P0308 — Cylinder 8 Misfire Detected

P0308 — Cylinder 8 Misfire Detected. A fault in the engine misfire monitoring. Short, gentle driving to a shop is usually possible. Typical causes: incorrect fuel pressure or a weak fuel pump, vacuum leak in the intake, low compression in the affected cylinder(s).

powertrain (engine/transmission)

Is it safe to drive?

Prompt diagnosis; avoid extended driving P0308 should be diagnosed promptly. Brief low-load driving may be necessary to reach a repair facility, but continued operation with an active cylinder 8 misfire can damage the catalytic converter and increase fuel consumption.

Typical symptoms

  • The check-engine light may illuminate with P0308 stored in the powertrain controller.
  • The engine may idle roughly, hesitate, or shudder when P0308 is active.
  • Cylinder 8 misfire may cause a noticeable vibration that increases with engine load.
  • Power may fall during acceleration or uphill driving because fuel is not burning correctly.
  • Unburned fuel exiting cylinder 8 can produce loud exhaust pops or afterburn.
  • Fuel consumption may increase while the engine continues to report a cylinder 8 misfire.

Common causes

  • A failed or weakened ignition coil on cylinder 8 may prevent the spark from reaching the combustion chamber.
  • A worn, damaged, oil-fouled, or incorrectly gapped spark plug in cylinder 8 can create an incomplete burn.
  • An open, shorted, or corroded connector in the cylinder 8 ignition circuit may interrupt coil power or signal operation.
  • A clogged, leaking, or electrically defective fuel injector on cylinder 8 may supply the wrong amount of fuel.
  • Low cylinder 8 compression caused by worn piston rings, a damaged valve, a bent valve, or a head gasket defect can cause repeated misfires.
  • Incorrect valve timing or an engine-control problem that disrupts ignition and fueling on cylinder 8 may store P0308.

How to diagnose with an OBD scanner

  1. Confirm P0308 with an OBD2 scanner Connect an OBD2 scanner, such as an ELM327 interface used with AIScanAuto, and read stored, pending, and permanent codes. Confirm that cylinder 8 is still contributing misfires and note freeze-frame RPM, load, and fuel-trim data for conditions under which P0308 appears.
  2. Inspect the cylinder 8 spark plug Remove the spark plug from cylinder 8 and inspect the electrode, ceramic insulator, and threads. Look for a worn center electrode, excessive carbon, oil contamination, cracks, or a gap that does not match the engine specification; verify that the correct heat range is installed.
  3. Test the cylinder 8 ignition coil and wiring Inspect the coil, connector, and wiring for heat damage, corrosion, or pin movement. Perform a manufacturer-approved coil load test. If a controlled coil swap is used, monitor whether P0308 follows the component; on waste-spark systems, test the paired coil instead of assuming it serves only cylinder 8.
  4. Check cylinder 8 fuel delivery Inspect the injector connector and verify that the injector receives a valid command. Where equipment is available, scope the injector pulse and compare it with an adjacent cylinder. Also check fuel pressure and short-term plus long-term fuel trims for signs that cylinder 8 is receiving too little or too much fuel.
  5. Measure cylinder 8 compression Perform a compression test and, if needed, a leak-down test on cylinder 8. Low dry compression that improves with added oil suggests ring or piston wear, while strong leakage points toward a valve or head-gasket problem. Confirm valve timing before condemning internal components.
  6. Repair and verify the misfire After correcting the confirmed fault, clear P0308 with the scanner and run the engine through several load and acceleration cycles. Confirm that no new cylinder 8 misfire counts appear and review fuel trims and catalytic-converter temperature data for abnormal operation.

Approximate repair cost

Typical independent-shop estimates in USD are $100–$250 for P0308 diagnosis and cylinder isolation, $80–$180 for a cylinder 8 spark plug, $180–$350 for an ignition coil, and $350–$700 for a fuel injector. Valve, head-gasket, piston-ring, or other internal engine repairs may range from $1,200–$3,500 or more. These figures are estimates and vary by region.

Common questions

Does P0308 always mean the cylinder 8 spark plug is bad?

No. P0308 identifies cylinder 8 as the misfiring cylinder, but the fault may be a plug, coil, injector, wiring problem, low compression, valve issue, or engine-control fault.

Can the vehicle be driven with P0308?

Short trips at low load may be acceptable, but an active P0308 misfire can overheat the catalytic converter. Extended or high-load driving should be avoided until the fault is corrected.

