P0301: Cylinder 1 Misfire Detected
Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.
What P0301 means
The ECM has counted enough misfires on cylinder 1 specifically to set a code.
What sets it
Crankshaft-speed misfire detection can attribute a misfire to a single cylinder because it knows which cylinder was due to fire. When cylinder 1's count passes the calibrated threshold, P0301 is stored. A single-cylinder code is good news for diagnosis: the fault is almost always local to that cylinder.
Symptoms
- Steady rough idle or a rhythmic "miss"
- Loss of power, especially under load
- Flashing check-engine light if the misfire is heavy
Is it safe to drive?
General guidance, not a guarantee: A flashing light means a catalyst-damaging misfire: minimize driving and get it repaired quickly. A steady light with an occasional misfire can usually be driven to a shop, but unburned fuel still stresses the converter. If in doubt, have it towed and diagnosed.
Common causes
Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.
- Spark plug worn, fouled or cracked: the most common single-cylinder cause
- Ignition coil or coil boot: coil-on-plug units fail individually; boots can crack and track to ground
- Fuel injector: clogged, leaking or not being pulsed
- Intake leak near cylinder 1: intake gasket or a vacuum port near that runner
- Mechanical: low compression from a valve or ring problem
Test before you replace: diagnostic sequence
General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.
- Find cylinder 1. Numbering is set by the manufacturer and differs between engines (front, rear, or one bank of a V engine). Check the service information before touching anything.
- Record freeze frame and the cylinder 1 misfire count.
- Swap the cylinder 1 coil with a neighboring cylinder, clear codes and re-test. If the misfire moves (for example to P0302), the coil is the cause.
- If it stays, pull and inspect the plug: wear, gap, oil or coolant fouling, cracked insulator. Swap plugs the same way if in doubt.
- If ignition is clear, check injector operation with a noid light or scope, or swap the injector where practical.
- Spray-test or smoke-test around the intake gasket near cylinder 1.
- If the misfire stays on cylinder 1 after all of that, run compression and leak-down tests on that cylinder.
- Clear codes and confirm with the misfire monitor or Mode $06 data after repair.
Bench note: The swap test is the cheapest diagnostic step there is. Move one part at a time so you know which part moved the misfire.
Related codes
FAQ
Is P0301 always the coil?
No. Coils are common, but plugs, injectors, intake leaks and low compression all cause single-cylinder misfires. A swap test tells you in minutes whether the coil is to blame.
Where is cylinder 1?
It depends on the engine. Manufacturers number cylinders differently, so check the service information or the intake manifold casting for numbering marks.
Oil on the spark plug: what does that mean?
Oil in the plug well usually means a leaking valve cover or tube seal. Oil can degrade the coil boot and cause misfires, so fix the seal as well.
Sources
- SAE J2012, Diagnostic Trouble Code Definitions (standard page; code titles only)
- California Code of Regulations, 13 CCR 1968.2, OBD II requirements (via Cornell LII)
- US EPA, "What To Do With the Check Engine Light" (EPA420-F-00-041)
- Generic code title cross-check: obd-codes.com generic code list
Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.