P1684 Code: Here's How to Solve It Fast

The P1684 trouble code is a diagnostic code that indicates a power interruption to the Powertrain Control Module (PCM) in your vehicle. This code is commonly seen in Chrysler, Dodge, and Jeep vehicles, but can also appear in other makes. Understanding this code is crucial for maintaining your vehicle's performance and preventing potential issues.

P1684 Code Meaning	P1684 Code Common Causes
Battery Disconnection	Recent battery replacement or disconnection
PCM Power Interruption	Faulty battery or alternator
Memory Reset Indicator	Corroded or loose battery terminals
Electrical System Issue	Damaged wiring or connectors to PCM
PCM Communication Error	Blown fuses or faulty relay

Battery Disconnection and Replacement

The most common cause of the P1684 code is a recent battery disconnection or replacement. This is not necessarily a problem, but rather an informational code to alert you that the PCM's memory has been cleared.

Symptoms:

• Check Engine Light (CEL) illumination
• No drivability issues

Technical Explanation:
When the battery is disconnected, the PCM loses power and its adaptive memory is cleared. The P1684 code is set to inform technicians that learned values and long-term fuel trims have been reset.

Diagnosis:

1. Check if the battery has been recently replaced or disconnected.
2. Verify the date of the last battery service.
3. Inspect battery terminals for proper connection.

Solution:
If the code appears after a battery service, no action is typically required. The code should clear itself after approximately 50 drive cycles.

Cost Estimate: $0 (if due to recent battery service)

Warning: Do not clear this code immediately after a battery service, as it serves as a reminder for technicians about the PCM's memory status.

Faulty Battery or Alternator

A failing battery or alternator can cause intermittent power supply issues to the PCM, triggering the P1684 code.

Symptoms:

• Difficulty starting the vehicle
• Dimming lights
• Electrical system malfunctions

Technical Explanation:
The PCM requires a stable power supply to function correctly. A weak battery or failing alternator can cause voltage fluctuations, leading to PCM power interruptions.

Step-by-step Diagnosis:

1. Test battery voltage (should be 12.6V when off, 13.7-14.7V when running).
2. Perform a load test on the battery.
3. Check alternator output voltage.
4. Inspect drive belt for wear or slippage.

Solution Methods:

• Replace the battery if it fails the load test.
• Replace the alternator if its output is below specifications.
• Adjust or replace the drive belt if it's loose or worn.

Cost Estimate:

• Battery replacement: $100-$300
• Alternator replacement: $300-$500
• Belt replacement: $50-$200

Recommendations:
Regular battery and alternator checks during routine maintenance can prevent unexpected failures and related codes.

Corroded or Loose Battery Terminals

Corrosion or loose connections at the battery terminals can cause intermittent power loss to the PCM, setting the P1684 code.

Symptoms:

• Intermittent starting issues
• Flickering lights
• Occasional electrical system resets

Technical Explanation:
Corroded or loose terminals create resistance in the electrical system, leading to voltage drops that can interrupt PCM power supply.

Diagnosis:

1. Visually inspect battery terminals for corrosion.
2. Check the tightness of terminal connections.
3. Test voltage drop across terminals under load.

Solution:

1. Clean terminals with a wire brush and baking soda solution.
2. Tighten terminal connections to proper torque specifications.
3. Apply dielectric grease to prevent future corrosion.

Cost Estimate: $10-$50 for cleaning supplies and dielectric grease

Warning: Always disconnect the negative terminal first and reconnect it last to avoid short circuits.

Damaged Wiring or Connectors to PCM

Wiring issues or damaged connectors can cause intermittent power loss to the PCM, triggering the P1684 code.

Symptoms:

• Intermittent check engine light
• Random electrical issues
• Occasional stalling or no-start conditions

Technical Explanation:
Damaged wires or connectors can create open circuits or high resistance paths, interrupting the power supply to the PCM.

Step-by-step Diagnosis:

1. Visually inspect wiring harnesses for damage, particularly near moving parts or heat sources.
2. Check PCM connectors for corrosion, bent pins, or loose connections.
3. Perform continuity tests on suspect wires.
4. Use a scan tool to monitor PCM voltage in real-time while wiggling harnesses.

Solution Methods:

• Repair or replace damaged wiring.
• Clean and secure connectors.
• Replace faulty PCM connectors.
• Apply dielectric grease to connections to prevent future issues.

