P1668

What Does Code P1668 Mean?

DTC P1668 signifies a communication error between the Powertrain Control Module (PCM) and the Injection Driver Module (IDM). This code is predominantly found in diesel-powered vehicles, particularly Ford models equipped with a separate IDM, such as the 7.3L and 6.0L Power Stroke engines. The PCM is the central control unit responsible for overall engine management, while the IDM is a dedicated module that receives commands from the PCM to control the high-voltage circuits necessary for activating the fuel injectors. When the PCM attempts to communicate with the IDM and does not receive the expected response, or detects corrupted data packets over the dedicated serial data line, it registers P1668. This indicates a critical breakdown in data integrity or a complete loss of communication, directly impacting the PCM’s ability to command precise fuel injection timing and quantity, leading to severe engine performance degradation or a no-start condition.

Common Symptoms

• No-start condition: A complete loss of communication often prevents the engine from starting.
• Rough idle or poor engine running: Intermittent communication or corrupted data can lead to erratic fuel injection.
• Engine misfire: Inconsistent injector firing due to communication issues can cause misfires.
• Reduced engine power: Compromised fuel delivery directly impacts engine output.
• Malfunction Indicator Lamp (MIL) illumination: The Check Engine Light will be illuminated.
• Excessive exhaust smoke: Incorrect fuel injection can result in black (rich) or white (unburnt fuel) smoke.

What Causes the Code P1668?

• Wiring Harness Issues: This is a very common cause, including open circuits, short circuits to ground or voltage, high resistance in the communication wires, or corroded/loose terminals within the connectors between the PCM and IDM.
• Faulty Injection Driver Module (IDM): Internal circuit board failure, processor malfunction, or a power regulation issue within the IDM can prevent it from communicating or responding to PCM commands.
• Faulty Powertrain Control Module (PCM): While less common for this specific code, an internal PCM failure affecting its communication circuit can lead to P1668.
• Power or Ground Supply Issues to the IDM: Insufficient or intermittent power supply (e.g., blown fuse, high resistance in power circuit) or a poor ground connection for the IDM can prevent it from operating and communicating correctly.

How to Diagnose and Troubleshoot

Diagnosis of P1668 requires a methodical approach, utilizing a digital multimeter (DMM), an OBD-II scanner capable of monitoring enhanced PIDs (Parameter Identifiers), and access to a vehicle-specific wiring diagram.

1. Verify Code and Related DTCs: Connect an OBD-II scanner to confirm P1668 is active. Check for any other related codes, particularly those pertaining to IDM power, injector circuits, or other module communication errors, which may provide additional diagnostic direction. Clear all codes and attempt to start the engine to see if P1668 immediately returns.
2. Visual Inspection:
   • Carefully inspect the entire wiring harness connecting the PCM and the IDM. Look for obvious signs of damage, chafing, pinching, corrosion, or loose connections.
   • Pay close attention to the integrity of the connectors at both the PCM and IDM. Check for bent, corroded, or pushed-out pins.
   • Ensure the IDM is securely mounted and its housing appears intact, free from signs of water intrusion or physical damage.
3. IDM Power and Ground Verification:
   • With the ignition in the ON position, use a DMM to measure voltage at the IDM’s primary power input pin(s). Refer to the vehicle’s wiring diagram for specific pin locations. Verify that battery voltage (B+) is consistently present.
   • Measure the resistance between the IDM’s ground pin(s) and a known good chassis ground point. The resistance should be extremely low, typically less than 0.5 ohms, indicating a solid ground connection. High resistance grounds can mimic communication failures.
4. Communication Circuit Integrity Test:
   • Disconnect the main electrical connectors from both the PCM and the IDM to isolate the communication circuit.
   • Using the wiring diagram, identify the specific communication wire(s) between the PCM and IDM (e.g., J1850, CAN bus if applicable, or a dedicated serial data line).
   • Perform a continuity test on the communication wire(s) end-to-end with the DMM. Resistance should be negligible (typically less than 1 ohm).
   • Check for short circuits: Measure resistance from each end of the communication wire to chassis ground and to B+ power wires (with the battery disconnected for safety). Resistance should be infinite (open circuit) in both tests.
5. IDM Self-Test and Data Monitoring: If available with an advanced scanner or factory diagnostic tool (e.g., Ford IDS), attempt to perform an IDM self-test or monitor IDM-specific PIDs. A complete lack of response or erroneous data from the IDM during these tests strongly suggests an internal IDM fault.
6. PCM Communication Output Verification (Advanced): For experienced technicians, an oscilloscope can be used to observe the communication signal output from the PCM on the dedicated data line. This helps determine if the PCM is actually transmitting data, even if the IDM isn’t responding. Requires knowledge of the expected waveform.

Recommended Repairs and Solutions

Addressing DTC P1668 typically involves focused repair or replacement of the identified faulty component:

• Wiring Harness Repair/Replacement: The most frequent resolution for P1668. Any detected opens, shorts, or high-resistance sections in the PCM-to-IDM communication wiring must be meticulously repaired. Use proper automotive wiring repair techniques, including soldering and heat-shrinking for splices, and replacement of corroded or loose terminal pins. Ensure all connectors are fully seated and locked.
• Injection Driver Module (IDM) Replacement: If comprehensive testing of the wiring, power, and ground circuits confirms their integrity, and the IDM shows no signs of communication or fails self-tests, then replacement of the IDM is indicated. Ensure the replacement IDM is the correct part number and is compatible with the vehicle’s application.
• Powertrain Control Module (PCM) Replacement: This is a less common solution for P1668, typically pursued only after all other potential causes (wiring, IDM, power/ground) have been definitively ruled out. A new PCM will require programming with the vehicle’s VIN and potentially other critical data (e.g., immobilizer details) using a factory-level diagnostic tool.
• Mechanic’s Tips:
  • Always consult the specific factory service manual and wiring diagrams for the vehicle year, make, and model you are working on, as pinouts and wire color codes can vary significantly.
  • When diagnosing communication errors, pay extra attention to ground circuits. A high-resistance ground can easily mimic an open communication line.
  • Given that IDMs handle high voltages for injector operation, always disconnect the battery before performing any electrical repairs or disconnections to prevent potential electrical shock or damage to sensitive electronic components.
  • After any repair, clear the Diagnostic Trouble Codes (DTCs) and perform an extended drive cycle under various conditions to confirm that P1668 does not return and that the communication between the PCM and IDM is stable. Monitoring IDM-specific PIDs with an enhanced scanner can help confirm proper function.