P0301 Code: Cylinder 1 Misfire Detected
Last reviewed May 2026 · Reviewed by the Find This Code Editorial Team
Quick Answer
Cylinder 1 isn't firing correctly — combustion isn't happening as it should in that specific cylinder.
At a Glance
What this code means
P0301 is a specific cylinder misfire code — unlike P0300 which indicates random misfires, this code pinpoints cylinder 1 as the problem. The ECU detects the misfire by monitoring small variations in crankshaft speed. When cylinder 1 misfires, that power stroke is weak or absent, causing the crankshaft to slow slightly at that point.
A misfire happens when combustion fails in one cycle. The crankshaft loses speed at that cylinder, which the ECU detects as a misfire event.
Common causes
- Worn or fouled spark plug in cylinder 1
- Failed ignition coil for cylinder 1
- Leaking or clogged fuel injector on cylinder 1
- Low compression in cylinder 1 (worn rings, damaged valve, or blown head gasket)
- Vacuum leak at intake manifold near cylinder 1
- Faulty spark plug wire (on older vehicles with distributor-based ignition)
Symptoms you might notice
- Rough idle or engine vibration
- Check engine light — may flash if misfire is severe
- Engine hesitation on acceleration
- Lower fuel economy
- Possible smell of unburned fuel
Can you still drive?
If the check engine light is flashing, stop driving immediately. A flashing light means an active misfire is damaging the catalytic converter. With a steady light, limit driving and get it checked soon.
How serious is this code?
High if the light is flashing (stop driving). Moderate if the light is steady. Single-cylinder misfires are easier to diagnose and fix than random multi-cylinder misfires.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Check for related codes — a companion code may point to the true root cause
- Inspect wiring, connectors, and grounds around the affected sensor or component
- Look for obvious physical damage: cracks, disconnected hoses, or corrosion
- Clear the code, drive the vehicle, and see if it returns — intermittent codes can come from loose connections
- Verify the part is actually faulty by testing it before replacing it
How to troubleshoot it
- Pull the spark plug from cylinder 1 and inspect it — look for wear, fouling, or damage
- Swap the cylinder 1 ignition coil to another cylinder and clear the code — if the misfire code follows, the coil is bad
- Check cylinder 1 compression with a compression gauge — low compression indicates an internal engine issue
- Use a noid light or injector pulse tester to verify the fuel injector is firing
- Inspect the cylinder 1 area for vacuum leaks
Common mistakes to avoid
These are the most frequent diagnostic errors when dealing with P0301. Avoiding them can save time and money.
- Swapping the ignition coil to another cylinder without also swapping the spark plug — if the misfire follows the coil but not the plug, the plug is fine
- Replacing spark plugs on all cylinders when only cylinder 1 is misfiring — replace and test cylinder 1 first
- Not performing a cylinder-specific compression test to rule out a mechanical cause for a persistent single-cylinder misfire
- Ignoring injector issues — a stuck-closed or leaking injector on cylinder 1 causes the same symptom as ignition failure
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- OBD-II scanner
- Compression tester
- Spark plug socket set
- Multimeter or coil tester
Estimated repair cost range
Spark plug replacement: $20–$100 for all plugs. Ignition coil: $50–$150 per coil. Compression test (shop): $80–$150. Fuel injector replacement: $150–$350.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
The coil-swap test is one of the easiest ways to diagnose a single-cylinder misfire. If P0301 changes to P0302 after swapping coils, you've confirmed the coil is the problem. No special tools needed for this test.
Learn more
How we write these guides
Find This Code guides are written based on OBD-II specification documentation, established automotive diagnostic practices, and real-world patterns observed across vehicle makes and models. Content is reviewed for accuracy and plain-English clarity before publication. Pages are marked with a “Last reviewed” date and updated when diagnostic guidance or code interpretation changes. All content is educational — it is not a substitute for hands-on diagnosis by a qualified mechanic.
Last reviewed May 2026 · Find This Code Editorial Team · Editorial standards →
Sources & references
Our guides are written based on OBD-II specifications, automotive engineering principles, and established diagnostic best practices. Key reference sources include:
- U.S. EPA — On-Board Diagnostics (OBD)Official EPA documentation on OBD-II standards and emissions monitoring requirements for passenger vehicles.
- ASE — Automotive Service ExcellenceIndustry certification body for automotive technicians; sets best-practice diagnostic and repair standards.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.