P0301 — Cylinder 1 Misfire Detected
The engine control module has detected a combustion misfire isolated to cylinder 1.
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What the P0301 code means
P0301 means the misfire is confined to one cylinder — cylinder 1. That precision makes diagnosis easier: the problem is in that cylinder's spark, fuel, air, or compression.
Cylinder numbering varies by manufacturer, so confirm the firing order and cylinder layout for the specific engine before testing. What is consistent is the process: verify spark, fuel, and compression at cylinder 1 and compare against a healthy cylinder.
Common symptoms
- Check-engine light on or flashing
- Rough idle with a rhythmic shudder
- Unsteady acceleration or a stumble under load
- Slight loss of power
- In some vehicles, a misfire counter shows cylinder 1 leading
Possible causes
- Worn or fouled spark plug at cylinder 1
- Failing ignition coil for cylinder 1
- Clogged or failing fuel injector
- Burned valve or low compression specific to cylinder 1
- A small vacuum leak near that cylinder's intake runner
- Rarely, a wiring or engine-computer issue affecting that cylinder's coil or injector
Diagnostic considerations
Single-cylinder codes reward component swapping: move the coil to a neighboring cylinder, clear the code, and see whether the misfire moves to a new code (P0302, P0303, etc.). If it moves, the coil is the problem. If it stays, compare the plug, injector, and compression for cylinder 1.
A misfire that appears only when the engine is cold points toward an injector sealing or valve issue; one that grows with engine temperature often tracks ignition breakdown under heat.
What a technician may inspect
- Cylinder 1 spark plug condition and gap
- Coil swap test between cylinders
- Injector operation and balance
- Compression and leak-down test for cylinder 1
- Wiring and connector condition at that coil and injector
Common mistakes
- Buying a full set of coils when one cylinder is at fault and untested
- Confusing cylinder numbering between makes — always verify the engine's layout
- Overlooking a minor vacuum leak near the misfiring cylinder
When professional inspection is appropriate
Related codes
Related glossary terms
FAQ
Frequently Asked Questions
Which cylinder is cylinder 1?
It depends on the engine. Manufacturers number cylinders differently — some start at the front of the engine, others at the crank pulley side. Verify the layout in service data for your specific vehicle.
Is P0301 easier to fix than P0300?
Usually, yes. A single-cylinder code narrows the search to one spark plug, coil, injector, or compression issue instead of a whole-engine hunt.
Can a bad fuel injector cause P0301?
Yes — a clogged or leaking injector at cylinder 1 is a common cause when ignition components test good.
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