P1645 Code: ECM/Anti-Theft Communication Fault, Causes, Symptoms & Fixes

Anti-Theft Communication Fault, Causes, Symptoms & Fixes

Table of Contents 1. What is P1645? 2. Common Causes 3. Key Symptoms 4. Affected Models 5. Diagnosis with ICARZONE UR1000 6. Fixes & Solutions 7. Repair Costs 8. Preventive Maintenance 9. FAQ 10. Summary
Diagnostic Guide

P1645: Immobilizer Communication Fault

Solve P1645 in BMW 3 Series & Mercedes C-Class. Learn causes, symptoms, and fixes for this security system error using the ICARZONE UR1000 diagnostic tool.

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P1645

1. What is P1645?

P1645 is a manufacturer-specific Diagnostic Trouble Code indicating a communication failure between the Engine Control Module (ECM) and the immobilizer/security module. It prevents the engine from starting if the security check fails


2. Common Causes

  • Damaged CAS/EIS modules due to water ingress or corrosion.
  • Faulty or low battery key fob.
  • ECM firmware mismatch or corrupted immobilizer data.
  • Loose or corroded wiring harness connections.
  • Failed ICARZONE UR1000 recommended to verify signal integrity.

3. Key Symptoms

  • Engine cranks but does not start.
  • Immobilizer warning light or symbol on dashboard.
  • Key fob not recognized.
  • Intermittent start issues.
  • Error code P1645 displayed on ICARZONE UR1000.

4. Affected Models

Commonly reported in:

Make Models Years
BMW 3 Series (E90, F30) 2005-2015
Mercedes-Benz C-Class (W204, W205) 2007-2019

5. Diagnosis with ICARZONE UR1000

ICARZONE UR1000 allows:

  • Read & clear P1645 codes.
  • Check immobilizer module communication.
  • Verify key fob recognition.
  • Monitor ECM and EIS/CAS signal integrity.
  • Perform key programming & reset functions.

6. Fixes & Solutions

  • Replace or repair damaged CAS/EIS modules.
  • Check and replace key fob battery if required.
  • Reflash or update ECM/immobilizer firmware.
  • Repair or clean wiring harness connections.
  • Use ICARZONE UR1000 to reset codes and program keys.

7. Repair Costs

  • CAS/EIS Module Replacement: $400-$800
  • Key Fob Battery Replacement: $15-$25
  • ECM Reflash/Update: $80-$150
  • Wiring Harness Repair: $50-$120
  • ICARZONE UR1000 Diagnostic: $220 (one-time purchase, reusable)

8. Preventive Maintenance

  • Keep CAS/EIS modules dry and free from corrosion.
  • Replace key fob batteries periodically.
  • Perform regular ECM/immobilizer firmware updates.
  • Use ICARZONE UR1000 for routine diagnostics.

9. Frequently Asked Questions

Q1: Can I start the car with P1645 active? No, the immobilizer prevents engine start until the issue is resolved.
Q2: Do I need professional tools? ICARZONE UR1000 provides full DIY diagnostic & reset capability for P1645.
Q3: Is this error common in older models? Yes, older BMW 3 Series E90 and Mercedes W204 are more prone due to module aging.

10. Summary

P1645 immobilizer faults in BMW and Mercedes can prevent engine start. Diagnosis with ICARZONE UR1000 is recommended for quick code reading, module communication checks, and key programming. Preventive care like dry module housing and battery maintenance can reduce occurrences.

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