P0304 — Cylinder 4 Misfire Detected
P0304 sets when cylinder 4 misfire rate exceeds the threshold based on crankshaft speed variation. On transverse 4-cylinder engines cylinder 4 is often the rear-most and hardest to access. Always check for oil in the plug well before installing a new coil. After repair, verify with a complete drive cycle that includes the freeze-frame conditions.
Fault Overview
Ignition component failure on cylinder 4, injector problem, compression loss, wiring issues, carbon tracking, and oil contamination are the primary causes.
Engine & Platform Coverage
Symptoms
- •P0304
- •cylinder 4 misfire
- •#4 misfire
- •P0304 Toyota
- •P0304 Honda
- •P0304 Ford
- •P0304 GM
Possible Causes
- 1.Ignition component failure on cylinder 4
- 2.Injector problem on cylinder 4
- 3.Compression loss on cylinder 4
- 4.Wiring fault specific to cylinder 4
- 5.Carbon tracking
- 6.Oil contamination in plug well
Wiring & Circuit Notes
Cylinder 4 coil and injector circuits and connectors.
DVOM Specs & Sensor Tests
Electrical values for coil and injector. Relative compression. Scope analysis.
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 — Isolate to Cylinder 4: Confirm misfire is isolated to cylinder 4 via scan data.
- 2Step 2 — Spark-Plug Inspection: Remove and inspect the cylinder 4 spark plug.
- 3Step 3 — Coil Swap Test: Swap the coil with another cylinder and retest.
- 4Step 4 — Injector Test: Test or swap the cylinder 4 injector.
- 5Step 5 — Compression Test: Perform a relative compression or leak-down test.
- 6Step 6 — Wiring Check: Inspect wiring and connectors for cylinder 4.
- 7Step 7 — Confirm Repair: Repair the failed component and confirm 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 $40–200; injector $100–500; mechanical 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.