P0301 means the engine control module counted misfires on cylinder 1 above its threshold. The module watches crankshaft speed; a cylinder that does not contribute a power stroke shows as a momentary slowdown at the moment it should have fired, and the camshaft sensor tells the module which cylinder that was. P0300 is the random or multiple version; P0302 through P0312 name the other cylinders. P0301 with P0300 means cylinder 1 contributed most to a wider count.
Severity: moderate, high if the light flashes. Can you keep driving? Only if the light is steady. A flashing light means the misfire rate is high enough to overheat the catalytic converter; stop and have it towed. A steady light with a slight roughness can be driven to a shop within days; see flashing versus steady.
Where cylinder 1 is
The number comes from the manufacturer's numbering, not from any universal rule, and the swap test below depends on getting it right.
- Inline four, five and six cylinders: cylinder 1 is at the end with the timing chain or belt, which on most transverse engines is the passenger side of the car and on longitudinal engines the front. The exceptions are a few older engines numbered from the flywheel end; the service manual settles it.
- V6 and V8, GM, Chrysler and most Asian: cylinder 1 is the front cylinder of the bank that sits slightly forward of the other, usually the driver's side on GM V8s and the passenger side on Chrysler V8s; on transverse V6 engines bank 1 is usually the rear bank against the firewall, and cylinder 1 is at its end nearest the belt.
- Ford V8: cylinder 1 is the front cylinder on the passenger side.
- Subaru flat-four and flat-six: cylinder 1 is the front cylinder on the passenger side.
- Porsche and other flat-six engines: manufacturer numbering, check the manual.
The swap test
Because the code names a cylinder, the fastest diagnosis is to move a part and see whether the code follows it:
- Clear the code and note the misfire counter for each cylinder on the scan tool.
- Swap the cylinder 1 ignition coil (or plug wire) with cylinder 2's or 3's, drive until the misfire returns. P0302 or P0303 now means the coil; still P0301 means it is not.
- Swap the spark plug the same way if the coil moved nothing.
- Swap the injector if the vehicle's design allows it in reasonable time, or compare injector balance on the scan tool.
- If the code never moves, the cause is in the cylinder: compression, a valve, a head gasket, or a leak at that cylinder's intake runner or injector seal.
A relative compression test with the scan tool, or a gauge on cylinder 1 and a neighbor, finishes the job.
Why cylinder 1 has causes of its own
Cylinder 1 is an end cylinder on most engines, and end cylinders run a little differently:
Coolant and oil at the end of the head. The timing-chain end of the head is where the chain tensioner and the cam phaser oil feeds live; a head gasket that fails between cylinder 1 and a coolant passage is common on some engines, and a misfire that appears on a cold start and clears as the engine warms, with a sweet smell from the exhaust, is the signature.
The intake runner at the end of the plenum. On many manifolds the end runners get the least even mixture and the first vacuum leak when the manifold gasket ages, because the end is where the manifold flexes.
The first cylinder in the injector rail, on port-injected engines, sometimes runs slightly leaner with a weak pump; it is also the last to receive fuel when the rail is partly blocked.
Access. Cylinder 1's coil and plug are usually the easiest to reach, and also the ones most often disturbed during other repairs; a coil not seated after an intake job is a real cause.
What the manufacturers documented
Honda CR-V 2017 to 2018 (Service Bulletin 19-037, June 2019): P0300, P0301 through P0304 or P0172 with a whirling noise, and a powertrain warranty extension to six years, unlimited mileage, for camshafts, rocker arm assemblies and spark plugs damaged by the underlying condition.
Toyota Highlander Hybrid 2017 to 2019 (T-SB-0148-19, October 2019): P0301 through P0306 corrected by reprogramming the engine computer and replacing an ignition coil under the emissions warranty.
Dodge Hornet 2023 with the 2.0 L turbo built January 28 to March 9, 2023 (TSB 18-079-23, June 2023): P0300 through P0304 caused by a spark plug cracked during installation.
