Last updated: July 6, 2026. P0301 means Cylinder 1 Misfire Detected. The engine computer has detected misfire activity on cylinder number one. Common causes include a worn spark plug, weak ignition coil, damaged coil boot or plug wire, fuel injector issue, vacuum leak near cylinder 1, low compression, wiring fault, or mechanical engine problem.
P0301 can be serious. If the check engine light flashes, the engine shakes hard, power drops, or raw fuel smell appears, stop driving when safe. Active misfire can overheat and damage the catalytic converter. Save freeze frame and diagnose before clearing codes.
P0301 Quick Facts
| Item | What it means |
|---|---|
| Code | P0301 |
| Definition | Cylinder 1 Misfire Detected |
| Main area | Ignition, fuel, air, compression, or mechanical issue on cylinder 1 |
| Common causes | Spark plug, coil, boot/wire, injector, vacuum leak, compression, wiring |
| Severity | High if check engine light flashes or engine shakes |
| Best scanner data | Freeze frame, misfire counters, fuel trims, RPM/load, pending codes |
P0301 is different from P0300. P0300 means random or multiple cylinder misfire. P0301 points to cylinder 1 specifically. That makes diagnosis more focused, but you still need data before replacing parts.
What P0301 Means
A misfire happens when a cylinder does not burn the air-fuel mixture correctly. The computer detects crankshaft speed changes and stores a code when the misfire pattern points to a specific cylinder. With P0301, cylinder 1 is the cylinder being flagged.
Cylinder 1 location depends on engine design. Do not guess. Confirm cylinder numbering for your exact engine before swapping coils, plugs, or injectors. Replacing the wrong cylinder part wastes time and can hide the real fault.
Common Symptoms
- Check engine light on.
- Flashing check engine light under load.
- Engine shaking or rough idle.
- Hesitation during acceleration.
- Loss of power.
- Fuel smell from exhaust.
- Poor fuel economy.
- Hard start or stumble after startup.
If P0301 appears with lean codes like P0171 or P0174, check fuel trims and vacuum leaks before replacing several ignition parts. A lean condition can trigger misfire on one cylinder first.
Common Causes of P0301
| Cause | Why it creates P0301 | First check |
|---|---|---|
| Worn spark plug | Weak or inconsistent spark on cylinder 1 | Plug age, gap, fouling, cracks |
| Weak ignition coil | Spark breaks down under load | Coil swap test if accessible |
| Bad coil boot or plug wire | Spark leaks before reaching plug | Boot tracking, oil, burns, cracks |
| Fuel injector issue | Cylinder 1 gets wrong fuel amount | Injector connector, balance, click, resistance where applicable |
| Vacuum leak near cylinder 1 | One cylinder runs lean | Intake gasket, hose, smoke test |
| Low compression | Cylinder cannot make power | Compression or leak-down test |
| Mechanical problem | Valve, ring, cam, or head-gasket issue | Compression, leak-down, borescope clues |
How to Diagnose P0301
Use a cylinder-specific process. The goal is to see whether the misfire follows a moved part, stays on cylinder 1, or changes with fuel trim and load.
- Scan all codes. Record P0301 plus any P0300, fuel trim, oxygen sensor, MAF, injector, cam/crank, or catalyst codes.
- Save freeze frame. Note RPM, load, speed, coolant temperature, fuel trims, and whether the code set at idle or under load.
- Confirm cylinder 1 location. Use correct firing order/cylinder layout for exact engine.
- Inspect spark plug and coil boot. Look for worn plug, wide gap, oil, coolant staining, carbon tracking, cracks, or loose connection.
- Swap ignition coil if safe and accessible. Move cylinder 1 coil to another cylinder. If code follows, coil is suspect.
- Swap spark plug if needed. If plug is questionable, move or replace with correct type and gap.
- Check fuel trims. High positive trims can point to lean condition instead of pure ignition failure.
- Check injector and wiring. Inspect connector, harness, and injector operation before replacing injector.
- Perform compression test if misfire stays on cylinder 1. Mechanical faults must be ruled out before repeated parts replacement.
- Clear codes after repair and road test. Confirm P0301 and pending misfire do not return.
If your scanner can show misfire counters, use them. A scanner with live data can help show whether cylinder 1 misfires mostly at idle, under load, when cold, or when warm. Compare tool features in our OBD2 scanner with live data guide.
Scanner Data to Check
| Data point | Why it matters |
|---|---|
| Freeze frame | Shows condition when P0301 set |
| Misfire counters | Confirms cylinder 1 pattern if supported |
| RPM and load | Separates idle misfire from load misfire |
| STFT and LTFT | Shows lean or rich correction clues |
| Coolant temperature | Shows cold or hot misfire pattern |
| MAF data | Helps with airflow and lean clues |
| Pending codes | Shows early return before light comes back |
Phone-based scanners can help when they graph fuel trims and save freeze frame. If you prefer app diagnostics, compare the best Bluetooth OBD2 scanner options.
Common Mistakes
- Replacing all coils before testing cylinder 1.
- Not confirming cylinder numbering.
- Clearing code before saving freeze frame.
- Ignoring fuel trims and vacuum leaks.
- Replacing injector before checking spark plug, coil, and connector.
- Skipping compression test when ignition and fuel checks pass.
- Driving with flashing check engine light.
Can I Drive With P0301?
Do not keep driving if check engine light flashes, engine shakes badly, power drops, or raw fuel smell appears. Active misfire can damage catalytic converter fast.
If light is steady and engine runs mostly normal, short gentle driving may be possible, but diagnose soon. A single-cylinder misfire can worsen and cause expensive converter damage if ignored. If catalyst code appears later, read the P0420 code guide before replacing converter parts.
Related Codes
P0301 can appear with P0300, P0302-P0308, P0171, P0174, P0101, injector circuit codes, cam/crank sensor codes, or catalyst codes. If airflow data looks wrong, use the P0101 code guide. If the scanner shows multiple code types, read stored vs pending vs permanent codes before clearing anything.
Final Advice
For P0301, confirm cylinder 1 location, save freeze frame, inspect plug and coil, use swap testing where possible, check trims, then test injector and compression if misfire stays on cylinder 1. Replace parts only when test results point there.
If you need a scanner for freeze frame, pending codes, fuel trims, and misfire clues, start with the main best OBD2 scanner guide.
FAQ
What is most common cause of P0301?
Common causes include worn spark plug, weak ignition coil, damaged coil boot or plug wire, injector issue, vacuum leak near cylinder 1, or low compression.
Can I swap coils to diagnose P0301?
Yes, when coils are accessible and same type. Move cylinder 1 coil to another cylinder. If misfire follows coil, coil is likely problem.
Can bad injector cause P0301?
Yes. A clogged, leaking, or electrically faulty injector can cause cylinder 1 misfire. Check ignition and wiring before injector replacement.
Is P0301 safe to drive?
Not if check engine light flashes or engine shakes badly. Stop driving when safe because active misfire can damage catalytic converter.
Written by Michael Hayes
Written by Michael Hayes, editor of Best OBD2 Scanner. Michael researches OBD2 scanners, diagnostic apps, trouble codes, live data features, and vehicle compatibility so readers can choose tools with fewer surprises.