Cost Estimate:

• DIY wire repair: $20-$100
• Professional wiring repair: $100-$500
• PCM connector replacement: $50-$200

Recommendations:
When repairing wiring, always use the correct gauge wire and proper splicing techniques. Consider using split loom to protect repaired areas.

Blown Fuses or Faulty Relay

A blown fuse or faulty relay in the PCM power circuit can cause the P1684 code by interrupting power to the module.

Symptoms:

• Sudden loss of engine power
• Multiple electrical system failures
• No-start condition

Technical Explanation:
Fuses and relays protect the electrical system from overloads. A blown fuse or stuck relay can cut power to the PCM, causing it to reset and set the P1684 code.

Diagnosis:

1. Locate the PCM fuse and relay in the vehicle's fuse box (consult the owner's manual).
2. Visually inspect the fuse for signs of failure.
3. Test the relay by swapping it with an identical, working relay.
4. Check for voltage at the fuse and relay terminals.

Solution:

1. Replace any blown fuses with the correct amperage.
2. Replace faulty relays.
3. Investigate the root cause of any fuse or relay failures to prevent recurrence.

Cost Estimate:

• Fuse replacement: $5-$20
• Relay replacement: $20-$100

Warning: Never replace a blown fuse with a higher amperage fuse, as this can cause electrical fires.

PCM Failure

In rare cases, the P1684 code may be caused by an internal PCM failure. This should only be considered after ruling out all other potential causes.

Symptoms:

• Multiple, persistent check engine codes
• Severe drivability issues
• Complete loss of engine management functions

Technical Explanation:
Internal PCM failures can cause the module to reset unexpectedly or lose critical memory, triggering the P1684 code along with other symptoms.

Step-by-step Diagnosis:

1. Perform a complete system scan to check for multiple codes.
2. Verify power and ground connections to the PCM.
3. Check PCM communication with a scan tool.
4. Perform specific PCM functional tests as outlined in the service manual.

Solution:
If a PCM failure is confirmed, replacement of the module is necessary. This process often requires programming and configuration by a dealership or specialized shop.

Cost Estimate: $500-$2000, depending on vehicle make and model

Recommendations:
Always ensure that a PCM failure is definitively diagnosed before replacement, as misdiagnosis can lead to unnecessary expenses.

In conclusion, the P1684 code, while often not immediately serious, serves as an important indicator of your vehicle's electrical system health. In many cases, it's a simple notification of recent battery service. However, it can also point to more significant issues that require attention. By following the diagnostic steps outlined above, you can quickly identify the root cause and implement the appropriate solution, ensuring your vehicle's continued reliable operation.

Frequently Asked Questions About P1684

• Will the P1684 code clear itself?
  Yes, in most cases, the P1684 code will clear itself after about 50 drive cycles if it was caused by a battery disconnection or replacement. However, if the underlying issue persists, the code may reappear.
• Can I drive my car with the P1684 code?
  Generally, you can drive with the P1684 code present, especially if it's due to a recent battery service. However, if you notice drivability issues, it's best to have the vehicle checked to rule out more serious problems.
• How do I reset the P1684 code?
  You can reset the code using an OBD-II scanner. However, if the underlying issue isn't resolved, the code will likely return. It's best to address the root cause before clearing the code.
• Does the P1684 code affect fuel economy?
  In most cases, the P1684 code itself doesn't directly affect fuel economy. However, if it's caused by an issue that's affecting the PCM's ability to manage the engine efficiently, you might see a decrease in fuel economy.
• Can a bad alternator cause the P1684 code?
  Yes, a failing alternator can cause voltage fluctuations that may trigger the P1684 code. If you're experiencing electrical issues along with this code, have your charging system checked.
• Is the P1684 code serious?
  The P1684 code itself is often not serious, especially if it appears after battery service. However, it can sometimes indicate more significant electrical system issues that should be addressed promptly.
• How much does it cost to fix the P1684 code?
  The cost can vary widely depending on the cause. If it's due to a recent battery service, there may be no cost. For electrical issues, repairs can range from $50 to $500, while PCM replacement could cost up to $2000.
• Can I clear the P1684 code by disconnecting the battery?
  While disconnecting the battery may clear the code temporarily, it will likely return if there's an underlying issue. Additionally, disconnecting the battery can trigger the code in the first place, so this method is not recommended.