P0301
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
A single misfiring cylinder still sends raw fuel into the catalytic converter. At a high enough rate the light flashes and the converter can be destroyed in a short drive. At lower rates it is a driveability and emissions problem that reliably gets worse rather than better.
- Safe to drive
- Flashing light means stop and tow. Steady light with mild roughness: short, gentle trips only, and get it diagnosed this week.
Cylinder 1 Misfire Detected
Cylinder 1 is failing to burn its fuel properly. Because the problem is confined to one cylinder, the cause is something that cylinder does not share with the others — its coil, plug, injector, wiring, valves, or compression.
The same crankshaft-acceleration monitor that produces P0300, but the misfire counter for cylinder 1 alone crossed the threshold while the others stayed normal. The PCM uses cam position to know which cylinder should be firing and attributes missing crank acceleration to it. Cylinder 1 is not necessarily the one nearest the front of the car: numbering is manufacturer-specific, and on transverse V engines cylinder 1 can sit on the rear bank against the firewall. Confirm the numbering from service information before touching anything.
How the car detected itContinuous monitor. Crank speed is sampled through each power stroke and misfire counts are kept per cylinder over rolling windows of roughly 200 and roughly 1,000 revolutions. Crossing the catalyst-damaging rate flashes the MIL immediately; crossing the emissions rate sets the code, usually after two consecutive failed trips, often appearing as a pending code after the first. Rough-road filtering suppresses false counts, and Mode 06 exposes the raw counts, which is the quickest confirmation that the fault really is on cylinder 1 and not bleeding over from a neighbor.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Flashing light means stop and tow. Steady light with mild roughness: short, gentle trips only, and get it diagnosed this week.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- Raw fuel keeps reaching the converter, risking meltdown and a large bill. Expect the affected plug to foul, the engine oil to be diluted with fuel, and eventually additional codes as the damage spreads. If low compression is the cause, continuing to drive can turn a valve problem into a cylinder head rebuild.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Check engine light on or flashing
- Rough idle and uneven running
- Power loss, hesitation under acceleration
- Increased fuel consumption
- Possible smell of unburned fuel from exhaust
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Ignition coil for cylinder 1 failed | Very common | — |
| 2 | Spark plug for cylinder 1 worn or fouled | Very common | — |
| 3 | Cylinder 1 injector clogged | Common | — |
| 4 | Low compression on cylinder 1 | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether the check engine light is flashing
Flashing means a catalyst-damaging misfire rate is happening right now, which changes the correct action from 'book a shop' to 'stop and tow'.
Which physical cylinder is number 1 on this engine
Numbering varies by manufacturer and engine layout. Working on the wrong cylinder is the single most common wasted hour on this code.
The cylinder 1 coil and spark plug, visually
A cracked boot, carbon tracking down the insulator, oil pooled in the plug well, or an obviously worn plug covers the two most likely causes and takes five minutes to see.
Any other codes stored, especially lean, rich, or knock codes
A lean code on the same bank suggests an intake runner leak or a failing injector at that cylinder rather than an ignition fault.
When the misfire appears — cold only, at idle, or under load
Cold-only points to a plug or injector on the edge. Load-only points to ignition breaking down under cylinder pressure. Constant points to a dead injector or a mechanical fault.
Confirm the misfire really is on cylinder 1
Tools Scan tool with Mode 06 or misfire counter live data
Read per-cylinder misfire counts. If neighboring cylinders also carry meaningful counts, the picture is closer to P0300 and shared causes come back into play.
ExpectedCylinder 1 counts far above every other cylinder.
Swap the coil to an adjacent cylinder and re-test
Tools Basic hand tools, scan tool
On coil-on-plug engines this is the cheapest and most decisive test available. Swap the cylinder 1 coil with a neighbor, clear codes and counters, then drive under the conditions that produce the misfire.
ExpectedIf the code becomes P0302 or whichever cylinder received the coil, that coil is the fault. If it stays P0301, the coil is good and the diagnosis moves on.
Remove and read the cylinder 1 spark plug, and swap it the same way
Tools Spark plug socket, gap gauge, torque wrench
Compare it against plugs from the other cylinders. Oil fouling points to valve stem seals or rings. White, crusty coolant deposits point to a head gasket or cracked head. A dry, washed-clean appearance points to excess fuel. Swapping the plug and seeing whether the misfire follows confirms it directly.
