P0306
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
A flashing lamp indicates a misfire rate that can destroy the catalytic converter in minutes, which is the critical scenario. A steady lamp with an intermittent misfire is less urgent but not optional — misfire causes reliably worsen, and each event pushes unburned fuel and oxygen into the exhaust system.
- Safe to drive
- Flashing light means stop and tow. Steady light with a mild stumble: drive gently and get it diagnosed promptly, avoiding heavy load.
Cylinder 6 Misfire Detected
Cylinder 6 is misfiring — the computer saw the crankshaft fail to get its expected push from that cylinder often enough to flag it. The code identifies which cylinder, not what is wrong with it.
Misfire detection works by measuring crankshaft acceleration through each firing event and attributing shortfalls to the cylinder that should have produced them. Because the fault is confined to cylinder 6 rather than spread across the engine, causes shared by all cylinders — fuel pressure, the mass airflow sensor, a central vacuum leak — move down the list, and components dedicated to that cylinder move up: its coil, plug, injector, wiring, valvetrain, and compression. On a transverse V6, cylinder 6 may sit on either bank depending on the manufacturer's numbering — it is the rear, firewall-side bank on some engines and the front bank on others. Confirm bank assignment in service information before removing anything. If it does land on a rear bank, expect the plug and coil there to have been skipped at past services, and check for oil in the plug tube. Which physical cylinder is number 6 varies by manufacturer.
How the car detected itCrankshaft velocity variation counted over two rolling windows — a short one at a catalyst-damaging misfire rate that flashes the malfunction indicator lamp, and a longer one at the emissions-threshold rate that lights it steady. Detection is normally masked over rough roads, at low fuel level, and through hard shifts to prevent false sets. Rates and window lengths are vehicle-specific, and a cylinder can misfire noticeably before it accumulates enough events to set the code.
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 a mild stumble: drive gently and get it diagnosed promptly, avoiding heavy load.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- Catalyst substrate overheats and eventually melts, adding a converter to the bill. Fuel washes down the cylinder bore and contaminates the oil. If the cause is a leaking valve or a failing head gasket, running it keeps eroding the sealing surface and turns a head repair into a bigger one.
- 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 6 failed | Very common | — |
| 2 | Spark plug worn or fouled | Very common | — |
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 malfunction indicator lamp is flashing or steady
It is the difference between calling a tow truck and driving to the shop, and you can see it from the driver's seat.
Freeze frame conditions — speed, load, coolant temperature
Cold-only misfire, idle-only misfire, and load-only misfire each point at a different family of causes. Load-only misfire in particular is the classic signature of a coil breaking down under cylinder pressure.
Misfire counters across every cylinder
On a V6 or V8, a partner cylinder also counting can point at a shared coil pack, an intake gasket spanning two runners, or a bank-wide problem instead of one dedicated part.
Oil in the spark plug tube or wells for that bank
On many transverse V6 and overhead-cam engines, failed plug tube seals fill the well with oil, which tracks up the boot and shorts the spark. It looks like a bad coil and replacing the coil fixes nothing.
Fuel trims for that bank compared against the other bank
One bank running lean while the other is normal points at an intake leak or an exhaust leak upstream of that bank's oxygen sensor, rather than at an ignition component.
Confirm the physical location of cylinder 6 and which bank it is on
Tools Service information for the specific engine
Cylinder numbering and bank assignment vary by manufacturer, and on a transverse V6 cylinder 6 can be on either the front or the rear bank. If it turns out to be on the rear bank it is often the least accessible plug on the engine, so it is worth being certain before starting.
ExpectedA confirmed cylinder position, bank, and firing order
Pull the coil and inspect the plug well before anything else
Tools Basic hand tools, inspection light
Check for oil, coolant, water, or debris in the well and for a boot that is carbon-tracked, split, or swollen. Look at the coil's boot spring too.
ExpectedA dry, clean well and a supple, unmarked boot. Oil in the well means plug tube seals or a valve cover gasket, and that must be fixed or the new coil fails the same way.
Swap the coil and plug with another cylinder, clear codes, and re-run
Tools Basic hand tools, spark plug socket, torque wrench, scan tool
Move one part at a time where practical, so the result is unambiguous. This is faster and more conclusive than any bench test.
ExpectedThe code following the swapped part identifies that part. The code staying on cylinder 6 means the cause is in the cylinder, its injector, or its wiring.
