P1668 Code? Here’s How to Fix It Quickly and Easily

The P1668 diagnostic trouble code is a manufacturer-specific powertrain code. Its exact definition depends heavily on the vehicle make, pointing to a module communication error on Ford diesel models, an alternator circuit fault on GM vehicles, or a security link issue on Land Rover.

Because this code targets entirely different systems across manufacturers, the exact vehicle application must be verified before diagnosis. The code alone does not prove a specific module or generator has failed without proper electrical testing.

Key Takeaways

• P1668 is a manufacturer-specific code with differing definitions depending on the vehicle make.
• On Ford 7.3L Powerstroke applications, it indicates a communication error between the PCM and the Injector Driver Module (IDM).
• On GM applications, it typically points to a generator L-terminal control circuit or alternator turn-on signal failure.
• On Land Rover applications, it indicates an anti-theft alarm serial link fault.
• Diagnosis requires verifying the exact factory definition and testing the relevant communication buses, alternator circuits, or security modules.

P1668 Code Description and Implications

The P1668 code indicates that a control module has detected a specific fault within a powertrain or auxiliary system. Because it is a manufacturer-specific code, the exact monitored component varies by brand.

Some Ford diesel applications: P1668 indicates a communication error between the Powertrain Control Module (PCM) and the Injector Driver Module (IDM). The PCM detects that expected messages from the IDM are missing or invalid.

Certain General Motors applications: On these vehicles, P1668 indicates a Generator L-Terminal Control Circuit fault. The ECM detects an issue with the alternator turn-on signal used to regulate charging system output.

Specific Land Rover applications: The code indicates an anti-theft alarm serial link fault, meaning the security module has lost communication or detected a signal out of range.

Common Causes of the P1668 Code

Because P1668 applies to different systems across different brands, the root cause depends on the verified vehicle application.

• Ford (IDM Communication) Causes: A failed Injector Driver Module, an open or shorted wiring harness between the IDM and PCM, blown IDM fuses, or a faulty IDM relay.
• GM (Alternator L-Terminal) Causes: A failed alternator, damaged wiring in the generator L-terminal circuit, poor electrical connections at the ECM, or a blown charging system fuse.
• Land Rover (Security Link) Causes: A faulty anti-theft alarm module, damaged serial link wiring, or poor connections in the security system harness.

Symptoms Associated With P1668

When the control module detects a critical system fault, it alters operation or disables the affected feature.

• Ford Symptoms: The engine may crank but fail to start, or it may stall unexpectedly. The check engine light will illuminate, and a buzz test may fail.
• GM Symptoms: The battery may not charge properly, leading to a “Check Charging System” message on the dashboard, dimming lights, or eventual stalling due to low voltage.
• Land Rover Symptoms: The anti-theft system may malfunction, potentially preventing the vehicle from starting or causing the security indicator light to behave erratically.

Diagnosing the P1668 Trouble Code

Accurate diagnosis begins by verifying the exact vehicle application and reviewing manufacturer-specific service information.

1. For Ford applications: Check the IDM relay and fuses. Inspect the wiring harness between the PCM and IDM for opens, shorts, or corrosion. Verify that the IDM is receiving proper power and ground.
2. For GM applications: Inspect the alternator L-terminal wiring for damage. Use a multimeter to verify the PWM signal or voltage on the L-terminal circuit between the alternator and the ECM.
3. For Land Rover applications: Inspect the serial link wiring between the anti-theft alarm module and the relevant control units for continuity and proper voltage signals.

Repairing the P1668 Issue

Repairs must follow the confirmed root cause identified during diagnosis.

• Ford Repairs: Repair any damaged wiring in the IDM communication circuit. If the wiring, fuses, and relay test correctly but communication is still lost, replace the Injector Driver Module.
• GM Repairs: Repair damaged wiring in the generator control circuit. If the wiring is intact but the L-terminal signal is missing or out of range, replace the alternator or address ECM issues.
• Land Rover Repairs: Repair any damaged serial link wiring. If the circuit is intact but the fault persists, replace the anti-theft alarm module and perform necessary programming.

Cost Considerations for P1668 Repairs

Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.

For a GM alternator issue, parts and labor are generally straightforward. For a Ford IDM replacement or Land Rover security module replacement, parts costs and required module programming can increase the total repair expense.

Prevention Strategies for Future Occurrences

While electronic module failures cannot always be predicted, you can reduce the risk of circuit and communication faults.

• Inspect exposed engine wiring harnesses, particularly near the alternator and fuel system components, for signs of chafing, heat damage, or corrosion during routine service.
• Ensure all ground connections, especially main engine and module grounds, remain clean and tight to prevent communication dropouts and charging system faults.

Troubleshooting Tips and Best Practices

Follow these practices to avoid misdiagnosis and ensure accurate repairs.

• Do not assume module failure: On Ford applications, a blown fuse, faulty relay, or chafed wire is a frequent culprit. Test the circuit power and ground before replacing the IDM.
• Check for related codes: Look for other communication codes or charging system codes that might indicate a broader wiring harness or network issue.
• Verify your definition: When researching this trouble code, always verify whether your specific vehicle uses the Ford, GM, or Land Rover definition before ordering parts.

More OBD-II Codes

P1655	P1656	P1658	P1660
P1661	P1662	P1671	P1672
P1673	P1674	P1675	P1676
U0100	U0101	U0140	U0155
P0620	P0622	P0562	P0563
P1682	P1683	P1684	P1690

Frequently Asked Questions

Is P1668 a generic or manufacturer-specific code?

No. P1668 is a manufacturer-specific powertrain code. Its definition varies significantly by brand, indicating an IDM communication fault on Ford diesels, an alternator circuit fault on GM, or a security link fault on Land Rover.

Can I drive my vehicle with a P1668 code?

It depends on the application. On GM models, the alternator may fail to charge, risking a dead battery and stalling. On Ford diesels, the engine may not start or could stall unexpectedly, presenting a severe safety risk.

Will disconnecting the battery clear the P1668 code?

Disconnecting the battery might temporarily clear the code. However, it does not repair the underlying communication fault or charging circuit issue, and the code will return once the modules run their self-tests.

Can a bad ground cause a P1668 code?

Yes. Poor engine or module grounds can disrupt the serial communication between the PCM and IDM on Ford vehicles, or disrupt the alternator control signal on GM vehicles, triggering the code.

Does P1668 affect the transmission?

Generally, no. The P1668 code relates strictly to the injector driver module communication, alternator control circuit, or anti-theft serial link. It does not directly affect internal transmission operation.

What Should You Do About the P1668 Code?

When a P1668 code appears, confirm the exact factory definition for your specific vehicle make and model. Because this code targets entirely different systems depending on the manufacturer, applying the wrong diagnostic path will lead to misdiagnosis.

Follow the applicable manufacturer diagnostic procedure to test the relevant system and components. Once the root cause is confirmed, complete the necessary electrical repairs or module replacements before returning the vehicle to normal service.