P0308
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
An unburned charge sends raw fuel and oxygen into the catalytic converter, where it lights off and drives substrate temperature past its design limit. A steady MIL is a slower version of the same damage. A flashing MIL means the catalyst is being destroyed right now, and catalyst replacement is one of the most expensive emissions repairs on the car.
- Safe to drive
- Flashing light: stop driving, tow it. Steady light: short, gentle trips only, no heavy load or sustained highway speed.
Cylinder 8 Misfire Detected
The engine computer counted cylinder 8 failing to burn its fuel charge often enough to trip the misfire threshold. Because it named one cylinder, the fault is usually in something that serves only that cylinder — its coil, plug, injector, or compression.
The PCM infers misfire from crankshaft acceleration. Each power stroke should accelerate the crank by a predictable amount; the module measures instantaneous crank velocity between firing events and attributes any deficit to the cylinder that should have fired. When cylinder 8's deficit count crosses the calibrated threshold, P0308 sets. Note that cylinder 8's physical location and bank assignment depend entirely on the manufacturer's cylinder numbering, which is not standardized.
How the car detected itThe crankshaft position sensor feeds a high-resolution reluctor signal to the PCM. Camshaft position tells the module which cylinder is on its power stroke. Two thresholds run in parallel: a Type B threshold evaluated over roughly a 1000-revolution window, which stores a pending code on the first qualifying trip and commands a steady MIL if the fault repeats on a second trip under similar conditions; and a Type A threshold evaluated over a much shorter window, roughly 200 revolutions, which sets the code and flashes the MIL on the first occurrence because that misfire rate can dump enough raw fuel into the catalyst to overheat it. The same code number is used for both — the MIL behavior tells you which one you have.
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: stop driving, tow it. Steady light: short, gentle trips only, no heavy load or sustained highway speed.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- Expect fuel economy loss and rough running to continue. The most likely expensive outcome is catalytic converter failure, which then sets its own code and costs far more than the original coil or plug. Raw fuel washing the cylinder wall also dilutes oil film and can accelerate bore and ring wear. On a severe misfire, unburned fuel reaching the exhaust can also damage the downstream oxygen sensor.
- 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 failed | 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.
Freeze frame data for the moment the code set
Load, RPM, coolant temperature and fuel trim at set time separate a cold-start-only misfire from one that appears under load, and that split changes the entire diagnostic path.
Whether the MIL is steady or flashing
This is the free triage step. Flashing means catalyst-damaging rate and changes the answer from 'drive it to the shop' to 'do not drive it'.
Per-cylinder misfire counters on a scan tool
Confirms the misfire is genuinely isolated to cylinder 8 and not one cylinder standing out inside a general misfire. Counters climbing on several cylinders points back to shared causes.
Coil and plug boot at cylinder 8, visually
Water in the plug well, a carbon-tracked boot, or a chafed coil connector are visible in a minute and are common on engines with deep plug wells.
Any other stored codes, especially lean, EVAP, or fuel system codes
A P0308 sitting alongside a lean bank code often means a vacuum leak or injector problem rather than a coil, and chasing the ignition side first wastes the coil.
Record freeze frame and misfire counters before clearing anything
Tools Scan tool with live data and Mode $06 or per-cylinder misfire PIDs
Capture RPM, calculated load, coolant temperature, short and long term fuel trim, and the per-cylinder misfire counts. Then monitor counters live while the engine idles and while you snap the throttle.
ExpectedCylinder 8 counts climbing while other cylinders stay near zero
Swap the cylinder 8 ignition coil with a neighbouring cylinder
Tools Basic hand tools, scan tool
Move the coil to another cylinder, clear codes, run the engine under the conditions the freeze frame showed, and see which cylinder now misfires. This is the single highest-value test on a coil-on-plug engine because it proves the coil rather than guessing at it.
ExpectedIf the misfire follows the coil to the new cylinder, the coil is confirmed faulty. If it stays on cylinder 8, the coil is good.
Remove and read the spark plug
Tools Plug socket, extension, torque wrench, inspection light
Inspect the electrode and insulator. Oil fouling, coolant deposits, fuel washing, a bridged gap, or a cracked insulator each point somewhere different. Compare it side by side against a plug from a healthy cylinder.
ExpectedLight tan or grey deposits and an intact insulator on a healthy cylinder. Gap and torque specs are vehicle-specific — use the service data for the engine.