ExpectedPlug in good condition, correctly gapped, and matching the other cylinders. Misfire moving with the swapped plug confirms the plug.
Test or swap the cylinder 1 injector
Tools Stethoscope, noid light on port injection, scan tool with injector control
With ignition cleared, move to fuel. Listen with a stethoscope for even operation, run a scan-tool injector balance test, or swap the injector to another cylinder and see whether the misfire follows. Before opening any fuel connection, relieve fuel system pressure, disconnect the battery, keep ignition sources away with an extinguisher within reach, and fit new O-rings on reassembly. On direct-injection engines this means high-pressure fuel lines, new sealing and support rings, and a specified torque on refit — use scan-tool injector contribution data instead and leave removal to a workshop.
ExpectedEven injector operation across cylinders. Misfire following a swapped injector identifies it.
Check the wiring and connectors serving that cylinder's coil and injector
Tools Digital multimeter or lab scope, backprobe leads
Backprobe for supply voltage and a clean control signal, then wiggle-test the harness while watching live data. Chafed harness sections and rodent damage are common, and cheaper than any part on the list.
ExpectedSteady supply voltage and clean switching signal with no dropouts during wiggle testing.
Smoke test for an intake leak specific to that cylinder
Tools Smoke machine
A leak at that cylinder's intake runner gasket or injector seal leans out one cylinder only and produces exactly this pattern with perfectly good ignition parts.
ExpectedNo smoke escaping at that cylinder's intake runner, port gasket, or injector seal.
Compression and leak-down test cylinder 1
Tools Compression gauge, leak-down tester
If spark and fuel are proven good, the cause is mechanical. Cranking compression compares cylinder 1 against the others. Leak-down then tells you where the pressure goes: hiss at the intake means an intake valve, at the exhaust means an exhaust valve, into the crankcase means rings, bubbles in the coolant mean the head gasket. On engines with hydraulic lifters, a collapsed lifter or tight valve clearance can also do this.
ExpectedCylinder 1 within roughly 10 to 15% of the other cylinders. Absolute values are vehicle-specific.
Confirm the repair under the original conditions
Tools Scan tool with Mode 06
Clear codes and misfire counters, then drive through the RPM and load recorded in freeze frame. Recheck counts rather than relying on the light staying off.
ExpectedCylinder 1 counts at or near zero and the misfire monitor reporting complete.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the cylinder 1 ignition coil | Misfire follows the coil when it is swapped to another cylinder. Check access first — on a transverse V engine cylinder 1 can sit on the rear bank, where reaching the coil may require intake manifold removal | $150–$450 | DIY |
| Replace spark plugs — the full set on a high-mileage engine | Cylinder 1 plug worn, out of gap, cracked, or fouled | $120–$600 | DIY |
| Replace valve cover gasket and spark plug tube seals | Oil pooling in the plug well and degrading the coil boot | $200–$700 | DIY |
| Clean or replace the cylinder 1 fuel injector | Misfire follows the injector when swapped, or a balance test shows it flowing low | $200–$800 | Hard |
| Repair coil or injector wiring, connector, or ground | Voltage drop, open circuit, or a signal dropout appearing during wiggle testing | $100–$450 | DIY |
| Repair intake leak at that cylinder's runner or injector seal | Smoke escaping at that port with the intake pressurized | $150–$600 | DIY |
| Repair mechanical compression loss on cylinder 1 — valve, seat, rings, or head gasket | Compression low on cylinder 1 with leak-down locating the escape path | $900–$4,000 | Shop |
This is one of the cheaper codes to fix when it is ignition — a coil plus a set of plugs is commonly $200 to $600 installed. It is one of the more expensive when it turns out to be mechanical. That gap is exactly why the free tests come first: the coil and plug swap costs nothing but a little labor and eliminates the two most likely causes before any part is bought.
- Buying the coil before doing the swap test. The swap costs nothing and answers the question outright.
- Replacing only the cylinder 1 plug on a high-mileage engine. If one is worn out, the others are close behind.
- Ignoring oil in the spark plug well. Valve cover gasket oil destroys coil boots repeatedly until the gasket is replaced, so the new coil fails too.
- Condemning the coil when the real fault is a chafed wire or a corroded connector feeding it.
- Counting cylinders from the wrong end of the engine, or assuming cylinder 1 is on the near bank of a transverse V6.
- Driving with a flashing light 'just to get it home'. That drive is often what costs the converter.
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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