P0301 – Misfire No.1 cylinder

Last updated: April 10, 2026

P0301 means the engine computer detected a misfire on cylinder 1. You will usually feel a rough idle, shaking on acceleration, or a lack of power. Continued driving can damage the catalytic converter and increase fuel use. The official definition for P0301 is “Misfire No.1 cylinder.” According to OEM factory diagnostic logic, the PCM sets this code when it sees crankshaft speed changes that match a missing combustion event on cylinder 1. The code points to a problem area, not a confirmed bad part. You must verify spark, fuel, air, and mechanical integrity.

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P0301 Quick Answer

P0301 points to a cylinder 1 misfire. Start by confirming the misfire with live data, then check cylinder 1 ignition and basic engine condition before replacing parts.

What Does P0301 Mean?

P0301 means the PCM has detected a misfire event attributed to cylinder 1. In plain terms, cylinder 1 did not contribute power consistently. That causes a rough idle, hesitation, and a flashing or steady MIL depending on severity. The definition does not identify a failed component. It only identifies the cylinder where the misfire pattern appears.

Technically, the PCM infers misfire by monitoring crankshaft speed variation from the CKP sensor signal. A healthy cylinder speeds the crank slightly during its power stroke. When cylinder 1 fails to burn the mixture, the crank slows at the wrong time. The PCM then logs P0301 based on that pattern. This matters because a CKP-based misfire flag can result from ignition, fuel delivery, air mixture, compression, or even a mechanical timing fault.

Theory of Operation

Under normal conditions, cylinder 1 receives the correct air charge, the correct fuel mass, and a timed spark. The cylinder then produces a predictable torque pulse. The PCM uses the CKP signal to track those torque pulses across all cylinders. It also uses fuel trims, O2 feedback, and load inputs to maintain mixture control.

P0301 sets when cylinder 1 torque contribution drops enough for the PCM to recognize a repeated misfire pattern. Ignition breakdown often shows up under load. Fuel delivery faults often show up on cold start or at high demand. Air or mixture faults can show up at idle and light throttle. Mechanical faults can misfire steadily and resist normal tune-up improvements.

Symptoms

P0301 symptoms usually show up as a drivability complaint tied to roughness and loss of power.

• Rough idle: Engine shakes most at idle or in gear at a stop.
• Hesitation: Stumble or flat spot when you tip into the throttle.
• Flashing MIL: Light flashes during active misfire that can overheat the catalyst.
• Reduced power: Noticeable lack of acceleration, especially on hills.
• Fuel smell: Raw fuel odor from the exhaust after a cold start.
• Hard start: Longer crank time when the misfire occurs on startup.
• Poor fuel economy: MPG drops as the PCM adds fuel and the cylinder fails to burn it.

Common Causes

• Worn or fouled spark plug on cylinder 1: A worn gap or contamination weakens the spark and leaves the mixture unburned, which the PCM interprets as a cylinder 1 misfire.
• Ignition coil or coil-on-plug boot issue on cylinder 1: Internal coil breakdown, carbon tracking, or a poor boot-to-plug connection reduces spark energy and creates a repeatable miss under load or at idle.
• Fuel injector #1 flow or control problem: A restricted injector, poor spray pattern, or an electrical control issue delivers the wrong fuel mass and pushes the mixture lean or rich enough to misfire.
• Intake air leak affecting cylinder 1 runner: A leaking intake gasket, cracked runner, or a vacuum leak near cylinder 1 adds unmetered air and leans that cylinder more than the others.
• Carbon buildup on intake valves (GDI/direct injection engines): On GDI/TGDI/FSI/EcoBoost/Skyactiv-G engines, fuel does not wash the intake valves — carbon accumulates on valve faces and stems over time, disrupting airflow and causing cylinder-specific or random misfires, especially at idle or light load. On vehicles over 60,000 miles this is one of the most common real-world misfire causes. Intake valve cleaning (walnut blasting or chemical decarbonisation) is required — replacing spark plugs or ignition coils will not fix it.
• Low fuel pressure or volume: A weak pump, restricted filter, failing pressure regulator, or supply issue can starve one or more cylinders, with cylinder 1 often showing first in misfire counters.
• Mechanical fault in cylinder 1: Low compression from a burned valve, ring wear, or a head gasket issue reduces cylinder contribution and triggers consistent misfire detection.
• Wiring/connector fault at coil or injector for cylinder 1: Corrosion, terminal tension loss, or harness damage creates voltage drop or intermittent control, which causes an erratic spark or injector pulse.

Diagnosis Steps

Tools: a scan tool with Mode $06 misfire data and live data, a DVOM for voltage-drop tests, an ignition spark tester, and basic hand tools. A fuel pressure gauge helps on returnless and return-style systems. Use a borescope if the engine uses direct injection. A compression gauge or leak-down tester confirms mechanical faults.