Swap or test the cylinder 8 fuel injector
Tools Stethoscope, DMM, noid light or scope; scan tool with injector balance or contribution test on direct injection
On a port-injected engine, listen with a stethoscope for the click, measure winding resistance and compare across all cylinders, and check the drive signal; if the coil and plug are cleared, swap the injector to another cylinder the same way you swapped the coil and see if the misfire follows. On a direct-injected engine do not casually swap injectors — they need new combustion seals and a torque-to-spec reinstall, many vehicles require the injector trim codes to be reprogrammed to the new position, and piezo units cannot be judged by resistance. Use the manufacturer's injector balance or contribution test instead, and follow the service procedure to relieve fuel pressure before opening any line.
ExpectedOn port injection, resistance closely matched across all injectors, and the misfire following the injector when swapped. On direct injection, cylinder 8 standing out in the balance or contribution test. Specific values are vehicle-specific.
Test for a vacuum or intake leak feeding that cylinder alone
Tools Smoke machine, or a regulated propane tester or non-flammable leak spray, plus a scan tool
A failed intake manifold gasket at one runner, a cracked runner, or a leaking injector O-ring leans out one cylinder only. Smoke test the intake — this is the safe and more conclusive method. If you use a spray test instead, understand that solvents are flammable and a hot exhaust or an ignition arc will ignite them: use a non-flammable option such as propane from a regulated tester or a dedicated non-flammable vacuum-leak spray, keep it away from exhaust components, and have an extinguisher within reach.
ExpectedNo smoke escaping the intake, and no change in idle speed or fuel trim when the suspect area is tested
Run a compression test on cylinder 8, compared against the rest
Tools Compression gauge, or a leakdown tester
If ignition and fuel both check out, the cylinder is not sealing. Absolute numbers vary by engine, so the meaningful reading is the spread: a cylinder well below its neighbours indicates a valve, ring, or head gasket problem.
ExpectedAll cylinders within a tight band of each other; a large outlier is the finding
If compression is low, follow with a leakdown test to locate the leak
Tools Leakdown tester, compressed air
Pressurize the cylinder at TDC compression and listen. Air at the throttle body means an intake valve, at the tailpipe an exhaust valve, at the oil filler ring or bore wear, bubbles in the coolant a head gasket.
ExpectedLeakage percentage varies by engine and mileage; the location of the escaping air is what identifies the failure
Check the coil control and power circuits before condemning the PCM
Tools DMM, oscilloscope, wiring diagram
Only if a known-good coil still misfires on cylinder 8. Verify the coil's ignition feed, ground, and the driver signal from the PCM, and wiggle-test the harness with the engine running.
ExpectedClean supply voltage and a consistent drive pulse; missing drive with good supply implicates the harness or the PCM driver
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the cylinder 8 ignition coil | Misfire follows the coil when swapped to another cylinder | $70–$320 | DIY |
| Replace the spark plug, or the full set if they are at their service interval | Plug shows a cracked insulator, bridged or excessive gap, or heavy fouling, and a new plug clears the misfire | $90–$450 | DIY |
| Replace the cylinder 8 fuel injector | On port injection, the misfire follows the injector when swapped, or the injector fails resistance and flow testing. On direct injection, the cylinder fails the manufacturer's injector balance or contribution test. | $180–$700 | DIY |
| Repair the coil harness connector or replace a chafed section of the coil control wiring | A known-good coil still misfires and the drive signal or supply is missing or drops during a wiggle test | $90–$450 | DIY |
| Replace the intake manifold gasket or leaking injector O-ring | Smoke test shows a leak at that runner, or a non-flammable leak test changes idle speed or fuel trim at that runner | $250–$900 | DIY |
| Internal engine repair — valve, valve seat, ring, or head gasket work | Compression on cylinder 8 is well below its neighbours and leakdown identifies the escape path | $1,200–$5,000 | Shop |
Ranges are US independent-shop, parts plus labour. Coil-on-plug engines are cheap to service; engines using a coil pack, plug wires, or with the intake manifold over the plugs cost more in labour. An eight-or-more-cylinder engine needs a plug per cylinder, so a full set costs proportionally more than a four-cylinder job. Diagnostic time to confirm the cause typically runs one hour.
- Replacing every coil and plug on the engine because one cylinder misfired, when a five-minute coil swap would have identified the single bad part.
- Clearing the code before recording freeze frame data, throwing away the one piece of evidence that says whether the misfire happens cold, hot, at idle, or under load.
- Assuming cylinder 8 is on a particular bank or in a particular physical position. Cylinder numbering varies by manufacturer, and the wrong cylinder gets a new coil.
- Ignoring a flashing MIL and driving the car in, then paying for a catalytic converter on top of the original repair.
- Treating an intermittent misfire that only appears in rain as a coil failure without checking the plug wells and boots for water and carbon tracking.
- Replacing the catalytic converter for a downstream efficiency code while the misfire is still active, which destroys the new converter too.
- 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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