P1266

What Does Code P1266 Mean?

DTC P1266 indicates a detected electrical fault within the ignition coil circuit for cylinder #6, specifically identifying a “High To Low Side Short.” This means the Engine Control Module (ECM), also commonly referred to as the Powertrain Control Module (PCM), has identified an unexpected electrical connection or path between the high-voltage (secondary) side of the ignition coil and the low-voltage (primary) side or ground circuit for cylinder #6. The ECM monitors the primary winding’s current flow and the secondary circuit’s integrity to ensure proper spark delivery. When an abnormal current draw, voltage drop, or continuity is detected that suggests a direct short circuit between the high-voltage output (which supplies power to the spark plug) and the coil’s low-voltage input or chassis ground, the ECM registers P1266. This short circuit prevents the coil from building sufficient magnetic field to generate a high-voltage spark, leading to a misfire condition in cylinder #6. The subsystem primarily affected is the ignition system, directly impacting combustion efficiency and engine performance for that specific cylinder.

Common Symptoms

• Check Engine Light (CEL) illumination: The most immediate and common symptom.
• Engine Misfire: Noticeable rough running, especially at idle or under load, specifically originating from cylinder #6.
• Reduced Engine Performance: A noticeable loss of power, hesitation, or stumbling during acceleration.
• Rough Idling: The engine may shake or vibrate excessively due to the absence of combustion in one cylinder.
• Decreased Fuel Economy: Due to incomplete combustion and the ECM attempting to compensate.
• Engine Hesitation or Stalling: In severe cases, particularly if the misfire is constant and significant.
• Unusual Exhaust Odor: Unburnt fuel passing into the exhaust due to lack of ignition in cylinder #6.

What Causes the Code P1266?

• Faulty Ignition Coil (Cylinder #6): This is the most prevalent cause. An internal short circuit can develop within the coil itself, between the primary and secondary windings, or from the secondary winding to the coil’s housing or internal ground. This prevents proper voltage buildup and discharge.
• Damaged Wiring Harness: Chafed, cut, or otherwise compromised wiring in the ignition coil primary or secondary circuit for cylinder #6 can lead to a direct short to ground or to another circuit. This includes wires that may have become brittle, cracked, or melted due to heat or abrasion.
• Corroded or Damaged Electrical Connector: The connector plug for ignition coil #6 can suffer from bent pins, corrosion (oxidation), or loose terminals, creating an unintended electrical path or short circuit within the connector itself.
• Faulty Spark Plug (Cylinder #6): While less direct, a severely fouled or cracked spark plug can sometimes lead to a secondary circuit short, putting excessive strain on the coil and potentially causing an internal breakdown or mimicking a short.
• Engine Control Module (ECM/PCM) Failure: Although rare, an internal fault within the ECM’s ignition driver circuit for cylinder #6 could lead to this code. This would typically involve a short to ground within the driver itself, constantly applying power or creating an erroneous ground path.

How to Diagnose and Troubleshoot

Diagnosis of P1266 requires careful electrical testing and methodical troubleshooting:

