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2018 CT6 3.6L Rough Idle and Misfire - P0301, P0303, P0305 Codes

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Angela R. · rep 0 · May 22, 3:56 AM · 1 replies · solved · Question

Hey everyone,

My 2018 Cadillac CT6 with the 3.6L V6 has developed a really rough idle lately. It feels like it's shaking quite a bit, especially when I'm stopped at a light. The check engine light came on, and when I scanned it, I got P0301, P0303, and P0305 – multiple cylinder misfire codes, specifically for cylinders 1, 3, and 5.

The car still drives, but acceleration feels a bit sluggish, and I'm worried about potential damage. I've heard these engines can be sensitive. What should I be looking at first? Could it be spark plugs, or something more serious like coil packs or even injectors?

Any advice on diagnosing this further before I take it to a shop would be greatly appreciated. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:56 AM

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ClearTheCode guidance (Cadillac · Diagnostics & codes)

Summary: For 2018 CT6 3.6L Rough Idle and Misfire - P0301, P0303, P0305 Codes, 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: Hey everyone,

My 2018 Cadillac CT6 with the 3.6L V6 has developed a really rough idle lately. It feels like it's shaking quite a bit, especially when I'm stopped at a light. The check engine light came on, and when I scanned it, I got P0301, P0303, and P0305 – multiple cylinder misfire codes, specifically for cylinders 1, 3, and 5.

The car still drives, but acceleration feels a bit sluggish, and…

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

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