Engine Misfire P030X on 2017 GMC Terrain 2.4L - Spark Plugs or Coils?

ClearTheCode admin answer:

Experiencing P0301 and P0303 misfire codes on your 2017 GMC Terrain 2.4L at 85,000 miles points directly to an ignition system issue, most commonly involving spark plugs or ignition coils. The rough idle and decreased fuel economy are classic symptoms.

1. Understanding P030X Codes:
   • P0301: Misfire detected in cylinder 1.
   • P0303: Misfire detected in cylinder 3.
   • These codes indicate the Engine Control Module (ECM) is detecting a lack of combustion in those specific cylinders.
2. Common Causes:
   • Worn Spark Plugs: Over time, spark plugs wear out, their electrodes erode, and they can't produce a strong enough spark. This is very common around your mileage.
   • Failing Ignition Coils: Each cylinder has its own ignition coil (coil-on-plug setup). If a coil fails, it can't deliver the high voltage needed to fire the spark plug, leading to a misfire.
   • Other Causes (Less Likely for Specific P030X): Fuel injector issues, low compression, vacuum leaks, or wiring problems.
3. Recommended Maintenance Intervals:
   • Spark Plugs: For your 2.4L engine, GMC typically recommends spark plug replacement every 97,500 miles (150,000 km). While you're a bit early, worn plugs can definitely cause misfires.
   • Ignition Coils: Coils generally don't have a strict replacement interval and are usually replaced as needed when they fail.
4. DIY Diagnosis (Swap Test): This is the best way to pinpoint the issue without replacing everything.
   • Tools: Basic socket set, spark plug socket, torque wrench, dielectric grease.
   • Steps:
     • Identify Cylinders: Locate cylinders 1 and 3. (Cylinder 1 is usually at the front of the engine).
     • Swap Coils: Move the ignition coil from cylinder 1 to cylinder 2 (or 4). Do the same for cylinder 3, moving it to cylinder 2 (or 4) if you need another good cylinder. Clear the codes with your OBDII scanner.
     • Drive and Re-scan: Drive the vehicle until the Check Engine light reappears (or for a good 15-20 minutes). Re-scan for codes.
     • Interpret Results:
       • If the misfire codes move with the coils (e.g., now you get P0302 and P0304), then the coils are bad.
       • If the misfire codes remain P0301 and P0303, then the coils are likely good, and the issue is either the spark plugs in those cylinders or something else (like fuel injectors or compression).
     • Swap Plugs (if coils are good): If coils are good, replace the spark plugs in cylinders 1 and 3. If misfires persist, you can try swapping the new plugs with known good ones from other cylinders to confirm.
5. When to See a Shop:
   • If the misfire persists after replacing the suspected faulty plugs and/or coils.
   • If the diagnosis points to fuel injector issues, low compression, or more complex electrical problems.
   • If you're uncomfortable performing the diagnostic swap or replacement yourself. Continued misfires can damage the catalytic converter over time.