Misfire: What It Is and How It's Detected

Combustion in a cylinder requires three things at once: a spark of sufficient energy at the right crank angle, the right quantity of fuel mixed with the right quantity of air, and a cylinder that holds compression. Degrade any one of them badly enough and the charge either does not ignite or burns too poorly to do useful work.

The result is a cylinder that takes its turn and contributes nothing. The engine loses a fraction of its power and gains a shake at the frequency of that cylinder's firing event. Unburned fuel and unconsumed oxygen leave through the exhaust valve.

That last part is why misfire is not just a comfort problem. Raw fuel and oxygen arriving together at a hot catalytic converter will burn inside it.

There is no sensor watching for combustion. The computer infers misfire from crankshaft speed.

The crankshaft carries a toothed reluctor wheel, and the crank position sensor produces a precisely timed pulse for each tooth. That gives the computer a very fine measurement of instantaneous crank speed. Each firing event should produce a small acceleration. The computer measures the acceleration contributed by each event, compares it against its neighbors, and uses the camshaft position signal to work out which cylinder was due. An expected acceleration that does not arrive is counted as a misfire event.

OBD-II evaluates that count over two standard windows:

• A 200-revolution window, used to catch misfire rates high enough to damage the catalytic converter. This is what makes the malfunction indicator lamp flash.
• A 1,000-revolution window, used for the emissions threshold. This typically sets a code and turns the lamp on steady, usually after the fault repeats on a second drive cycle.

Because detection is inferred rather than measured directly, things unrelated to combustion can fool it. Rough road surfaces shake the driveline and can mimic the signature — many vehicles cross-check wheel speed data to suppress that. A damaged reluctor wheel, a crank sensor with a marginal signal, or a slipping harmonic balancer can all produce misfire counts on cylinders that are burning perfectly well.

A flashing malfunction indicator lamp means the misfire rate has crossed the catalyst-damage threshold. Unburned fuel is being pushed into the converter and burning there, and substrate temperature can rise high enough to melt the honeycomb inside during a short drive. This is not a warning that ages gracefully over a week.

What to do: get off the road somewhere safe. Reduce load — no highway speed, no hills under power, no towing. If the engine is shaking hard, running on obviously fewer cylinders, or smells strongly of fuel, have it towed rather than driven. A short careful drive to a shop a mile away is a judgment call; a thirty-mile commute is not.

There is a safety side as well as a cost side, and it is the reason to get off the road rather than finish the trip. A converter burning raw fuel can reach temperatures at which it glows, and vehicles have been set alight by parking over dry grass or leaves in that condition — so once you have stopped, park on pavement or bare ground, not on vegetation. A misfire bad enough to flash the lamp can also worsen into a stall, and if the engine stops while you are moving, power steering assist is lost immediately and brake assist is gone after the first pedal application: steering and braking both still work, but both take far more effort than you are expecting, at the worst possible moment.

The economics point the same way. A converter typically runs $900 to $3,000 or more installed, and vehicles with two banks have at least two of them. A set of spark plugs runs $150 to $500 depending on engine and access; an ignition coil $150 to $450. Continuing to drive converts the cheap repair into the expensive one.

A severe misfire also washes fuel down the cylinder walls, thinning the oil film and diluting the crankcase oil. That is a second, slower kind of damage running in parallel.

P0301 through P0308 name a specific cylinder. That points directly at what only that cylinder has: its ignition coil or plug wire, its spark plug, its fuel injector, and its own valves, rings, and head gasket sealing surface.

P0300 is random or multiple-cylinder misfire. That points at something shared across cylinders: fuel pressure or volume, a large vacuum leak, ignition or camshaft timing, an EGR valve stuck open flooding the intake with inert gas, contaminated or very low-octane fuel, a crank or cam sensor signal problem, or low compression across the engine.

P0300 stored alongside a specific cylinder code means start with the named cylinder — the shared code is often collateral.

P0316 is misfire detected on startup, within the first 1,000 revolutions. It commonly points toward fueling or valve timing behavior at start rather than a dead cylinder.

P0313 is misfire with low fuel level, which is exactly what it sounds like and worth ruling out before anything else. P0314 is a single-cylinder misfire where the computer could not identify which cylinder.

Map misfiring cylinders back onto their banks. Several misfires clustered on one cylinder head points at that bank's cam timing, that bank's intake sealing, or a head gasket — not at a coincidence of failed coils.

1. Read freeze frame before clearing anything. Engine speed, load, coolant temperature and fuel trims at the moment it set. A misfire that only appears cold, only at idle, or only under load each point in a different direction.

2. Watch live misfire counters per cylinder. Most scan tools will show the running count for each cylinder. That gives you the distribution and the conditions that provoke it, which the code alone does not.

3. Swap test. Let the engine cool before working around the exhaust manifold and coil boots — both get hot enough to burn, and a hot manifold is the last thing you want to brace a hand against. Then move the ignition coil from the misfiring cylinder to a known-good cylinder, clear the codes, and reproduce the misfire at idle or on a short low-load run, not a normal drive — and especially not if the lamp had been flashing, since deliberately reproducing a catalyst-damaging misfire keeps damaging the converter. Watch the live misfire counters rather than waiting for a code to reset; the counters move in seconds. If the misfire follows the coil, the coil is the fault. If the misfire stays on the original cylinder, the coil is not — put it back. Do the same with injectors where they are accessible, after depressurising the fuel system to the manufacturer's procedure. This is the cheapest conclusive test available and it is how you avoid replacing parts on a hunch.

4. Read the spark plug you removed. Its condition is physical evidence: soaked in fuel, oil-fouled, unusually clean and steam-scoured (coolant), eroded gap, cracked insulator, or physically damaged.

5. If spark and fuel both test good, test the cylinder itself. Compression first, then a cylinder leakdown test on the offending cylinder. A cylinder under leakdown pressure can turn the engine over, so bring the piston to top dead center on the compression stroke and keep the vehicle in park or neutral with the parking brake set and the wheels chocked before applying air. Leakdown tells you where the pressure is escaping: air at the throttle body means an intake valve, air at the tailpipe means an exhaust valve, air at the oil filler means rings, and bubbles in the coolant means head gasket or a crack.

6. Replace a part only on the basis of a specific failed test. Compression and leakdown diagnosis typically runs $100 to $300 at an independent shop and routinely saves several times that in parts.

Where a fault is genuinely intermittent, the counters plus freeze frame conditions are your best chance of catching it in the act.

A shake or stumble is not proof of a misfire. Torque converter lockup shudder, worn engine or transmission mounts letting normal vibration into the body, driveshaft or CV joint problems, and out-of-balance wheels all produce complaints that get described as "missing." None of them sets a misfire code, and that absence is the clue.

The reverse also happens: a genuine misfire can be present and felt while staying below the count threshold that sets a code. Live misfire counters will show it even when no code has stored. A vehicle with no misfire code and a real stumble is not a vehicle with nothing wrong — it is a vehicle whose fault has not yet crossed a threshold someone chose.

A flashing lamp means catalyst damage is happening now — and a converter burning raw fuel can glow hot enough to be an ignition source, while a misfire that severe can worsen into a stall that takes power steering and brake assist with it. That is the reason to stop, and it does not depend on what the repair costs. The cost side matters too: misfire is where the most money gets wasted on guessed-at parts, because plugs and coils are easy to reach and easy to sell. The swap test settles which part is responsible in minutes and costs nothing, and stopping while the light is flashing is the difference between a plug-and-coil repair and a converter several times its price.