What P0308 Means

P0308 is a diagnostic trouble code defined by SAE J2012 as "Cylinder 8 Misfire Detected." The engine control module (ECM) sets this code when it detects that cylinder 8 is not firing properly, based on irregularities in crankshaft rotation speed measured by the crankshaft position sensor.

During normal combustion, each cylinder fires in sequence and contributes equally to crankshaft acceleration. When a cylinder misfires—whether due to lack of spark, fuel, or compression—the crankshaft slows momentarily during that cylinder's power stroke. The ECM monitors these variations continuously. If the misfire rate in cylinder 8 exceeds a calibrated threshold over a specified number of crankshaft revolutions, the ECM logs P0308 and illuminates the check engine light.

The code may appear as pending after a single trip or become confirmed after multiple detection cycles, depending on the vehicle's onboard diagnostics calibration. Manufacturer-specific definitions and thresholds can vary, so always consult your vehicle's service information for precise criteria.

Symptoms

The most common symptom of P0308 is a steady or flashing check engine light. A flashing light indicates a severe misfire that can damage the catalytic converter and requires immediate attention.

Other symptoms you may notice include:

  • Rough idle or noticeable engine vibration, especially at idle
  • Loss of power or hesitation during acceleration
  • Increased fuel consumption
  • Raw fuel smell from the exhaust
  • Engine stumble or surge under load

In some cases, particularly if the misfire is intermittent or occurs only under specific conditions, you may experience no symptoms beyond the illuminated check engine light. Freeze frame data captured when the code set can help identify the operating conditions that triggered the misfire.

Causes, Most Likely First

P0308 points to a problem affecting cylinder 8 specifically. The most common causes, ranked by frequency, are:

  • Faulty ignition coil for cylinder 8 — weak or intermittent spark due to coil breakdown
  • Worn or fouled spark plug in cylinder 8 — carbon buildup, oil fouling, or excessive electrode wear prevents proper ignition
  • Vacuum leak affecting cylinder 8 — intake manifold gasket leak or cracked vacuum hose introduces unmetered air
  • Low compression in cylinder 8 — worn piston rings, burned or bent valves, or head gasket failure reduce cylinder pressure
  • Clogged or faulty fuel injector for cylinder 8 — restricted flow or electrical fault prevents adequate fuel delivery
  • Damaged or corroded spark plug wire or boot (on vehicles with distributor ignition) — arcing or high resistance interrupts spark
  • Intake manifold gasket leak at cylinder 8 — localized gasket failure creates a lean condition
  • Worn camshaft lobe for cylinder 8 — reduced valve lift affects air and fuel flow
  • Manufacturer-specific causes — some engines have known issues such as valve seat recession, injector harness faults, or coil-on-plug connector corrosion; consult technical service bulletins for your vehicle

Because P0308 isolates the fault to a single cylinder, diagnosis often involves comparing cylinder 8 components and readings to those of other cylinders.

How to Diagnose P0308

Diagnosing a misfire systematically saves time and prevents unnecessary parts replacement. Follow these steps from simplest to most involved:

  1. Read freeze frame data. Capture the engine speed, load, coolant temperature, and fuel trim values present when the code set. This context helps identify whether the misfire occurs at idle, under load, or when cold.

  2. Inspect the spark plug and ignition coil for cylinder 8. Remove the spark plug and examine it for carbon fouling, oil deposits, or worn electrodes. Check the ignition coil and connector for cracks, corrosion, or oil intrusion. Replace the spark plug if it shows wear or fouling.

  3. Swap the ignition coil with another cylinder. If the misfire moves to the cylinder where you installed the suspect coil, the coil is faulty. If P0308 remains, continue diagnosis.

  4. Check for vacuum leaks. Inspect intake manifold gaskets, vacuum hoses, and PCV connections around cylinder 8. Use a smoke machine or propane enrichment method to locate leaks. A localized vacuum leak will create a lean condition that affects only nearby cylinders.

  5. Perform a compression test on cylinder 8. Compare the reading to other cylinders and to the manufacturer's specification (typically within about 10% across all cylinders). Low compression indicates internal engine wear or damage. A leak-down test can further isolate whether the leak is past the rings, valves, or head gasket.

  6. Inspect the fuel injector for cylinder 8. Check injector resistance and compare it to specification. Use a noid light or oscilloscope to verify the injector pulse signal. Listen for injector clicking with a mechanic's stethoscope. Swap the injector with another cylinder to see if the misfire follows.

  7. Check live data for fuel trim and oxygen sensor response. Short-term and long-term fuel trims should be within roughly ±10% at idle and under light load. Excessive positive trim (above about +10%) indicates a lean condition; excessive negative trim suggests a rich condition. Confirm these values in your vehicle's service information, as thresholds vary. Fuel trim diagnosis can reveal system-wide issues that contribute to misfires.

Manufacturer-specific diagnostic procedures may include additional tests such as coil primary circuit checks, injector flow testing, or camshaft position correlation. Always consult your vehicle's service information for detailed test procedures and specifications. Diagnostic output is a decision-support estimate, not a substitute for professional inspection.

Is It Safe to Drive With P0308?

Limited driving to a repair facility may be acceptable if the check engine light is steady, the engine runs smoothly enough to maintain safe control, and you avoid hard acceleration or high speeds. However, continuing to drive with a misfire risks further damage.

Stop driving immediately if:

  • The check engine light is flashing. A flashing light indicates a severe misfire that sends unburned fuel into the exhaust, where it can overheat and destroy the catalytic converter.
  • The engine stalls, loses significant power, or runs so roughly that safe vehicle control is compromised.
  • You notice overheating, unusual noises, or smoke from the exhaust.

Even a steady light warrants prompt diagnosis. Unburned fuel from a misfiring cylinder washes oil from cylinder walls, accelerates wear, fouls the catalytic converter, and reduces fuel economy. Address P0308 as soon as practical to prevent costly secondary damage.

Repair Options

Repair complexity and cost depend on the underlying cause. Many P0308 repairs are straightforward; others require significant disassembly or professional tools.

DIY repairs:

  • Replace the spark plug in cylinder 8. This requires only basic hand tools. Use the spark plug type specified in your owner's manual and gap it correctly.
  • Replace the ignition coil. This is the most common fix. On coil-on-plug systems, it is a simple swap. Ensure the replacement coil matches your vehicle's specification.
  • Inspect and repair vacuum leaks. Replace cracked hoses or damaged gaskets. Some intake manifold gasket jobs are accessible to experienced DIYers; others require special tools or engine disassembly.

Repairs best performed by a qualified mechanic:

  • Compression test and internal engine inspection. Low compression may require valve adjustment, valve replacement, piston ring replacement, or head gasket repair—all of which involve significant labor and specialized knowledge.
  • Fuel injector replacement or professional cleaning. Injector service often requires a fuel rail removal and, in some cases, ultrasonic cleaning equipment.
  • Head gasket or valve repair. These repairs involve cylinder head removal, resurfacing, and precise reassembly with torque specifications and sequences.

If you lack the tools, service information, or experience for a particular repair, consult a qualified mechanic. Misdiagnosis or improper repair can lead to additional damage and expense.

P0308 is part of a family of cylinder-specific misfire codes. You may see one or more of these codes alongside P0308:

If multiple cylinder misfire codes appear together, suspect a common cause such as a vacuum leak, fuel pressure problem, or ignition system fault rather than individual component failures in each cylinder.