A misfire code tells you combustion failed in a specific cylinder — it does not tell you why. The cause could be ignition, fuel, mechanical, or a wiring fault, and each one requires a different repair. The fastest path to the right answer is a structured approach using misfire counters, live data, and a logical test sequence — starting with the tests most likely to confirm the cause and working toward the more invasive ones only when needed. This guide walks through that sequence from scan tool data to confirmed diagnosis.

How the ECM detects misfires

The engine control module monitors crankshaft speed variation using the crankshaft position sensor. During normal operation each cylinder’s power stroke causes a small, predictable acceleration of the crankshaft. When combustion fails in a cylinder, that acceleration doesn’t happen — the crankshaft slows slightly where it should have sped up. The ECM detects this deviation, counts it as a misfire event, and increments the misfire counter for that cylinder. When the count exceeds a threshold within a defined window of crankshaft revolutions, a DTC is stored.

OBD-II requires two misfire thresholds. The first — a high rate of misfires within 200 crankshaft revolutions — triggers a flashing MIL because misfire at that rate can damage the catalyst within seconds. The second — a lower sustained rate over 1,000 revolutions — triggers a standard stored DTC with the MIL on steady. A flashing check engine light during a misfire is a catalyst protection warning and means you should reduce load immediately.

Common misfire codes and what they tell you

• P0300 — Random or multiple cylinder misfire. The ECM is detecting misfires but cannot attribute them consistently to one cylinder, or multiple cylinders are misfiring simultaneously.
• P0301 through P030x — Specific cylinder misfire. The number after P030 is the cylinder number. These are the most actionable misfire codes because they localise the fault immediately.
• P0316 — Misfire detected on startup (first 1,000 revolutions). Often indicates a fuel or cold-start related cause rather than a mechanical failure.

A P0300 alongside specific cylinder codes (e.g., P0300 + P0302 + P0304) usually means those two cylinders are misfiring heavily enough to cause the random misfire threshold to also trigger. The specific codes are more useful — focus on those cylinders first.

Reading misfire counters

Most scan tools can display per-cylinder misfire counters in live data — usually listed as “Misfire Cyl 1,” “Misfire Cyl 2,” and so on. These counters show how many misfire events have been counted per cylinder since the last clear, or in some scan tools, the rate per 1,000 revolutions in real time. Reading these during a road test or load test while the symptom is present is far more useful than reading stored codes alone.

Watch the counters while driving under the conditions that trigger the misfire. A counter that climbs rapidly in one cylinder while the others stay near zero confirms a cylinder-specific fault — ignition, injector, compression, or wiring for that cylinder. Counters climbing across multiple cylinders simultaneously point to a shared cause — fuel delivery, vacuum leak, timing, or a network issue affecting multiple coils.

Reading the misfire pattern

Single cylinder misfire. Misfire events concentrated in one cylinder are almost always ignition, fuel injector, compression, or wiring. The coil swap test (described below) resolves ignition vs. non-ignition within minutes. This is the most straightforward misfire pattern to diagnose.

Adjacent cylinder misfires. On engines with coil-on-plug ignition, two adjacent misfiring cylinders sometimes point to a shared ground or a wiring harness fault affecting both coil circuits rather than two simultaneous component failures. Check the wiring and connector for those coils before replacing both coils.

Multiple random cylinders or P0300 only. Misfires spread across multiple cylinders with no consistent pattern point to a system-level fault affecting combustion quality globally — low fuel pressure, a vacuum leak pulling the mixture lean, contaminated fuel, ignition timing error, or a MAF sensor fault causing the base fuel map to be wrong across all cylinders. Fuel trim data is the key tool here — see fuel trim diagnostics.

Load-dependent misfires. Misfires that only appear under acceleration or high load and disappear at idle point to fuel delivery — a pump or pressure regulator that can supply adequate fuel at idle but cannot keep up under demand. They can also indicate a spark plug or coil that is marginal and breaks down under the higher cylinder pressure of a loaded engine.

