P0303 — Cylinder 3 Misfire Detected
P0303 indicates misfire detected specifically on cylinder 3 above the calibrated threshold using crankshaft acceleration data. Swap the coil and plug first — the majority of single-cylinder misfires are ignition related. On EcoBoost engines, check for carbon on the intake valves of the affected cylinder.
Fault Overview
Failed plug or coil on cylinder 3, injector issue, low compression, wiring fault, carbon tracking, oil in plug well, and vacuum leak near cylinder 3 are the primary causes. Use a borescope if compression is low to inspect for valve or piston issues.
Engine & Platform Coverage
Symptoms
- •P0303
- •cylinder 3 misfire
- •#3 cylinder misfire
- •P0303 Ford F150
- •P0303 Chevy
- •P0303 Honda
- •rough running cylinder 3
Possible Causes
- 1.Failed plug or coil on cylinder 3
- 2.Injector issue on cylinder 3
- 3.Low compression on cylinder 3
- 4.Wiring fault specific to cylinder 3
- 5.Carbon tracking
- 6.Oil in plug well
- 7.Vacuum leak near cylinder 3
Wiring & Circuit Notes
Cylinder 3 coil and injector connectors. Check for oil, corrosion, and pin tension.
DVOM Specs & Sensor Tests
Coil and injector electrical values. Relative compression. Scope patterns for ignition and injection.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Step 1 — Confirm Cylinder 3 Misfire Counts: Confirm cylinder 3 misfire counts on the scan tool.
- 2Step 2 — Inspect and Replace Spark Plug: Inspect and replace the spark plug if worn or fouled.
- 3Step 3 — Swap Coil and Retest: Swap the coil with another cylinder and retest misfire counts.
- 4Step 4 — Test Injector: Test or swap the cylinder 3 injector if ignition is proven good.
- 5Step 5 — Relative Compression Test: Perform a relative compression test on cylinder 3.
- 6Step 6 — Wiring Inspection: Inspect wiring and connectors for cylinder 3.
- 7Step 7 — Repair and Verify: Repair the failed component and verify zero misfire counts.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Plug/coil replacement $40–200; injector service $100–500; gasket repair $100–400; compression repair $500–2,000+.
Related Codes & Pages
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.