OBD2 Code P1351: Understanding and Troubleshooting Ignition Coil Control Circuit High Voltage

Navigating the complexities of your vehicle’s onboard diagnostics system can be daunting, especially when you’re faced with a trouble code you don’t recognize. Among these codes, OBD2 code P1351, often indicating “Ignition Coil Control Circuit High Voltage,” can be a cause for concern. But what does this code really mean, and how can you address it? As an auto repair expert at carparteu.com, I’m here to break down this code, helping you understand its implications and guide you through the troubleshooting process.

Understanding OBD2 Code P1351

Code P1351 is triggered when your car’s Powertrain Control Module (PCM) detects an abnormally high voltage in the ignition coil control circuit. To grasp this, let’s first understand the role of the ignition system. In a distributor ignition system, like the one mentioned in the original document for a 1996 Chevrolet Caprice, the PCM relies on signals to calculate and manage spark timing. It receives timing inputs from crankshaft sensors and sends signals to the ignition coil module via the Ignition Control (IC) circuit. These IC circuit pulses prompt the coil module to fire the ignition coil, generating the high voltage needed for spark plugs to ignite the air-fuel mixture in your engine’s cylinders.

The PCM expects the voltage on the IC circuit to be within a specific range, typically between 0.5 and 4.5 volts. Code P1351 sets when the PCM registers a voltage exceeding 4.6 volts on this circuit specifically during engine cranking. This high voltage anomaly signals a potential fault within the ignition coil control system that needs investigation.

Symptoms Associated with Code P1351

When code P1351 is present, you might experience several symptoms, including:

• Check Engine Light (MIL) Illumination: This is the most immediate sign. The Malfunction Indicator Lamp will light up on your dashboard to alert you to a detected issue.
• Engine No-Start Condition: In some cases, the high voltage issue can prevent the engine from starting altogether. The diagnostic steps in the original document start with checking if the engine starts, indicating this is a possible symptom.
• Engine Misfires or Rough Running: If the ignition system is compromised, you might experience engine misfires, leading to rough idling, reduced power, and poor fuel economy. Though not explicitly mentioned as a direct symptom in the provided document, ignition issues often manifest as misfires.
• Injector Disabled for the Ignition Cycle: The original document states, “The injector will be disabled for the entire ignition cycle.” This is a fail-safe action by the PCM to protect the engine or manage the detected fault, and while not directly felt as a symptom by the driver, it’s an important consequence of the code.

Potential Causes of Code P1351

Several factors can lead to a high voltage condition in the ignition coil control circuit and trigger code P1351:

• Faulty Ignition Coil Module: The ignition coil module itself could be malfunctioning, sending out an incorrect voltage signal.
• Wiring Issues in the IC Circuit: Problems such as shorts to voltage, open circuits, or high resistance in the Ignition Control (IC) circuit wiring can cause voltage irregularities. This includes issues with the wiring harness or connectors leading to the ignition coil module.
• PCM Malfunction (Less Likely): While less common, a faulty PCM could incorrectly detect or report high voltage on the IC circuit. This is usually diagnosed after ruling out other potential causes.
• Ground Circuit Issues: An open or poor ground connection for the ignition coil module can also lead to voltage problems in the control circuit. Step 6 in the diagnostic table specifically mentions “Repair the open Ignition Coil Module ground circuit.”

Diagnosing and Troubleshooting Code P1351

Diagnosing code P1351 requires a systematic approach. Here’s a simplified breakdown based on the diagnostic steps from the service manual, geared for a DIY enthusiast or car owner:

1. Initial OBD System Check: Always start with a general OBD system check to see if any other codes are present. Address any other codes first as they might be related or provide further clues.

2. Attempt to Start the Engine: Check if the engine starts. If it doesn’t, this is a critical symptom to note.

3. Voltage Measurement at Ignition Coil Module Connector (Cranking):

   • Turn the ignition OFF and disconnect the ignition coil module harness connector.
   • Use a digital voltmeter (DVM) to measure the DC voltage between terminal B of the connector and ground while cranking the engine.
   • The expected voltage range should be approximately 1.0-4.0 volts, as indicated in Step 3 of the diagnostic table.
     • If the voltage is within range: Proceed to Step 4, which checks the power supply to the ignition coil module.
     • If the voltage is outside the range or excessively high: This indicates a fault in the IC circuit itself. Proceed to Step 5, which directs you to “Repair the IC circuit.” This might involve checking for shorts to voltage in the wiring between the PCM and the ignition coil module.

4. Power Supply Check to Ignition Coil Module:

   • Turn the ignition ON, engine OFF.
   • Use a test light connected to battery positive (B+) to probe terminal C of the ignition coil module harness connector.
     • If the test light illuminates (ON): This confirms power is reaching the module. Proceed to Step 7, suggesting replacement of a potentially faulty Ignition Coil Module.
     • If the test light does not illuminate (OFF): This indicates a problem with the power supply to the module. Proceed to Step 6, “Repair the open Ignition Coil Module ground circuit.” Although Step 6 mentions ground circuit, in the context of Step 4, it’s more likely referring to an open in the power supply circuit to terminal C. Reviewing the specific wiring diagram for your vehicle is crucial here to confirm terminal C function.

5. Circuit and Component Repair/Replacement: Based on the voltage readings and test light results, you’ll be directed to:

   • Repair IC Circuit (Step 5): Address wiring faults like shorts or opens in the IC circuit.
   • Repair Ground Circuit (Step 6): Fix any open ground connections for the ignition coil module.
   • Replace Ignition Coil Module (Step 7): If power and ground are confirmed, and the issue persists, replacing the ignition coil module is the next step.

6. Clear DTC and Verify Repair: After any repair, use a scan tool to clear the DTC (P1351). Start the engine and allow it to reach normal operating temperature. Then, use the scan tool to run the specific DTC test again (if your scan tool has this capability) or operate the vehicle under the conditions that typically set the code.

7. Final DTC Check: After testing, re-scan for DTCs.

   • If code P1351 is gone and no new codes appear: The repair is successful.
   • If code P1351 returns or other codes are present: Continue troubleshooting based on the new findings or repeat the diagnostic steps.

Conclusion

OBD2 code P1351 points to a high voltage issue within the ignition coil control circuit, potentially disrupting your vehicle’s ignition system and engine performance. By understanding the symptoms, potential causes, and following a logical diagnostic process, you can effectively troubleshoot and resolve this issue. Remember to always consult your vehicle’s specific service manual for detailed wiring diagrams and component locations, and if you’re uncomfortable performing these diagnostic steps yourself, it’s best to seek assistance from a qualified automotive technician.