P0301
Cylinder 1 Misfire DetectedP0301 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 1 Misfire Detected. It is logged by the engine control unit when the misfire monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0301 means
Code P0301 is set when the Engine Control Module (ECM) detects a misfire condition on cylinder number one. The ECM monitors combustion quality by tracking crankshaft rotational velocity via the crankshaft position sensor — each time a cylinder fires correctly, the crankshaft accelerates slightly. When cylinder 1 fails to contribute its expected power pulse, the ECM registers the velocity drop, increments the misfire counter for that cylinder, and stores P0301. If the misfire rate crosses the threshold for catalytic converter damage (roughly 1–2% of firing events), the MIL will flash rather than illuminate steadily.
P0301 belongs to the SAE generic family P0300–P0312, where the last digit identifies the specific cylinder. Unlike P0300 (random/multiple cylinders), P0301 is always confined to cylinder 1, which makes diagnosis more focused. The root cause falls into one of three broad categories: ignition delivery (spark plug, coil, wires), fuel delivery (injector clog, low fuel pressure), or mechanical integrity (compression loss from worn rings, burnt valves, or a failed head gasket). Variable valve timing faults and vacuum leaks can also produce single-cylinder misfires that initially appear ignition-related.
Left unaddressed, a persistent cylinder-1 misfire forces unburned fuel into the exhaust stream, which can destroy the catalytic converter within 50–100 miles of sustained misfiring, and raw fuel washing cylinder walls accelerates piston and ring wear. Prompt diagnosis is strongly recommended; in most cases the repair is straightforward and low-cost (spark plug or coil replacement), but ignoring the code risks escalating to significantly more expensive engine or catalyst damage.
Common causes
Most-frequently reported root causes when P0301 is logged.
-
1
Faulty or worn spark plug on cylinder 1
-
2
Failed or weak ignition coil on cylinder 1
-
3
Damaged or open-circuit spark plug wire (if applicable)
-
4
Clogged or electrically faulty fuel injector on cylinder 1
-
5
Low compression due to worn piston rings, burnt valve, or failed head gasket
-
6
Vacuum leak near cylinder 1 intake port
-
7
Low fuel pressure (failing fuel pump or blocked fuel filter)
-
8
Variable valve timing system fault (low oil, incorrect viscosity, or faulty VVT actuator)
Symptoms drivers notice
How to diagnose P0301
A typical diagnostic flow when this code is present.
-
1
Connect an OBD-II scanner, record all stored and pending codes, and note freeze-frame data (RPM, load, coolant temp) to understand conditions when misfire was detected
-
2
Perform a cylinder swap test: move the cylinder-1 spark plug to a different cylinder and re-scan — if the misfire code follows the plug (e.g. P0302), replace the spark plug; if it stays on P0301, the plug is not the cause
-
3
Swap the cylinder-1 ignition coil with an adjacent cylinder's coil and re-scan — if the misfire follows the coil to the new cylinder, replace the coil
-
4
Use a noid light or injector pulse tester to verify the cylinder-1 fuel injector is receiving a signal; measure injector winding resistance (typically 11–17 Ω for high-impedance injectors) and compare to adjacent cylinders
-
5
Perform a compression test on cylinder 1; a reading more than 10–15% below adjacent cylinders or below 120 psi (on most petrol engines) indicates a mechanical fault — follow up with a leak-down test to isolate rings vs. valves vs. head gasket
-
6
Inspect vacuum lines and intake manifold gaskets near cylinder 1 for cracks, disconnections, or hissing sounds; use a smoke machine or careful propane enrichment test to locate leaks
-
7
Check fuel pressure at idle and under snap-throttle; verify it holds steady after key-off (pressure drop indicates a leaking injector or faulty fuel pressure regulator)
Related powertrain codes
Frequently asked questions
Can I keep driving with a P0301 code?
Short distances to a workshop are generally safe if the MIL is on steadily and the car drives without severe shaking. However, if the MIL is flashing, pull over safely and stop driving — a flashing MIL means the misfire rate is high enough to damage the catalytic converter within minutes of continued operation. In either case, the fault should be diagnosed and repaired as soon as possible.
Will a bad oxygen sensor cause P0301?
No. An oxygen sensor monitors exhaust gas composition for the entire bank, not individual cylinders. A faulty O2 sensor typically sets its own sensor-related codes and causes a rich or lean condition across all cylinders, not a single-cylinder misfire. P0301 specifically indicates cylinder 1 is not firing correctly, so diagnosis should focus on ignition, fuel delivery, and compression for that cylinder.
How much does it cost to fix a P0301?
Repair cost depends on the root cause. Replacing a spark plug is typically $20–$80 in parts and is often a DIY job. An ignition coil runs $40–$150 per coil. A fuel injector service or replacement ranges from $150–$400. Compression-related repairs (valves, head gasket, rings) are the most expensive, often $800–$2,500 or more depending on the engine. Starting with the cheapest and most accessible parts (spark plug, coil) before moving to mechanical tests is the recommended approach.
Why does P0301 come back after I replaced the spark plug?
If the misfire returns after a plug swap, the spark plug was either not the root cause or a secondary victim of an underlying problem. Common culprits include a weak ignition coil that repeatedly damages plugs, a leaking fuel injector flooding the cylinder, a compression leak washing oil or coolant onto the plug, or a vacuum leak leaning out only that cylinder. The coil swap test (moving the coil to another cylinder) and a compression / leak-down test are the best next steps.
Disabling P0301 in software
RaceTune can permanently disable P0301 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
Got P0301 in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
Upload your file