer/BCM/ECM codes
Verify symptom: attempt to start and note any immobilizer dash messages and behavior
Check spare/known-good key: if spare key is recognized, problem likely with original key transponder or key battery
Inspect antenna/reader and ignition barrel area for corrosion/damage; unplug connector and inspect pins for corrosion or bent pins
Measure antenna coil resistance and compare to service spec; check for open/short in wiring harness
Verify power and ground to the antenna/immobilizer module with ignition on; repair any missing supply/ground
Use a data capture or scan tool to see if the immobilizer/BCM reports a key ID when key is present; if no key ID, trace antenna/reader to module
Check CAN/LIN bus health (bus voltages, error frames); if network messages from immobilizer/BCM are absent, trace wiring and inspect other modules
Perform a wiggle test on harnesses while monitoring for intermittent communication or DTCs
Try reprogramming/reinitializing key and immobilizer per manufacturer procedure; ensure the module firmware/calibration level is correct
If module failure is suspected after wiring and key checks, consider module bench test or replacement and reprogramming per procedure
Clear codes and retest after each repair step; document results and repeat tests to confirm repair

Likely causes

Key transponder not being read by the antenna/reader
Antenna power or ground missing, open or intermittent
Damaged antenna coil (high/low resistance) or damaged wiring/connectors at ignition barrel/reader
BCM/immobilizer module not receiving or sending key ID over vehicle network
Weak key battery (for proximity keys) reducing signal strength
Intermittent CAN/LIN communication preventing key ID transfer

Fault status

⚠️ Status

Immobilizer/Transponder communication error: immobilizer module or BCM reports no valid key transponder response or lost communication with the transponder reader. Key may not be recognized and engine start may be inhibited.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5–2.5 hours