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OBD-II Diagnostic Trouble Code P1685: Wrong Immobilizer Key
Complete Technical Guide to Diagnosis, Repair, and System Understanding of Vehicle Immobilizer Security Systems
1
Code P1685 Technical Definition & System Overview
1.1 Official OBD-II Definition
P1685 is a manufacturer-specific diagnostic trouble code (DTC) defined as “Wrong Immobilizer Key” or “Immobilizer Key Not Programmed.” This code is set when the Powertrain Control Module (PCM) or Engine Control Module (ECM) receives an invalid or unrecognized security key code from the vehicle’s immobilizer system.
Technical Note: P1685 is part of the P1680-P1699 range, which are manufacturer-controlled codes typically related to vehicle computer systems and networks. Unlike generic P0xxx codes, these require manufacturer-specific diagnostic procedures.
1.2 Immobilizer System Architecture
The modern vehicle immobilizer system consists of three primary components working in unison:
| Component | Function | Communication Protocol | Typical Location |
|---|---|---|---|
| Transponder Key | Contains unique encrypted code chip, transmits via RFID when energized by antenna ring | 125 kHz RFID (Passive) | Ignition cylinder or key fob |
| Immobilizer Control Module (ICM) | Authenticates key code, communicates with PCM to enable engine start | CAN Bus, K-Line, or dedicated serial | Under dash, near steering column |
| Antenna Ring | Generates electromagnetic field to power transponder, receives key code | Inductive Coupling | Around ignition lock cylinder |
| Powertrain Control Module (PCM) | Receives enable signal from ICM, allows fuel injection and ignition | CAN Bus | Engine compartment |
1.3 System Operation Flow
When the key is inserted and turned to “ON”:
- Antenna ring emits 125 kHz electromagnetic field
- Key transponder chip is energized and transmits its unique 40-bit encrypted code
- Immobilizer module receives and decrypts the code
- ICM compares code with stored values in EEPROM memory
- If match is found, ICM sends “enable” signal to PCM via CAN bus
- PCM enables fuel pump, injectors, and ignition system
- If no match or communication failure, P1685 is set and engine start is inhibited
2
Symptoms, Causes & Diagnostic Priority
2.1 Primary & Secondary Symptoms
Primary Symptoms (Immediate Effects)
- Engine no-start condition – Most common manifestation (crank but no start, or no crank at all)
- Security warning light illuminated – Typically a key-shaped icon or “SECURITY” message
- Check Engine Light active – With P1685 stored as current or pending code
- Engine starts then immediately stalls – Usually within 1-3 seconds of starting
Secondary Symptoms (System Effects)
- Intermittent starting issues – May work sometimes, fail other times
- Reduced power mode – Some vehicles enter limp mode with immobilizer faults
- Other electrical anomalies – Radio presets lost, clock resetting (indicates voltage issues)
- Multiple system communication faults – CAN bus errors due to immobilizer module issues
2.2 Root Cause Analysis Matrix
| Root Cause Category | Specific Fault | Probability | Diagnostic Complexity |
|---|---|---|---|
| Key/Transponder Issues | Unprogrammed key, Damaged transponder chip, Wrong key for vehicle | 45% | Low |
| Electrical/Voltage Issues | Low battery voltage, Poor grounds, Wiring harness damage | 25% | Medium |
| Module Communication | CAN bus faults, Serial data line issues, Module programming | 15% | High |
| Component Failure | Failed immobilizer module, Faulty antenna ring, PCM failure | 10% | High |
| Software/Programming | Corrupted memory, Incorrect programming, Software updates needed | 5% | Very High |
Important Safety Notice: Before beginning diagnostics, ensure the vehicle battery is fully charged (12.6V minimum). Low voltage is the #1 cause of false immobilizer codes and can lead to incorrect diagnosis and unnecessary parts replacement.