Ram 1500 2025 with the 3.0 L Hurricane built on or before January 15, 2025 (TSB 08-031-25, February 2025): P0301 through P0306 with the catalyst codes, corrected by an ignition system control module software update.
GM V6 and V8 pushrod engines 2002 to 2025 (bulletin 19-NA-219, November 2024): misfire codes with a chirp, squeak, squeal or tick, pointing at the valvetrain rather than ignition.
Ford F-150 2018 with the 2.7 L EcoBoost built on or before 12 July 2018 (TSB 18-2310): P0301 through P0306 with lean and rich codes, corrected by replacing the six direct injectors.
Causes, in the order they are found
A spark plug (worn, fouled, cracked) · an ignition coil or plug wire · an injector (clogged, dead, or leaking) · a vacuum leak at cylinder 1's intake runner or injector seal · low compression from a burnt valve, worn rings or a head gasket, with the head gasket more common on end cylinders · a collapsed lifter or a worn cam lobe on cylinder 1 · a coil or connector not seated after an earlier repair · a calibration on the engines above.
A diagnostic order
- Record the codes, the misfire counters per cylinder, and the freeze frame. A misfire only at cold start with a high count that fades is a head gasket or a cold-fouling plug; one under load is spark or fuel; one at idle only is a vacuum leak or a lifter. See freeze frame data.
- Check for a bulletin on the exact engine.
- Confirm which cylinder is number 1 from the manual before touching anything.
- Run the swap test: coil, then plug, then injector.
- Inspect the plug pulled from cylinder 1: fouling says rich or oil, white says lean or hot, coolant residue says gasket.
- Check for a vacuum leak at the runner with smoke or a careful spray with the engine running.
- Relative compression, then a gauge, then a leak-down test on cylinder 1 if nothing moved the code.
- Listen for valvetrain noise and check the lifter and cam lobe on cylinder 1 on pushrod engines, per GM's tip.
- Repair the cause, clear the code, and verify the cylinder 1 counter stays at zero at idle, cruise and load, then let the readiness monitors complete.
What P0301 does not tell you
It does not tell you the coil or the plug is bad. It tells you cylinder 1 did not contribute its share of power on enough cycles to count. Spark, fuel, compression, a leak at that runner and a lifter all produce that; the swap test separates the first two from the rest in ten minutes, and the manufacturers' bulletins show software and injectors on the list as well. The general principle is covered in what a trouble code does not tell you.
Frequently asked questions
Can I drive with a P0301 code?
If the check engine light is steady and the engine only feels slightly rough, yes, to a shop. If the light flashes, stop; a misfire at that rate damages the catalytic converter.
Which cylinder is number 1?
On most inline engines, the cylinder at the timing chain or belt end. On V engines it depends on the manufacturer: usually the front cylinder of the bank that sits slightly forward, the driver's side on GM V8s, the passenger side on Ford and Chrysler V8s. Confirm in the service manual before running the swap test.
Is P0301 always a spark plug or coil?
Most often, and the swap test tells you in ten minutes: move the coil to another cylinder and see whether the code follows. If it stays on cylinder 1 through coil, plug and injector, the cause is compression, a valve, a head gasket or a leak at that cylinder, and the bulletins add a valvetrain condition on the CR-V and software on the Ram and the Highlander Hybrid.
Why does P0301 appear only on cold starts?
Because a head gasket leaking coolant into cylinder 1 overnight, a plug that fouls when cold, or a phaser that lags on a cold start each misfire until the engine warms. A sweet exhaust smell and a cylinder 1 plug with coolant residue point at the gasket.
How much does it cost to fix P0301?
A spark plug or coil is inexpensive. An injector is a moderate part. A head gasket or a valve is a major repair. The swap test costs nothing and decides which.
Related codes
- P0300: random or multiple misfire; see P0300.
- P0302 to P0308: other cylinders; see P0304.
- P0201: injector circuit cylinder 1. P0351: ignition coil A primary or secondary circuit.
- P0171 / P0172: mixture codes stored alongside when fuel or air is the cause; see P0171.
- P0420: catalyst efficiency, the consequence of a misfire driven on; see P0420.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.