1. Retrieve and Analyze DTCs and Freeze Frame Data: Use an OBD-II scanner to confirm P1266. Check for any accompanying misfire codes (e.g., P0306 – Cylinder 6 Misfire) or other related DTCs. Review freeze frame data to understand engine conditions (RPM, load, temperature) when the fault occurred, which can provide clues. Clear codes after initial review.
2. Visual Inspection:
   • Inspect ignition coil #6 for any signs of physical damage, such as cracks, carbon tracking, burn marks, or oil contamination.
   • Examine the wiring harness leading to coil #6 for chafing, cuts, melting, or evidence of rodent damage. Pay close attention to areas where the harness might rub against engine components or sharp edges.
   • Check the electrical connector for coil #6 for corrosion, bent pins, pushed-out terminals, or loose connections. Ensure it is securely seated.
   • Inspect the spark plug for cylinder #6 for cracks in the ceramic insulator, heavy fouling, or excessive gap that could strain the coil.
3. Resistance Testing of Ignition Coil (using a Digital Multimeter – DMM):
   • Disconnect the electrical connector from ignition coil #6.
   • Following manufacturer specifications, measure the resistance of the coil’s primary winding (typically between the positive power terminal and the trigger/ground control terminal). Compare to known good specifications.
   • Measure the resistance of the coil’s secondary winding (often between the high-voltage output tower and a primary ground terminal or dedicated secondary ground). A “high to low side short” often manifests as abnormally low resistance or continuity where very high resistance (Megaohms) or an open circuit is expected. Some coils are designed as “coil-on-plug” units with integrated igniters and may not have easily accessible secondary resistance test points; consult service data.
   • Test for continuity or resistance between the high-voltage output terminal (spark plug boot side) and the coil’s mounting bracket or internal ground. Any continuity or very low resistance indicates an internal short within the coil.
4. Wiring Harness Continuity and Short-to-Ground Testing (using a DMM):
   • With the ignition OFF and the coil #6 connector disconnected from the coil and the ECM/PCM connector disconnected (to isolate the wiring from both components), perform continuity checks.
   • Check for continuity between the power wire at the coil connector and battery positive (with battery connected) or relevant fused power source (confirming power supply).
   • Check for continuity between the ground wire at the coil connector and a known good chassis ground (confirming ground integrity).
   • Check for continuity between the signal wire (from ECM) at the coil connector and its corresponding pin at the ECM connector.
   • Critically, check for continuity between the signal wire, power wire, and ground wire at the coil connector to chassis ground. Any continuity on the power or signal wire to ground indicates a short in the harness. Also check for shorts between individual wires in the harness.
5. Power and Ground Supply Verification (using a DMM):
   • With the ignition ON (engine OFF) and the coil #6 connector disconnected, verify proper voltage at the power supply wire terminal (usually battery voltage).
   • Verify a good ground connection at the ground wire terminal using the DMM’s continuity or voltage drop test to the battery negative post.
6. Ignition Coil Swap Test: If an identical ignition coil is used on another cylinder (e.g., cylinder #5), swap coil #6 with the known good coil from the other cylinder. Clear the DTCs, start the engine, and re-scan. If the P1266 code moves to the new cylinder (e.g., P1265), the coil itself is confirmed faulty. If P1266 persists for cylinder #6, the issue lies in the wiring harness, connector, or potentially the ECM.
7. ECM/PCM Driver Circuit Testing (Advanced): If all other tests prove inconclusive, and only for experienced technicians with an oscilloscope, monitor the primary trigger signal waveform from the ECM to coil #6. An improper or missing waveform, or a waveform indicating a constant short, could point to an internal ECM driver failure.

Recommended Repairs and Solutions

Once the root cause of P1266 is identified, the repair generally involves replacement or repair of the faulty component:

• Replace Ignition Coil #6: If resistance tests, visual inspection, or the swap test confirm the ignition coil is internally shorted or defective, replace it with a new, high-quality, OEM-spec coil. It’s often recommended to replace spark plugs at the same time, especially if they are due for service or if the old coil’s failure was potentially exacerbated by a worn plug.
• Repair or Replace Wiring Harness: If a short or damage is found in the wiring harness to coil #6, repair the affected section. This may involve soldering new wires, using heat-shrink tubing for insulation, and ensuring the harness is properly routed and secured to prevent future damage. If the damage is extensive, a section of the harness or the entire engine harness may need replacement.
• Repair or Replace Electrical Connector: If the connector for coil #6 is corroded, damaged, or has loose pins, clean the terminals thoroughly or replace the connector housing and its pins. Ensure a secure, clean connection.
• Replace Spark Plug (Cylinder #6): If the spark plug was found to be severely fouled, cracked, or excessively worn, replace it. While not the direct cause of a “high to low side short” in the coil itself, a bad plug can stress the coil and contribute to its failure or mimic a secondary circuit issue.
• ECM/PCM Replacement (Last Resort): If all other components have been thoroughly tested and verified as functional, and advanced diagnostics point specifically to an internal fault within the ECM’s ignition driver for cylinder #6, the ECM/PCM may need replacement. This is typically an expensive repair and often requires reprogramming the new module to the vehicle. Ensure all other possibilities are exhausted first.

Mechanic’s Tips:

• Always disconnect the vehicle’s battery before performing electrical repairs to prevent accidental shorts or damage to the ECM/PCM.
• When installing a new ignition coil, ensure the connection to the spark plug is firm and secure, and apply a small amount of dielectric grease to the spark plug boot to aid in installation and prevent moisture intrusion.
• Use OEM or reputable aftermarket brands for replacement ignition coils. Cheap, low-quality coils can fail prematurely or not perform to specification, leading to recurring issues.
• After any repair, clear the DTCs from the ECM/PCM and perform an extended test drive under varying load and speed conditions to ensure the fault does not return and the readiness monitors reset.