Step-by-step diagnostic workflow

1. Record all codes and misfire counters before clearing anything. Note which cylinders have active counters and whether the MIL was flashing or steady. Pull freeze frame data — the RPM, load, and coolant temperature at the moment the code set tells you the conditions you need to reproduce. A misfire that set at 3,200 RPM under 75% load will not show up idling in the workshop.
2. Check for obvious causes first. Inspect spark plugs if they are accessible — a plug that is fouled, cracked, or worn beyond its gap specification is a five-minute find that saves an hour of electrical testing. Check coil connectors for corrosion, backed-out pins, or heat damage. Look for oil in the spark plug wells on engines prone to valve cover gasket leaks — oil contamination destroys plugs and coils quickly and creates a recurring misfire until the source is fixed.
3. Perform the coil swap test for a single-cylinder misfire. This is the most efficient test for isolating ignition from non-ignition causes. Swap the suspect cylinder’s coil with an identical coil from a cylinder that is not misfiring. Clear the misfire counters and drive under the conditions that trigger the misfire. If the misfire moves to the cylinder you moved the coil to, the coil is faulty — replace it. If the misfire stays in the original cylinder, the coil is not the cause and you move to the injector and compression tests. This test works because it generates a definitive result rather than a probability. See how to test an ignition coil properly.
4. Check fuel trim data to assess mixture quality. With the engine running under the conditions that trigger the misfire, monitor STFT and LTFT on both banks. Significantly positive trims (above +10%) indicate the engine is running lean — vacuum leak, low fuel pressure, or MAF fault. Significantly negative trims (below -10%) indicate rich — leaking injectors, high fuel pressure, or restricted intake. Fuel trim data tells you whether the misfire is a combustion quality issue caused by mixture problems or a cylinder-specific ignition or mechanical fault. See fuel trim diagnostics.
5. Test fuel injector operation on the suspect cylinder. If the coil swap test cleared ignition and fuel trims are normal, the injector or its circuit is the next suspect. A noid light confirms the injector is being pulsed. Resistance testing at the injector connector checks the coil winding. A cylinder contribution test — available as a bidirectional function on many scan tools — kills each injector one at a time and measures the RPM drop. A cylinder that shows minimal RPM drop when its injector is killed is either not firing or has very low compression. See how to test a fuel injector electrically.
6. Test compression if ignition and fuel are ruled out. A cylinder-specific misfire that survives a coil swap test, has a confirmed injector pulse, and normal fuel trims is almost certainly mechanical — low compression from worn rings, a burned valve, or a damaged head gasket. A compression test gives you a number to compare against specification and against the other cylinders. A cylinder leak-down test tells you where the compression is going — rings, intake valve, exhaust valve, or head gasket. Compression below 75% of the highest cylinder, or more than 25% variation between cylinders, is a mechanical fault.
7. For random misfires — address fuel delivery and air metering first. If counters are climbing across multiple cylinders without a clear pattern, test fuel pressure at idle and under load (it should hold within spec during acceleration), check for vacuum leaks with a smoke machine or propane enrichment, and compare MAF g/s values against expected for the RPM and load. Fix the fuel or air metering fault before chasing individual cylinders — the pattern will change once the system-level cause is resolved.

Common misfire diagnostic mistakes

• Replacing the spark plug without checking the coil. Plugs and coils fail at similar rates and produce identical symptoms. The coil swap test costs nothing and takes two minutes — always perform it before buying parts.
• Replacing the coil without fixing oil contamination in the plug well. Oil in the spark plug well destroys plugs and coils. A new coil in an oil-filled well will fail within weeks. Fix the valve cover gasket first.
• Diagnosing a random misfire by replacing ignition components one at a time. A P0300 with counters spread across all cylinders is almost never caused by multiple failing coils or plugs simultaneously. Work on the fuel and air metering systems — fuel pressure, vacuum leaks, MAF — before touching ignition components.
• Not reproducing the misfire under the right conditions. A load-dependent misfire that only appears at 3,000 RPM under load will not show up at idle. Use freeze frame data to identify the conditions and reproduce them — a road test with a helper monitoring misfire counters, or a workshop load test that achieves the same RPM and load percentage.
• Ignoring downstream codes caused by the misfire. A sustained misfire sets O2 sensor codes, fuel trim codes, and eventually catalyst efficiency codes. These are effects of the misfire, not independent faults. Repair the misfire first and rescan before pursuing any other codes in the list.

Frequently asked

Can a bad crankshaft position sensor cause misfire codes?

Yes. The misfire detection system relies entirely on crankshaft position sensor data. A failing CKP sensor that produces a noisy or intermittent signal can cause the ECM to calculate false speed variations and count misfires that are not happening — usually appearing as a P0300 with counters spread randomly across all cylinders. A scope test of the CKP sensor output during the fault condition confirms or rules this out. A CKP fault also typically causes other symptoms alongside the misfire codes — rough running, stalling, or difficulty starting.

The misfire moves with the coil in the swap test. Does that definitely mean the coil is bad?

Almost always yes — but confirm the spark plug is also not the cause before ordering the coil. Swap the plug from the originally misfiring cylinder to the new location along with the coil on the first test, then swap just the coil back while leaving the plug in the new location. If the misfire follows the coil regardless of plug position, the coil is the fault. If it follows the plug, replace the plug. This two-step swap isolates the exact component on engines where plugs and coils are both suspect.

How many misfires per 1,000 revolutions is too many?

OBD-II sets a catalyst-damaging misfire threshold at roughly 1–2% of firing events within a 200-revolution window, and a general misfire threshold at around 0.2–0.5% over 1,000 revolutions — though exact thresholds vary by manufacturer. In practice, a misfire counter that is climbing in real time during a road test indicates an active fault regardless of the exact count. Any cylinder showing significantly more misfire events than the others during the same test period has a problem worth diagnosing.

My misfire only happens when the engine is cold. Where do I start?

Cold-start misfires that clear once the engine warms up point to a few specific causes. Spark plugs and coils that are marginal can misfire under the higher cylinder pressure of a cold, tight engine and recover once clearances open up with heat. Fuel injectors that are partially clogged may not deliver enough fuel during the cold-start rich phase when fuel atomisation is poor. A coolant temperature sensor reading incorrectly when cold causes the PCM to miscalculate the cold-start fuel enrichment. Check freeze frame data for the coolant temperature at the time of the misfire and compare it to actual engine temperature — a significant discrepancy points to the coolant temperature sensor.

Related articles

• Fuel trim diagnostics: short term vs long term explained
• How to test an ignition coil properly
• How to test a fuel injector electrically
• How to use OBD2 freeze frame data
• How to diagnose intermittent electrical faults
• P0300 — Random misfire detected
• P0301 — Cylinder 1 misfire detected