What information should an OBD2 scanner collect for P0308?

Record whether P0308 is stored, pending, or permanent, along with freeze-frame RPM, load, fuel trims, and live misfire counts. The scanner narrows the condition but cannot identify the failed part by itself.

Why does cylinder 8 misfire and misfire counts keep returning?

A failed repair or an uncorrected cause will let the controller detect cylinder 8 combustion misses again. Verify the repair under several load conditions, then confirm that no new P0308 events occur.

A P0308 diagnostic trouble code tells you that your engine's powertrain control module has identified a misfire occurring specifically in cylinder 8. The misfire detection system works by analyzing crankshaft acceleration; each combustion event should accelerate the crank by a predictable amount, and when one cylinder fails to fire properly, the module notices the irregular rotation speed and logs the fault after confirming it across multiple drive cycles.

Cylinder numbering varies by engine configuration. On most inline engines, cylinder 8 sits at the far end opposite the belt-driven accessories — this matters because reaching it for repairs may require disassembly of surrounding components. V-configured engines number their cylinders differently depending on the manufacturer, so consulting your vehicle's firing order diagram helps you locate the actual problem cylinder before spending money on parts or labor.

When the PCM detects misfire-related irregularities beyond its programmed threshold, it stores P0308, illuminates the Check Engine light, and captures freeze frame data. That snapshot records engine speed, load, temperature, and vehicle speed at the exact moment the fault occurred, giving you a precise starting point for troubleshooting rather than guessing blindly.

Symptoms

  • Noticeable increase in fuel consumption compared to normal driving patterns
  • Engine stuttering, jerking, or hesitation when pressing the accelerator
  • Rough idle with visible or audible vibration coming from the engine bay
  • Stronger-than-usual fuel smell from the exhaust
  • Check Engine light either steadily illuminated or flashing during operation
  • Reduced engine power and sluggish acceleration response

Causes

  • Defective ignition coil or damaged wiring feeding cylinder 8's ignition system
  • Worn, fouled, or incorrectly gapped spark plugs failing to ignite the air-fuel mixture
  • Vacuum leak somewhere in the intake manifold or associated hoses
  • Low compression in cylinder 8 due to worn rings, failed valves, or head gasket damage
  • Faulty crankshaft or camshaft position sensor producing incorrect timing signals
  • Clogged or malfunctioning fuel injector unable to deliver proper fuel volume
  • Problematic exhaust gas recirculation valve affecting mixture preparation

How to Diagnose

  1. Plug in an OBD2 scanner, retrieve stored P0308 codes, and record all freeze frame information before clearing anything — those conditions tell you exactly what the engine was doing when the failure occurred.

  2. Scan for any additional diagnostic trouble codes; misfire codes often appear alongside sensor failures or vacuum-related faults that point toward the root cause rather than the symptom.

  3. Inspect all wiring harnesses and electrical connectors in the ignition and fuel delivery circuits for frayed insulation, corrosion buildup, broken terminals, or loose connections that could interrupt signals to cylinder 8.

  4. Access live misfire counter data through your scanner to confirm cylinder 8 is the active misfire location and measure how severe the counter increments during each firing event.

  5. Perform a cylinder-to-cylinder swap test: exchange the ignition coil from cylinder 8 with another cylinder's coil, clear codes, and drive to see whether the misfire migrates to the different cylinder — if it follows the coil, you have your answer.

  6. After any repairs, clear the code, complete a drive cycle matching the freeze frame conditions, and verify that P0308 does not return before considering the job finished.

Repair Options with Costs

  • Replace spark plugs: $90–$250
  • Replace ignition coil (single): $150–$380
  • Clean or replace fuel injector: $200–$600
  • Engine mechanical repair addressing compression issues: $800–$2,500

Total Repair Cost

The final bill depends entirely on which underlying problem triggered P0308 in your vehicle. Parts and labor at independent shop rates typically fall between $90 for basic spark plug replacement and $2,500 for comprehensive engine mechanical work. Most misfire issues trace back to failed ignition components or dirty injectors — both relatively affordable fixes — so thorough diagnosis before purchasing expensive parts almost always saves money overall.

Can You Drive with P0308?

Use caution and minimize driving distance. An active misfire allows raw fuel to enter the exhaust stream, and that unburned fuel can rapidly overheat and destroy a catalytic converter if the condition persists. If your Check Engine light is flashing rather than steady, the damage risk is immediate — pull over safely and arrange diagnosis rather than continuing down the road.