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Home / DTC / B2798 — Immobilizer/Key Transponder Communication Error

B2798 — Immobilizer/Key Transponder Communication Error

Detailed page for trouble code B2798.

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B2798

Generic B — Body

Immobilizer/Key Transponder Communication Error

Brand: Generic
Type: B — Body
Views: UK: 1 EN: 2 RU: 1
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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

  1. Connect a factory-level or full-feature scan tool, read DTCs and freeze frame, and record related immobilizer/BCM/ECM codes
  2. Verify symptom: attempt to start and note any immobilizer dash messages and behavior
  3. Check spare/known-good key: if spare key is recognized, problem likely with original key transponder or key battery
  4. Inspect antenna/reader and ignition barrel area for corrosion/damage; unplug connector and inspect pins for corrosion or bent pins
  5. Measure antenna coil resistance and compare to service spec; check for open/short in wiring harness
  6. Verify power and ground to the antenna/immobilizer module with ignition on; repair any missing supply/ground
  7. 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
  8. Check CAN/LIN bus health (bus voltages, error frames); if network messages from immobilizer/BCM are absent, trace wiring and inspect other modules
  9. Perform a wiggle test on harnesses while monitoring for intermittent communication or DTCs
  10. Try reprogramming/reinitializing key and immobilizer per manufacturer procedure; ensure the module firmware/calibration level is correct
  11. If module failure is suspected after wiring and key checks, consider module bench test or replacement and reprogramming per procedure
  12. 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

Similar codes

B2797 B2799 U0100 U0073 P1620 B2780
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