P0301: Cylinder 1 Misfire Detected (Meaning, Causes, Fixes)
What P0301 means
P0301 is a generic OBD-II code, so it applies to all 1996-and-newer OBD-II vehicles, though repair steps vary by make and model. RepairPal explains how it's detected: the misfire monitor watches crankshaft speed through the crankshaft sensor, and when cylinder 1 isn't contributing its share (more than about a 2 percent variance in RPM between cylinders), it sets P0301 and turns on the check engine light. Above roughly a 10 percent variance, the light blinks to warn of a misfire harmful to the catalytic converter. Cylinder numbering differs between engines, so check your service information to find which cylinder is number 1. The two sources disagree on a couple of causes: OBD-Codes.com lists a faulty fuel injector and faulty oxygen sensors as possible causes, while RepairPal lists fuel injectors and oxygen sensors among the common misdiagnoses for P0301. OBD-Codes.com also mentions a failed catalytic converter (rotten-egg smell) and a defective computer, while RepairPal adds vacuum leaks, low fuel pressure, EGR faults, and MAF, crank/cam or throttle position sensor faults.
Severity: can I keep driving?
Stop and fix promptly
RepairPal rates P0301 'Urgent'. If the check engine light is flashing, a severe misfire is happening that can overheat and damage the catalytic converter, so ease off, avoid hard acceleration and get it checked as soon as you safely can. A steady light with a rough-running engine still needs prompt attention.
Common symptoms
- Check engine light on, or flashing during a severe misfire
- Rough running, hesitation or jerking when accelerating
- Harder starting (listed by OBD-Codes.com)
- Rough idle or dying at stop signs (listed by RepairPal)
- Lack of power, especially during acceleration, and lower fuel economy
- Often no adverse symptoms noticed by the driver (noted by RepairPal)
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Worn or faulty spark plug on cylinder 1
- Faulty spark plug wire, ignition coil (coil pack), or distributor cap and rotor where fitted
- Vacuum leak (listed by RepairPal)
- Low or weak fuel pressure, or running out of fuel
- Faulty fuel injector (listed by OBD-Codes.com; RepairPal calls injectors a common misdiagnosis)
- Incorrect ignition timing, EGR faults, or a defective MAF, crankshaft, camshaft or throttle position sensor (listed by RepairPal)
- Mechanical engine problems such as low compression, a leaking head gasket or a burned exhaust valve
- Defective engine computer, which is uncommon (listed by OBD-Codes.com)
Diagnostic steps you can do
Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.
- Read all stored codes. Other misfire codes (P0300, P0302 to P0304) or lean codes like P0171 point to a cause shared by several cylinders rather than a part on cylinder 1.
- Inspect the wiring and connectors to cylinder 1's coil and injector, as OBD-Codes.com suggests.
- Pull and inspect the cylinder 1 spark plug. If plugs and wires are due by mileage, replacing them is regular maintenance anyway.
- On coil-on-plug engines, a common check is to swap cylinder 1's coil with a neighboring cylinder, clear the code and see whether the misfire follows the coil.
- If there are no symptoms, OBD-Codes.com suggests clearing the code and seeing if it comes back.
Typical fixes
- Replace the spark plug, plug wire or ignition coil on cylinder 1
- Repair vacuum leaks
- Restore correct fuel pressure or repair a faulty injector once testing confirms it
- Repair ignition timing, EGR or sensor faults found during diagnosis
- Repair mechanical problems found by a compression or leak-down test
- Replace the catalytic converter if a long-running misfire has damaged it
Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.
🛠 When to call a pro
- The check engine light is flashing
- The misfire stays on cylinder 1 after replacing the plug and coil
- A compression or leak-down test is needed to check valves, rings or the head gasket
- You smell rotten eggs from the exhaust, which OBD-Codes.com links to a failing catalytic converter
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Can I drive with P0301?
RepairPal rates it 'Urgent'. If the check engine light is flashing, the misfire is severe enough to harm the catalytic converter, so limit driving and get it fixed soon. Many cars show only mild symptoms, but the cause should still be found promptly.
Is P0301 always a bad spark plug or coil?
No. Ignition parts are the most common cause, but RepairPal also lists vacuum leaks, low fuel pressure, sensor faults and mechanical problems like low compression. Swapping the coil to another cylinder is a cheap way to see if the misfire follows it.
What's the difference between P0300 and P0301?
P0301 pins the misfire to cylinder 1. P0300 means random or multiple cylinders are misfiring, which usually points to something shared by all cylinders, such as fuel pressure or a vacuum leak.
Sources
Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.