The P1685 diagnostic trouble code is a manufacturer-specific powertrain code. On verified Chrysler, Dodge, and Jeep applications, it indicates that the Powertrain Control Module received an invalid key message from the Sentry Key Immobilizer Module.
Because this code targets the vehicle’s anti-theft system, the exact application must be verified. The code alone does not prove the immobilizer module has failed; it simply indicates an unauthorized or unrecognized key signal was detected.
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Key Takeaways
- P1685 is a manufacturer-specific powertrain code primarily associated with Chrysler, Dodge, and Jeep applications.
- It indicates the Powertrain Control Module received an invalid key message from the Sentry Key Immobilizer Module (SKIM).
- Possible causes include an unprogrammed key, a damaged transponder chip, SKIM communication faults, or module failure.
- Symptoms include a no-start condition, engine stalling immediately after starting, and an illuminated security light.
- Diagnosis requires verifying the key programming and testing the communication circuit between the SKIM and PCM.
P1685 Code Description and Implications
The P1685 code indicates that the vehicle’s anti-theft system has detected an issue with the key authorization process. On verified Chrysler, Dodge, and Jeep applications, this code specifically relates to the Sentry Key Immobilizer Module (SKIM).
When you insert the key, the SKIM reads the transponder chip and sends a verification message to the Powertrain Control Module (PCM). If the PCM receives an invalid or missing message, it triggers the P1685 code and disables the fuel injectors to prevent unauthorized operation.
Common Causes of the P1685 Code
Because P1685 relates to the immobilizer system, the root cause involves the key, the module, or the communication circuit.
An unprogrammed or incorrect key is a frequent cause. If the transponder chip inside the key is damaged, missing, or not programmed to the vehicle’s SKIM, the system will reject it.
Wiring issues between the SKIM and the PCM can disrupt the verification message. A failing SKIM module or a PCM communication fault may also prevent the correct signal from being transmitted.
Symptoms Associated With P1685
When the immobilizer system rejects the key, the vehicle’s operation is directly affected.
The most immediate symptom is a no-start condition. The engine may crank normally but will not start because the PCM disables the fuel injectors.
In some cases, the engine may start and then stall immediately after one or two seconds. A security or key icon on the dashboard will typically illuminate or flash to indicate the immobilizer fault.
Diagnosing the P1685 Trouble Code
Accurate diagnosis begins by verifying the exact vehicle application and checking the key itself.
Try using a known-good spare key. If the spare key starts the vehicle, the original key’s transponder is likely damaged or unprogrammed.
If neither key works, inspect the wiring and connectors between the SKIM and the PCM. Use a scan tool capable of accessing the immobilizer module to check for internal SKIM faults or communication errors.
Repairing the P1685 Issue
Repairs must follow the confirmed root cause identified during diagnosis.
If the key transponder is damaged or unprogrammed, the key must be replaced and properly programmed to the vehicle using a compatible diagnostic tool.
If the wiring is damaged, repair the harness to restore communication. If the SKIM module itself fails testing, it must be replaced and programmed with the vehicle’s secret key code and PIN.
Cost Considerations for P1685 Repairs
Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.
Cutting and programming a new transponder key involves moderate costs. However, replacing and programming a new SKIM module requires specialized dealer-level equipment and access to the manufacturer’s security database, which increases the total expense.
Prevention Strategies for Future Occurrences
Protecting your key fob from physical damage and moisture intrusion helps prevent the internal transponder chip from failing.
Avoiding aftermarket key shells that lack the proper transponder chip ensures the immobilizer system continues to receive the correct authorization signal during every start cycle.
Troubleshooting Tips and Best Practices
Follow these practices to avoid misdiagnosis and ensure accurate repairs.
Do not assume the SKIM module has failed just because a P1685 error code is present: A simple unprogrammed key or a damaged transponder is a frequent cause of immobilizer faults. Always test a spare key first.
Check for related codes: Look for accompanying communication bus codes, which might help pinpoint a broader network issue preventing the SKIM from talking to the PCM.
Verify your definition: Always confirm whether your specific vehicle uses the SKIM definition before ordering immobilizer components.
More OBD-II Codes
| P1681 | P1680 | P1679 | P1678 |
| P1677 | P1676 | P1686 | P1687 |
| P1688 | P1689 | P1691 | P1695 |
| P1690 | P1662 | P1586 | P1600 |
| P1611 | P1609 | P1608 | P1606 |
| P1388 | P1386 | P1378 | P1387 |
Frequently Asked Questions
Is P1685 a generic or manufacturer-specific code?
No. P1685 is a manufacturer-specific powertrain code. On Chrysler, Dodge, and Jeep applications, it specifically indicates an invalid key message from the Sentry Key Immobilizer Module.
Can I drive my vehicle with a P1685 code?
No. Because the immobilizer system disables the fuel injectors when this code is active, the vehicle will either not start or will stall immediately, making it undrivable.
Will disconnecting the battery clear the P1685 code?
Disconnecting the battery might temporarily reset the module. However, it does not fix an unprogrammed key or a damaged transponder, and the immobilizer fault will return upon the next start cycle.
Can a bad key fob battery cause a P1685 code?
No. The immobilizer transponder chip inside the key is passive and does not require the key fob battery to communicate with the SKIM module. A dead fob battery only affects remote locking.
Does P1685 affect the transmission?
No. The P1685 code relates strictly to the engine immobilizer and anti-theft authorization system. It does not directly affect the automatic or manual transmission operation.
What Should You Do About the P1685 Code?
When a P1685 code appears, confirm the exact factory definition for your specific vehicle make and model. Because this code targets the vehicle’s anti-theft and immobilizer systems, applying the wrong diagnostic path will lead to misdiagnosis.
Follow the applicable manufacturer diagnostic procedure to test the key transponder and the relevant communication circuits. Once the root cause is confirmed, complete the necessary key programming, wiring repairs, or module replacements before returning the vehicle to normal service.
