OBD2 P1351 Code: Understanding Ignition Coil Control Circuit High Voltage

The Obd2 P1351 code indicates a problem within your vehicle’s ignition system, specifically pointing to a “Ignition Coil Control Circuit High Voltage” issue. This diagnostic trouble code (DTC) is triggered when your car’s Powertrain Control Module (PCM) detects an abnormally high voltage in the control circuit for the ignition coil. Understanding what this code means, its potential causes, symptoms, and how to diagnose and fix it is crucial for maintaining your vehicle’s performance and preventing potential engine damage.

Decoding DTC P1351: Ignition Coil Control Circuit High Voltage

To understand P1351, let’s break down the components involved. Your vehicle’s ignition system relies on ignition coils to generate the high voltage spark needed to ignite the air-fuel mixture in the engine cylinders. The PCM controls these coils, telling them when to fire to ensure proper engine timing and combustion. The “Ignition Coil Control Circuit” is the pathway through which the PCM sends signals to the ignition coil module.

When the PCM detects a voltage reading exceeding a certain threshold (in this case, above 4.6 volts during cranking, as specified in the service manual for some models like the 1996 Chevrolet/Geo Caprice), it sets the P1351 code. This high voltage reading suggests a potential fault in the control circuit itself or within the components connected to it.

Symptoms Associated with OBD2 P1351

While the P1351 code itself points to a specific electrical issue, it can manifest in various symptoms that affect your vehicle’s driveability. Common symptoms include:

• Check Engine Light (MIL) Illumination: This is the most immediate and obvious symptom. The malfunction indicator lamp will light up on your dashboard to alert you to a problem.
• Engine No-Start Condition: In some cases, a high voltage issue in the ignition control circuit can prevent the engine from starting altogether. If the ignition coils are not firing correctly, combustion cannot occur, and the engine won’t run.
• Engine Misfires: If the issue is intermittent or affecting only some cylinders, you might experience engine misfires. This can result in rough idling, reduced engine power, and poor fuel economy.
• Poor Engine Performance: Even if the engine starts and runs, a problem in the ignition control circuit can lead to overall poor engine performance, hesitation during acceleration, and lack of power.

Potential Causes of P1351 Code

Several factors can contribute to a P1351 code. Pinpointing the exact cause requires systematic diagnosis, but common culprits include:

• Faulty Ignition Coil Module: The ignition coil module itself could be malfunctioning, causing it to send or receive incorrect voltage signals. This is a prime suspect, especially given the diagnostic steps in the original document pointing to module replacement as a potential solution.
• Wiring Issues in the IC Circuit: Problems within the Ignition Control (IC) circuit wiring are a frequent cause. This includes:
  • Short to Voltage: A wire in the IC circuit could be shorting to a voltage source, leading to the high voltage reading detected by the PCM.
  • Open Circuit: Although the code is “high voltage,” an open circuit or high resistance in the ground circuit of the ignition coil module could indirectly cause a high voltage reading on the control signal wire as the PCM attempts to compensate.
  • Damaged Wiring or Connectors: Corrosion, damage, or loose connections in the wiring harness or connectors associated with the ignition coil module can disrupt the circuit and cause voltage irregularities.
• PCM Malfunction (Less Likely): While less common, a faulty PCM could also be the source of the problem. If the PCM is sending incorrect signals or misinterpreting voltage readings, it might incorrectly set the P1351 code.

Diagnosing the P1351 Code: A Step-by-Step Approach

Diagnosing the P1351 code effectively requires a systematic approach. Here’s a simplified diagnostic process based on the service manual steps, suitable for experienced DIYers and professional technicians:

1. OBD-II System Check: Begin by performing a general OBD-II system check using a scan tool. Verify that P1351 is the only code present or note any other related codes that might offer additional clues.

2. Attempt to Start the Engine: Try starting the engine. Note whether the engine starts and runs or if it fails to start. This helps narrow down the severity and nature of the problem.

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

   • Turn the ignition OFF and disconnect the ignition coil module harness connector.
   • Use a Digital Voltmeter (DVM) set to DC voltage and Peak Min/Max mode.
   • Measure the voltage between the Ignition Coil Module harness connector terminal B and ground while cranking the engine.
   • Expected Reading: According to the service manual, the voltage should be within the range of 1.0-4.0 volts.
   • Interpretation:
     • Voltage within range: Proceed to step 4.
     • Voltage outside range or no voltage: Indicates a problem with the IC circuit itself, potentially a short or open. Go to step 5.

