P0301 — Cylinder 1 Misfire Detected
P0301 sets when the ECM detects a misfire rate exceeding the threshold specifically on cylinder 1. Cylinder-specific swap testing isolates the exact failed component without replacing an entire bank of parts. After repair, clear misfire counters and verify with a road test under the freeze-frame conditions.
Fault Overview
Failed spark plug or coil on cylinder 1, clogged or failed #1 injector, low compression on cylinder 1, wiring fault specific to cylinder 1, vacuum leak nearest cylinder 1, and carbon tracking on the plug boot are the primary causes. On many engines the #1 plug well collects oil from valve-cover gasket leaks.
Engine & Platform Coverage
Symptoms
- •P0301
- •cylinder 1 misfire
- •#1 cylinder misfire
- •rough idle cylinder 1
Possible Causes
- 1.Failed spark plug or coil on cylinder 1
- 2.Clogged or failed #1 injector
- 3.Low compression on cylinder 1
- 4.Wiring fault specific to cylinder 1
- 5.Vacuum leak nearest cylinder 1
- 6.Carbon tracking on plug boot
- 7.Oil in plug well from valve-cover gasket
Wiring & Circuit Notes
Inspect coil-on-plug connector and injector connector for cylinder 1. Check for oil in the spark-plug well. Verify power, ground, and control signals at the coil and injector.
DVOM Specs & Sensor Tests
Coil primary resistance and current. Injector resistance. Relative compression on cylinder 1. Scope patterns for ignition and injector pulse.
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 Misfire Is Isolated to Cylinder 1: Confirm the misfire is isolated to cylinder 1 via scan-tool misfire counter data.
- 2Step 2 — Inspect and Replace Spark Plug if Worn or Fouled: Inspect and replace the spark plug if worn, fouled, or oil-contaminated. Check the plug well for oil.
- 3Step 3 — Swap Coil with Another Cylinder and Retest: Swap the #1 coil with another cylinder and retest. If the misfire moves to the new cylinder, the coil is bad.
- 4Step 4 — Swap or Test #1 Injector: Swap or test the #1 injector for proper resistance and contribution.
- 5Step 5 — Perform Relative Compression or Leak-Down on Cylinder 1: Perform relative compression or leak-down on cylinder 1 if plug, coil, and injector are proven good.
- 6Step 6 — Check Wiring and Connectors Specific to Cylinder 1: Check wiring and connectors specific to cylinder 1 for opens, shorts, or poor pin tension.
- 7Step 7 — Repair and Verify Zero Misfire Counts: Repair the failed component and verify zero misfire counts on a road test.
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
Replace spark plug $10–40; replace coil $40–200; clean or replace injector $80–400; repair valve-cover gasket $100–400; address compression issues $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.