3
Comprehensive Diagnostic Procedures
3.1 Preliminary Checks
Before advanced diagnostics, complete these mandatory preliminary checks:
| Step | Procedure | Acceptable Value | Tools Required |
|---|---|---|---|
| 1 | Measure battery voltage with key OFF | 12.4V – 12.8V | Multimeter |
| 2 | Measure battery voltage during cranking | > 10.0V | Multimeter |
| 3 | Test with all available keys | All should work | Vehicle keys |
| 4 | Check for aftermarket accessories | None affecting ignition | Visual inspection |
| 5 | Scan all modules for codes | Note all DTCs | Advanced scan tool |
3.2 Advanced Diagnostic Flowchart
// Diagnostic Logic Flow for P1685
START DIAGNOSIS
↓
[Test with spare key] → IF WORKS → FAULTY KEY → Replace/Reprogram key
↓ (If no spare or same issue)
[Check battery/charging system] → IF < 12.4V → CHARGE/REPLACE BATTERY
↓ (If voltage OK)
[Access immobilizer module with scan tool] → IF NO COMMUNICATION → Check power/ground to module
↓ (If communication OK)
[Check key status in data list] → IF "NOT RECOGNIZED" → Test antenna/transponder
↓ (If "RECOGNIZED")
[Check PCM enable signal] → IF "DISABLED" → Check CAN communication
↓ (If "ENABLED" but code persists)
[Check for PCM software updates] → IF AVAILABLE → Flash PCM
↓
[Perform module initialization/relearn] → TEST SYSTEM
↓
END DIAGNOSIS
3.3 Specific Manufacturer Procedures
Chrysler/Dodge/Jeep (SKIM System)
Special Tool Required: DRBIII or equivalent with security access
- Connect scan tool and access “Security” menu
- Select “SKIM” module and view “Key ID” status
- Valid keys show as “Learned” with key count
- To program new keys: Must have 2 already learned keys, or use PIN code from dealership
- Common issue: SKIM module loses sync after dead battery – requires PIN code reset
General Motors (Passlock System)
- System uses resistive pellet in key, not transponder chip
- Common failure: ignition lock cylinder wear changes resistance
- Diagnosis: Measure resistance between Passlock sensor pins (typically 600-5000 ohms)
- 30-minute relearn procedure often required after repair
4
Related Error Codes & System Interactions
4.1 Directly Related Immobilizer Codes
| Error Code | Description | Relationship to P1685 | Common Vehicles |
|---|---|---|---|
| P1686 | No SKIM Bus Message | Communication failure between SKIM and PCM | Chrysler, Dodge, Jeep |
| P1687 | No Clutch pedal position switch message | Often appears with P1685 in manual transmission vehicles | Various manufacturers |
| B2106 | Invalid Key Code Received | Body control module version of P1685 | GM, Ford, European vehicles |
| P0633 | Immobilizer Key Not Programmed | Alternative code for same condition | Toyota, Lexus |
| U0140 | Lost Communication With Body Control Module | Network failure affecting immobilizer communication | All modern vehicles |
| P0513 | Incorrect Immobilizer Key | Manufacturer-specific equivalent to P1685 | Nissan, Infiniti |
4.2 Network Communication Codes
Immobilizer systems in modern vehicles communicate via CAN bus networks. When P1685 appears with these codes, diagnose network issues first:
- U0100 – Lost Communication with ECM/PCM
- U0121 – Lost Communication with Anti-Lock Brake Module
- U0140 – Lost Communication with Body Control Module
- U0155 – Lost Communication with Instrument Panel Cluster
Network Diagnosis Tip: When multiple U-codes appear with P1685, always check CAN bus termination resistors (typically 120Ω at each end of bus), wiring harness for damage, and module power/ground connections before replacing any control modules.
5
Repair Procedures & Technical Specifications
5.1 Key Programming Specifications
| Vehicle Type | Maximum Keys | Programming Method | Special Tools Required | Security Access |
|---|---|---|---|---|
| Chrysler SKIM | 8 keys | DRBIII + PIN code | DRBIII/wiTECH | Dealer PIN required |
| GM Passlock III | 10 keys | 30-min relearn | Scan tool optional | None for relearn |
| Ford PATS | 8 keys | Two admin keys or IDS | Ford IDS/VCM | Admin keys required |
| Toyota Immobilizer | 7 keys | Techstream + master key | Techstream | Master key required |
| VW/Audi Immobilizer II/III | 8 keys | VCDS/VAS + SKC code | VCDS/VAS-PC | 7-digit SKC code |
5.2 Wiring & Electrical Specifications
Immobilizer Antenna Ring Testing
- Resistance: Typically 5-20Ω (check manufacturer spec)
- Inductance: 300-800 μH (requires LCR meter)
- Test Procedure:
- Disconnect antenna connector
- Measure resistance between pins
- Check for continuity to ground (should be open/infinite)
- Inspect for physical damage to wiring
Module Power & Ground Requirements
- Operating Voltage: 9V-16V DC
- Sleep Current: < 50mA after 30 minutes
- Ground Resistance: < 0.5Ω to chassis ground
- Communication Voltage: CAN High: 2.5-3.5V, CAN Low: 1.5-2.5V
Pro Tip: When replacing immobilizer components, always check for technical service bulletins (TSBs) first. Many manufacturers have updated parts or software fixes for common immobilizer issues that can save diagnostic time and prevent comebacks.
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