Engine misfire (skipping, stumbling, or a dead cylinder)
A misfire means one or more cylinders aren’t burning their fuel — the codes usually name the cylinder, which makes this unusually diagnosable.
What this usually means
Each cylinder needs three things to fire: spark, fuel, and compression. A misfire is any cycle where one of the three falls short and the cylinder contributes nothing — felt as a stumble, a rhythmic shake, lost power, and often a check-engine light. What makes misfires more tractable than most engine problems is that the computer counts them per cylinder: a P0301 means cylinder 1 specifically, P0300 means misfires scattered across cylinders. A single-cylinder code points at that cylinder’s own parts (plug, coil, injector, compression); a random-misfire code points at something shared, like fuel pressure, a vacuum leak, or exhaust restriction. That one distinction, read from the codes before any parts are bought, is the backbone of an efficient diagnosis.
Most likely causes
- highWorn or fouled spark plugsThe most common cause and the cheapest fix. Electrodes erode with mileage until the spark can’t reliably jump, especially under load.
- highFailing ignition coilCoils fail one at a time on modern coil-on-plug engines, producing a persistent single-cylinder misfire. Swapping the suspect coil to another cylinder and seeing if the code follows is the standard test.
- mediumClogged or failed fuel injectorA restricted injector leans out its cylinder; a dead one stops it entirely. Single-cylinder misfires that survive new plugs and coils often end here.
- mediumVacuum leak or lean conditionA shared lean mixture causes random misfires across cylinders, usually worst at idle, and often comes with lean codes alongside the misfire code.
- lowLow compression (valve, rings, or head gasket)If the cylinder can’t hold pressure, no ignition part will fix it. A compression or leak-down test is the definitive check when the usual suspects are exhausted.
How urgent is it?
Redline can’t determine from a symptom or code alone whether your car is safe to drive.
Typical fix & cost
Read the codes first: a numbered cylinder code narrows the work to one cylinder, where plug, coil, and injector get checked in that order — coil-swap tests make this fast. Random misfires get the shared systems checked: fuel pressure, vacuum leaks, intake. Most misfires resolve with plugs, a coil, or an injector; compression-related misfires are the rare, expensive tail.
Typical range: $100–$1,000
A set of plugs is the low end, a coil or injector the middle; compression-related repairs (valves, head gasket) exceed this range and get quoted separately once confirmed.
The price depends on which cause it turns out to be, so confirm the cause before paying. Diagnose this for my car →
Start a case with what you’re noticing. Redline weighs these causes against your car’s evidence and names the check that would settle it.
Common questions
What does the cylinder number in a misfire code actually tell me?
P0301 through P0308 name the specific misfiring cylinder, which localizes the problem to that cylinder’s plug, coil, injector, or compression. P0300 (random/multiple) means the cause is shared — think fuel pressure or a vacuum leak rather than one bad part.
Can I fix a misfire by just replacing all the spark plugs?
Often — plugs are the most common cause and due by mileage anyway. But if the misfire persists, resist replacing parts in bulk; a coil-swap test or injector check pinpoints the fault for less than a set of coils costs.
Will a misfire go away on its own?
Rarely, and intermittent ones usually return worse. Meanwhile every misfiring cycle pushes unburned fuel into the catalytic converter. Even a miss that comes and goes deserves a diagnosis.
Related OBD-II codes
If your car has stored a trouble code, these often come with this symptom:
Related symptoms
General guidance, not a substitute for a hands-on inspection. Cost ranges are broad reference ranges, not quotes. For safety-related issues, have the car inspected before driving.
Seeing this on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.