   Alt text: Measuring voltage at terminal B of the ignition coil module connector with a digital voltmeter during engine cranking to diagnose OBD2 P1351 code.

4. Check for Power at Ignition Coil Module Connector (Ignition ON):

   • Turn the ignition ON, engine OFF.
   • Use a test light connected to battery positive (B+).
   • Probe the Ignition Coil Module harness connector terminal C with the test light.
   • Expected Result: The test light should illuminate brightly.
   • Interpretation:
     • Test light ON: Power supply to the module is good. Proceed to step 7 (potential module replacement).
     • Test light OFF: Indicates a problem with the power supply circuit to the ignition coil module. Go to step 6.

5. Repair IC Circuit (If Voltage Out of Range in Step 3): If the voltage reading in step 3 was incorrect, the next step is to diagnose and repair the IC circuit. This may involve:

   • Checking for shorts to voltage in the wiring.
   • Inspecting for open circuits or high resistance in the ground wire or signal wire.
   • Repairing or replacing damaged wiring or connectors.

6. Repair Open Ignition Coil Module Ground Circuit (If Test Light OFF in Step 4): If the test light did not illuminate in step 4, it suggests an open or faulty ground circuit for the ignition coil module. Inspect and repair the ground wire and its connections.

7. Replace Faulty Ignition Coil Module (If Power and Circuit are OK): If steps 3 and 4 indicate that the wiring and power supply are functioning correctly, the ignition coil module itself is likely faulty and needs replacement.

8. Clear DTCs and Verify Repair:

   • After performing any repairs or component replacements, use a scan tool to clear the OBD2 DTCs, including P1351.
   • Start the engine and allow it to reach normal operating temperature.
   • Use the scan tool to run the diagnostic test for P1351 again (if available) or operate the vehicle under the conditions that originally set the code (if known).
   • Verification:
     • Test Passed (Scan Tool) or Code Does Not Return: The repair was successful. Proceed to step 9.
     • Code P1351 Returns: Return to step 2 and re-evaluate the diagnosis and repair steps.

9. Check for Other DTCs: Perform a final scan for any remaining DTCs that may not have been addressed. Diagnose and repair any new codes as necessary. If no other codes are present, the system is likely OK.

Resolving OBD2 Code P1351

The resolution to a P1351 code depends entirely on the underlying cause identified during the diagnostic process. Potential solutions include:

• Wiring Repair: Repairing shorts, opens, or damage in the Ignition Control circuit wiring and connectors. This could involve splicing wires, replacing connectors, or repairing chafed insulation.
• Ignition Coil Module Replacement: Replacing a faulty ignition coil module. Ensure you use a compatible replacement part for your vehicle.
• PCM Repair or Replacement (Rare): In rare cases, if the PCM is determined to be faulty, it may require reprogramming or replacement. This is generally a last resort after ruling out all other possibilities.

Prevention and Maintenance

While some electrical issues are unavoidable, regular vehicle maintenance can help prevent ignition system problems and potentially reduce the risk of P1351 and related codes. Consider these preventative measures:

• Regular Spark Plug Replacement: Follow your vehicle manufacturer’s recommended service intervals for spark plug replacement. Worn or fouled spark plugs can put extra stress on the ignition system.
• Inspect Wiring Regularly: Periodically inspect the wiring harness and connectors in the engine bay for signs of damage, corrosion, or wear. Address any issues promptly.
• Proper Battery Maintenance: Ensure your vehicle’s battery is in good condition and properly charged. A weak or failing battery can cause various electrical system problems.

Conclusion

The OBD2 P1351 code signals a “Ignition Coil Control Circuit High Voltage” issue, indicating a potential problem in your vehicle’s ignition system control circuitry. By understanding the symptoms, potential causes, and diagnostic steps outlined above, you can effectively troubleshoot and resolve this code. Remember to follow a systematic diagnostic approach, and if you’re not comfortable working with automotive electrical systems, it’s always best to consult a qualified mechanic for professional diagnosis and repair.

Alt text: An OBD2 scanner tool being used to diagnose car trouble codes, including P1351, for automotive repair and troubleshooting.