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2018 340i Rough Idle & Misfire Codes (P0301, P0300)

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David L. · rep 0 · May 22, 4:04 AM · 1 replies · solved · Question

My 2018 BMW 340i (B58 engine) recently started acting up. I'm getting a rough idle, especially when cold, and it feels like it's losing power under acceleration. The Check Engine Light came on, and when I scanned it, I got P0301 (Cylinder 1 Misfire Detected) and P0300 (Random/Multiple Cylinder Misfire Detected).

I've heard about ignition coil issues on these, but I'm not sure where to start. The car has about 65,000 miles on it. I replaced the spark plugs about 15,000 miles ago, so I'm hoping it's not them already.

Any ideas on what I should check first before taking it to the dealer? I'm comfortable doing some basic DIY.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:04 AM

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ClearTheCode guidance (Bmw · Diagnostics & codes)

Summary: For 2018 340i Rough Idle & Misfire Codes (P0301, P0300), work through these steps on your 2016+ vehicle:

  1. Confirm the symptom — note when it happens (cold start, highway, after fuel fill, etc.).
  2. Scan codes — save all pending/stored DTCs; don't clear until you've recorded them.
  3. Check TSBs & recalls — look up your VIN for open campaigns and known fixes.
  4. Inspect the obvious — related fuses, connectors, fluid level, and recent repairs.
  5. Test the common fix — follow model-specific service procedures; one change at a time.
  6. Escalate if needed — burning smell, no brakes, overheating, or stalling in traffic → shop visit.

Owner reported: My 2018 BMW 340i (B58 engine) recently started acting up. I'm getting a rough idle, especially when cold, and it feels like it's losing power under acceleration. The Check Engine Light came on, and when I scanned it, I got P0301 (Cylinder 1 Misfire Detected) and P0300 (Random/Multiple Cylinder Misfire Detected).

I've heard about ignition coil issues on these, but I'm not sure where to start. The …

This is educational guidance, not a substitute for hands-on diagnosis.

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