Read the removed spark plug
Tools Inspection light, gap gauge
Examine electrode wear, gap, insulator condition, and deposits, and compare it against a plug from a healthy cylinder on the same engine.
ExpectedEven light-brown deposits, intact insulator, gap within the vehicle's specification. Oil fouling points at stem seals or rings; a steam-cleaned white electrode points at coolant entering the chamber.
Verify coil power and trigger signal at the connector
Tools Digital multimeter or lab scope, backprobes, wiring diagram
Test with the circuit loaded and compare directly against a working cylinder's coil connector. Rear-bank harnesses on transverse engines take a lot of heat and vibration.
ExpectedBattery voltage on the feed and a clean trigger pulse matching the other coils. Terminal and pin identification is vehicle-specific.
Test the injector for that cylinder against its siblings
Tools Digital multimeter, mechanic's stethoscope
Compare winding resistance across all injectors and listen with a stethoscope for a clean, regular click while the engine runs.
ExpectedResistance consistent with the other injectors and a clear click. The absolute value is engine-specific; the comparison is what matters.
Smoke-test the intake if that bank's fuel trims are lean
Tools Smoke machine
Introduce smoke and inspect around the manifold gasket for that runner, the injector seals, and any vacuum port or PCV connection feeding that side.
ExpectedNo escaping smoke. A leak at that runner explains a lean bank trim and a single-cylinder misfire together.
Test compression on that cylinder if ignition and fuel check out
Tools Compression gauge, leakdown tester, or scope with amp clamp
Run a relative compression test first if you have an amp clamp, then a conventional compression or leakdown test to quantify and locate any loss.
ExpectedCylinders reasonably consistent with one another. Variation limits are engine-specific — many manufacturers allow up to about 25 percent between the highest and lowest cylinder, while a tighter spread is typical of a healthy engine. Use the manufacturer's stated variation limit and treat one cylinder clearly below its neighbors as a reason to run leakdown, not as a diagnosis. Leakdown air at the intake means an intake valve, at the tailpipe an exhaust valve, at the oil filler rings. Absolute specifications are engine-specific.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the spark plug for that cylinder, or the full set if they are at service age | A worn, fouled, cracked, or misgapped plug found, with the misfire ending once a known-good plug is fitted | $80–$450 | DIY |
| Replace the ignition coil for that cylinder | The misfire following the coil when swapped to another cylinder | $120–$400 | DIY |
| Replace the spark plug tube seals or valve cover gasket | Oil found standing in the plug well, with a carbon-tracked boot and a plug wet with oil on the insulator | $200–$700 | DIY |
| Repair the coil harness, connector, or trigger circuit | Missing power or a corrupted trigger at the coil connector while the coil itself is proven good on another cylinder | $100–$400 | DIY |
| Clean or replace the fuel injector for that cylinder | Injector resistance or flow clearly out of line with its siblings, or the misfire following the injector on a swap | $200–$700 | DIY |
| Repair an intake manifold or injector seal leak on that runner | Smoke escaping at that runner together with lean bank fuel trims that normalize once sealed | $200–$800 | DIY |
| Cylinder head or valve repair | Compression on that cylinder outside the manufacturer's stated variation limit, with leakdown air audible at the intake or exhaust port and no improvement from a wet compression test | $1,200–$3,500 | Shop |
US independent shop rates, parts plus labor. Rear-bank work on a transverse V6 can require removing the intake manifold or rocking the engine forward, which multiplies the labor on what looks like a simple plug job. Add catalytic converter replacement if sustained misfire has already damaged it.
- Replacing the coil while leaving an oil-filled plug tube untouched, so the new coil fails exactly the same way within weeks.
- Skipping the rear bank at service time on a transverse V6 and then diagnosing the resulting misfire as a coil failure.
- Replacing every coil on the engine when a single swap would have identified the failed one.
- Assuming cylinder 6 is on a particular bank. Bank assignment and cylinder numbering vary by manufacturer — on some transverse V6s cylinder 6 is on the rear bank, on others it is on the front.
- Continuing to drive with a flashing lamp, converting a plug-and-coil job into a catalytic converter job.
- Condemning the cylinder on a rule-of-thumb compression spread rather than the manufacturer's stated variation limit.
- Ignoring bank-specific fuel trims that were pointing at an intake leak the whole time.
- Not re-running the vehicle under the original freeze frame conditions after the repair, so a load-only misfire is declared fixed after an idle test.
- 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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