P1373

What Does Code P1373 Mean?

DTC P1373 signifies an “Ignition Coil – Cylinder 3 – Early Activation Fault.” This code is set by the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), when it detects that the ignition coil for cylinder 3 is activating or discharging significantly earlier than commanded. The ECM precisely controls ignition timing by monitoring crankshaft and camshaft position sensors, engine speed, load, and other parameters to determine the optimal moment to fire each spark plug. For a coil-on-plug (COP) or distributorless ignition system, the ECM directly manages the primary circuit of each ignition coil, dictating when the coil charges (dwell time) and when it discharges to create a spark. When P1373 is stored, the ECM has observed an electrical event (e.g., primary current draw or interruption, indicating a spark event) on cylinder 3’s ignition coil driver circuit occurring prematurely, outside of its calculated and commanded ignition timing window. This anomaly suggests an uncontrolled or incorrect activation of the coil, disrupting the intended combustion cycle for cylinder 3.

Common Symptoms

• Engine Misfire: The most prevalent symptom, leading to rough running, especially at idle or under load, due to incomplete or mistimed combustion in cylinder 3.
• Reduced Engine Performance: Noticeable loss of power and acceleration, as one cylinder is not contributing effectively to engine output.
• Illuminated Check Engine Light (CEL): The Malfunction Indicator Lamp (MIL) will illuminate once the fault is detected and confirmed by the ECM.
• Increased Fuel Consumption: Inefficient combustion due to mistimed spark can lead to more fuel being consumed for less power.
• Poor Idling Quality: A noticeable shake or vibration at idle, indicative of an engine imbalance.
• Potential Backfiring: If the spark occurs substantially early, it could ignite the air/fuel mixture during the intake stroke or prematurely during the compression stroke, potentially causing a backfire through the intake.

What Causes the Code P1373?

• Faulty Ignition Coil (Cylinder 3): An internal electrical short or insulation breakdown within the ignition coil itself can cause it to prematurely discharge or draw current, leading the ECM to detect an “early activation.”
• Wiring Harness Short Circuit: A short in the primary control circuit wire for cylinder 3’s ignition coil. This could be a short to voltage (e.g., battery positive) or a short to another signal wire, causing an unintended or premature trigger signal to the coil. Damage from chafing, heat, or corrosion can lead to such shorts.
• Engine Control Module (ECM/PCM) Failure: A rare but possible cause where the internal ignition driver circuit for cylinder 3 within the ECM has failed, causing it to send an early activation signal or misinterpret feedback from the coil.
• Damaged Coil Connector: Corrosion, loose terminals, or damaged wiring within the connector for cylinder 3’s ignition coil can intermittently short or incorrectly signal the coil.

How to Diagnose and Troubleshoot

Diagnosing P1373 requires a methodical approach, utilizing an OBD-II scanner, digital multimeter (DMM), and potentially an oscilloscope.

1. Retrieve and Analyze DTCs: Use an OBD-II scanner to confirm P1373 and check for any related or accompanying codes (e.g., misfire codes, other ignition-related DTCs). Review freeze frame data to understand engine conditions (RPM, load, temperature) when the fault occurred, which can provide valuable clues.
2. Visual Inspection:
   • Inspect the ignition coil for cylinder 3 for any signs of physical damage, cracks, oil saturation, or carbon tracking.
   • Carefully examine the wiring harness leading to cylinder 3’s ignition coil connector for chafing, exposed wires, signs of rodent damage, or corrosion.
   • Check the coil connector itself for bent or corroded pins and ensure it is securely seated.
3. Ignition Coil Swap Test: If accessible, swap the ignition coil for cylinder 3 with a known good coil from another cylinder (e.g., cylinder 2 or 4). Clear the DTCs and operate the vehicle. If the misfire or “early activation” fault moves to the cylinder where the swapped coil was installed, then the original cylinder 3 coil is faulty. If the fault remains on cylinder 3, proceed to wiring and ECM checks.
4. Electrical System Checks (Using DMM):
   • Power Supply: With the ignition on and the coil connector disconnected, use a DMM to verify battery voltage at the coil’s power supply terminal(s).
   • Ground Circuit: Check for a good ground connection at the coil connector’s ground terminal(s) with the ignition off, ensuring minimal resistance to chassis ground.
   • Control Circuit (Signal Wire): This is crucial. With the coil disconnected and the ignition on (and engine not running), check the control wire from the ECM for any unintended voltage. An “early activation” could be caused by this wire being shorted to voltage. A more advanced test involves using an oscilloscope to monitor the waveform on the control wire while cranking or running the engine (if possible), looking for premature trigger pulses or an incorrect duty cycle compared to other cylinders or specifications.
5. Wiring Harness Continuity and Short Testing:
   • Disconnect the battery and both the cylinder 3 ignition coil connector and the ECM connector (refer to service manual for correct pinouts).
   • Using a DMM, check for continuity on the ignition coil control wire from the coil connector to the ECM connector. Resistance should be very low (typically less than 1 ohm).
   • Test for shorts to ground: With one DMM lead on the control wire terminal at the ECM connector, test for continuity/resistance to a known good chassis ground. There should be infinite resistance (open circuit). Any continuity indicates a short to ground.
   • Test for shorts to voltage: Test the control wire for continuity/resistance to adjacent wires in the harness that carry voltage. There should be infinite resistance.
6. ECM Driver Check: If all other components (coil, wiring) test good, the fault might lie within the ECM’s internal driver for cylinder 3’s ignition coil. This requires specialized diagnostic equipment or a known good ECM for substitution testing. Consult a reputable automotive electrician or dealership for ECM diagnosis.

Recommended Repairs and Solutions

Based on the diagnostic findings, the appropriate repair typically involves one of the following:

• Replace Ignition Coil (Cylinder 3): If the swap test confirms the coil is faulty, replacement of the cylinder 3 ignition coil with an OEM or high-quality aftermarket unit is necessary. It is often recommended to replace spark plugs concurrently, especially if they are due for maintenance or show signs of wear/fouling, as a healthy spark plug contributes to optimal coil function.
• Repair or Replace Wiring Harness: If a short, open circuit, or damaged connector is identified in the wiring leading to cylinder 3’s ignition coil, repair the specific section of the harness or replace the entire harness if damage is extensive. Ensure all connections are secure and weatherproof.
• ECM Replacement or Reprogramming: If advanced diagnostics definitively point to an internal ECM fault (specifically a failure of the cylinder 3 ignition driver circuit), the ECM will need to be replaced and subsequently programmed to the vehicle. This is usually the most expensive solution and should only be pursued after thoroughly ruling out all external factors.

Important Mechanics’ Tips:

• Always disconnect the battery before performing any electrical repairs to prevent damage to sensitive electronic components.
• After any repair, clear the DTCs from the ECM and perform an extended test drive under various operating conditions to confirm the fault has been resolved and that the monitors have run and passed.
• When replacing ignition coils, consider replacing all coils if the vehicle has high mileage, as other coils may soon fail.
• Always use the correct dielectric grease on ignition coil boots to prevent moisture intrusion and ensure good electrical contact.
• Consult the vehicle’s specific service manual for precise wiring diagrams, component locations, and resistance/voltage specifications.