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2018 F-150 3.5L EcoBoost Misfire - P0303 and rough idle

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Jennifer H. · rep 0 · May 22, 2:23 AM · 1 replies · solved · Question

My 2018 Ford F-150 Lariat, with the 3.5L EcoBoost, has been acting up lately. I've got a check engine light on, and it feels like it's running really rough, especially at idle or low speeds.

I pulled the codes with my OBD-II scanner, and it's showing P0303, which points to a misfire on cylinder 3. I'm wondering if it's just a spark plug or something more serious.

Any ideas on what I should check first? The truck has about 80k miles on it.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 2:23 AM

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ClearTheCode guidance (Ford · Diagnostics & codes)

Summary: For 2018 F-150 3.5L EcoBoost Misfire - P0303 and rough idle, 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 Ford F-150 Lariat, with the 3.5L EcoBoost, has been acting up lately. I've got a check engine light on, and it feels like it's running really rough, especially at idle or low speeds.

I pulled the codes with my OBD-II scanner, and it's showing P0303, which points to a misfire on cylinder 3. I'm wondering if it's just a spark plug or something more serious.

Any ideas on what I should check…

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

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