Rough idle & CEL on my 2019 WRX - Misfire codes?

ClearTheCode admin answer:

It sounds like you're on the right track with suspecting spark plugs or ignition coils for your 2019 WRX with P0301 and P0303 misfire codes. At 55,000 miles, it's definitely within the range where spark plugs could be due for replacement, especially if they haven't been changed before.

Here's a breakdown and some steps:

1. Spark Plug Interval: For the FA20 DIT engine in your WRX, the recommended spark plug replacement interval is typically every 60,000 miles. So, at 55,000 miles, you're right on schedule.
2. Diagnosis Steps (DIY):
   • Confirm Misfire: Since you have codes P0301 and P0303, focus on cylinders 1 and 3.
   • Swap Coils: A common diagnostic step is to swap the ignition coils from the misfiring cylinders (1 and 3) with coils from non-misfiring cylinders (e.g., 2 and 4). Clear the codes, drive the car, and re-scan. If the misfire codes follow the coils (e.g., now you get P0302 and P0304), then the coils are the issue. If the codes remain P0301 and P0303, the problem is likely the spark plugs or fuel injectors.
   • Inspect Spark Plugs: If the codes don't follow the coils, remove the spark plugs from cylinders 1 and 3. Inspect them for wear, fouling, or damage. Worn plugs can cause misfires and rough idle.
3. DIY Spark Plug Replacement Tips for WRX (FA20):
   • Tools: You'll need a spark plug socket (thin-walled, 14mm for FA20), various extensions, a universal joint, and a torque wrench. A magnetic or rubber-insert spark plug socket is highly recommended.
   • Access: Yes, it's tight. You'll likely need to remove the airbox on the passenger side and potentially the battery/tray on the driver's side for better access. Some people also unbolt the engine mounts and slightly lower the engine for more clearance, but this is often not necessary if you have the right tools and patience.
   • Order: Work on one cylinder at a time to avoid mixing up coils or plugs.
   • Torque: Always torque spark plugs to the manufacturer's specification (typically around 15-18 ft-lbs for these plugs) to prevent damage to the cylinder head threads or plugs coming loose.
   • Anti-Seize: Some people use a tiny amount of anti-seize on the plug threads, but many modern plugs come with a special coating, so check the plug manufacturer's recommendations.

Causes of Misfires:

• Worn or fouled spark plugs.
• Faulty ignition coils.
• Clogged or faulty fuel injectors.
• Vacuum leaks.
• Low compression (less common at 55k miles).

When to See a Shop:

• If replacing spark plugs and swapping coils doesn't resolve the issue.
• If you're uncomfortable with the tight access or specialized tools required.
• If the issue persists and you suspect fuel injector problems or more serious engine issues. Ignoring misfires can lead to catalytic converter damage.