1. Confirm P0301 as pending or confirmed, then record freeze frame data. Focus on RPM, engine load, coolant temperature, fuel system status (open/closed loop), battery voltage, and STFT/LTFT to learn when the misfire set.
2. Check for related DTCs and review misfire counters. Use Mode $06 and live misfire data to verify cylinder 1 leads the count, not a secondary symptom from another fault.
3. Inspect the simple failure points before meter work. Check ignition and injector-related fuses, power distribution, and any ignition relay outputs that feed the coils or injectors.
4. Verify PCM and engine grounds under load with voltage-drop testing. Run the engine or crank it and measure ground drop at the main engine ground and the PCM ground circuits; keep drop under 0.1V during operation.
5. Perform a targeted visual inspection of cylinder 1 components and the harness route. Look for oil in the plug tube, cracked coil boot, loose connectors, rubbed-through wiring, and water intrusion near the coil or injector connector.
6. Identify whether the engine uses direct injection before swapping parts. Confirm injector placement using service data or by inspection; DI injectors mount into the cylinder head at the combustion chamber, not the intake port.
7. If the engine is DI and has higher mileage (often 60,000+), inspect intake valves before condemning ignition or the injector. Remove the intake ducting as needed and borescope the cylinder 1 intake port; heavy deposits support intake valve cleaning as the correct repair direction.
8. Use a scan tool snapshot during a road test if the concern is intermittent. Freeze frame shows what happened when the DTC set, while a snapshot captures live misfire counts, trims, load, and RPM at the moment the driver feels the miss.
9. Test the ignition system on cylinder 1 with evidence. Check plug condition and gap, then use a known-good spark tester or swap the coil and plug with another cylinder and recheck misfire counters to see if the misfire follows the component.
10. Test injector #1 control and delivery after ignition checks. Verify injector power feed, then check the control side for a consistent pulse; if available, perform an injector balance test or compare cylinder contribution to confirm a fueling problem.
11. If trims show lean at idle and improve with RPM, check for vacuum leaks near cylinder 1. Use smoke testing around the intake manifold gasket area, PCV connections, and brake booster plumbing to confirm unmetered air.
12. Confirm mechanical integrity if the misfire stays on cylinder 1 after spark, fuel, and air checks. Perform a compression and, if needed, a leak-down test to locate valve, ring, or head gasket faults.

Professional tip: Don’t rely on coil swapping alone when P0301 happens at hot idle on a DI engine. A borescope view of the intake valves saves hours and prevents repeat comebacks after plug and coil replacement.

Possible Fixes

• Replace the spark plug on cylinder 1 (and any others due by interval) after you confirm fouling, wear, or incorrect gap.
• Repair the ignition coil/boot issue on cylinder 1 only after testing shows weak spark or the misfire follows the coil during a controlled swap.
• Repair wiring, terminals, or connector fitment at the coil or injector when voltage-drop, pin tension, or intermittent control confirms a circuit fault.
• Restore correct fueling by cleaning/replacing injector #1 or correcting fuel pressure/volume issues after you verify a delivery or control problem.
• Repair vacuum leaks or intake manifold sealing problems when smoke testing confirms unmetered air near cylinder 1.
• Perform intake valve cleaning (walnut blasting/chemical decarbonisation) on DI engines when borescope inspection confirms heavy carbon buildup.
• Address mechanical faults with compression/leak-down guided repairs if testing confirms low compression or a leakage path in cylinder 1.

Can I Still Drive With P0301?

You can sometimes drive short distances with a P0301 code, but you should treat it as a “limp it to a shop” fault. A No. 1 cylinder misfire can turn into a flashing MIL. That indicates an active misfire that can overheat and damage the catalytic converter fast. If the engine shakes at idle, lacks power, or smells like raw fuel, stop driving and tow it. Keep driving only if the misfire feels minor and the MIL stays steady. Avoid hard acceleration and heavy loads. Do not ignore coolant loss, overheating, or loud mechanical noise, since those point to a different failure that can escalate quickly.

How Serious Is This Code?

P0301 ranges from an inconvenience to a catalyst-killer. A light, intermittent misfire may only cause a rough idle and worse fuel economy. It still raises emissions and can fail inspection. A steady misfire under load can create dangerous hesitation when merging or passing. A severe misfire can also dump fuel into the exhaust and melt the catalytic converter. That turns a tune-up sized repair into a four-figure bill. Treat a flashing check engine light as urgent. You should diagnose it the same day. When the misfire stays mild and only happens at idle, you still need a plan. Carbon buildup on direct-injection engines and small vacuum leaks often start that way.

Common Misdiagnoses

Parts swapping causes the most wasted money on P0301. Many people install plugs and coils first, then chase the same misfire. That happens when No. 1 has a fuel delivery issue, an injector wiring problem, or low compression. Techs also miss intake valve carbon on GDI engines. The misfire can improve with higher RPM, which tricks you into blaming ignition. Another frequent error involves cylinder numbering. The “No. 1” cylinder depends on engine design, not firing order guesses. Misfire counters on the scan tool often show the truth fast. Finally, some skip freeze-frame data. That loses the load, RPM, and temperature clues that separate vacuum leaks from ignition breakdown.

Most Likely Fix

The most common confirmed repair directions for P0301 start with basic ignition verification on cylinder 1, then move to mixture and mechanical checks. On many engines, a worn plug, a boot tracking issue, or a coil that fails under load causes the misfire. You confirm that with a swap test and misfire counters, not guesswork. On higher-mileage direct-injection engines, intake valve carbon buildup frequently drives an idle or light-load No. 1 misfire. A borescope inspection through the intake port confirms it. The real fix is intake valve cleaning, not more coils. After any repair, you must verify the misfire stays at zero on the road test and that the OBD-II misfire monitor completes under the correct enable conditions.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.