P0308 — Cylinder 8 Misfire Detected

Quick answer

• What the code means: The engine control module (ECM) has detected a misfire in cylinder 8.
• Severity: Get towed or drive carefully to a mechanic immediately. Prolonged driving can cause serious engine or catalytic converter damage.
• Most common cause: Faulty spark plug or ignition coil.

What the P0308 code means

The P0308 diagnostic trouble code indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a misfire in cylinder number 8. A misfire occurs when an engine cylinder does not produce the expected power stroke due to an incomplete or absent combustion event. The "8" in P0308 specifically points to cylinder 8 as the source of the misfire.

In plain English, your engine isn't firing on all cylinders, specifically cylinder number 8. This leads to rough running, reduced power, and decreased fuel efficiency.

Common symptoms

• Check Engine Light (CEL) illuminated, possibly flashing
• Rough idle or vibration
• Loss of engine power and acceleration
• Decreased fuel economy
• Engine hesitation or stumbling during acceleration
• Strong odor of unburnt fuel from the exhaust
• Engine knocking or pinging sounds

Common causes

1. Faulty Spark Plug: Worn, fouled, or incorrectly gapped spark plugs are a very common cause of misfires. If the spark plug in cylinder 8 isn't firing effectively, combustion won't occur.
2. Defective Ignition Coil: The ignition coil provides the high voltage necessary for the spark plug to ignite the air-fuel mixture. A failing coil on cylinder 8 will result in no spark.
3. Clogged or Faulty Fuel Injector: If the fuel injector for cylinder 8 is clogged or not delivering the correct amount of fuel, the air-fuel mixture will be too lean or too rich, preventing proper combustion.
4. Vacuum Leaks: A vacuum leak near cylinder 8 can lean out the air-fuel mixture, leading to a misfire. This is less common for a single-cylinder misfire but still possible.
5. Low Compression: Internal engine problems such as worn piston rings, damaged valves, or a blown head gasket can cause low compression in cylinder 8, preventing proper combustion.
6. Wiring Issues: Damaged or corroded wiring to the spark plug, ignition coil, or fuel injector for cylinder 8 can interrupt the necessary electrical signals.
7. ECM/PCM Failure: While rare, a faulty ECM/PCM could incorrectly identify a misfire or fail to control the ignition or fuel system properly for cylinder 8.

How a technician diagnoses it

1. Verify the code and check for other codes: The technician will connect an OBD-II scanner to confirm the P0308 code and check for any other related codes that might provide additional clues.
2. Freeze Frame Data Review: Analyze the freeze frame data to understand the engine conditions (engine speed, load, temperature, etc.) at the time the misfire occurred. This helps replicate the problem if it's intermittent.
3. Visual Inspection: Perform a thorough visual inspection of spark plug wires, ignition coils, spark plugs, and fuel injector connectors for cylinder 8. Look for obvious signs of damage, corrosion, or loose connections.
4. Swap Components Test: To isolate the issue between spark plug, ignition coil, and fuel injector:
   • Spark Plug: Swap the spark plug from cylinder 8 with one from a known good cylinder. If the misfire moves to the new cylinder, the spark plug is faulty.
   • Ignition Coil: Swap the ignition coil from cylinder 8 with one from a known good cylinder. If the misfire moves, the coil is faulty.
   • Fuel Injector: Move the fuel injector from cylinder 8 to another cylinder. If the misfire moves, the fuel injector is faulty.
5. Scan Tool Data Analysis: Monitor live data streams from the ECM/PCM, specifically focusing on misfire counters for each cylinder. A healthy cylinder will show very few or no misfires, while cylinder 8 will show a significantly higher count. Also, monitor fuel trims to detect lean or rich conditions.
6. Compression Test: If the above components check out, a compression test will be performed on cylinder 8 to check for internal engine issues. Good compression typically ranges from 150-180 PSI and should be within 10-15 PSI of other cylinders. Low or zero compression in cylinder 8 indicates an internal engine problem.
7. Fuel Pressure and Injector Pulse Test: Check fuel pressure to ensure adequate supply. An oscilloscope can be used to check the injector pulse and pattern for cylinder 8.
8. Vacuum Leak Test: If needed, a smoke machine can be used to detect vacuum leaks around the intake manifold or vacuum lines related to cylinder 8.

Typical repair cost

• Spark Plug Replacement: $100 - $300 (parts - $10-$30 per plug; labor - $80-$270 for general diagnosis and replacement)
• Ignition Coil Replacement: $150 - $400 (parts - $50-$150 per coil; labor - $100-$250)
• Fuel Injector Replacement: $200 - $600 (parts - $70-$200 per injector; labor - $130-$400, can be higher if access is difficult)
• Vacuum Leak Repair: $100 - $300 (parts - $10-$50; labor - $90-$250)
• Engine Internal Repair (e.g., head gasket, piston rings): $1000 - $4000+ (highly variable depending on severity and engine type)

These costs are estimates and can vary based on vehicle make and model, labor rates, and parts quality.

Can I keep driving with this code?

It is not recommended to keep driving with a P0308 code. A continuous misfire can lead to serious and expensive damage to your catalytic converter, as unburnt fuel enters the exhaust and overheats it. It can also cause further engine damage due to excessive vibration or stress on specific components. Get the vehicle to a qualified repair shop as soon as possible.

Related codes

• P0300: Random/Multiple Cylinder Misfire Detected
• P0301 - P0307: Misfire Detected in Cylinder 1 through 7 (respectively)
• P0313: Misfire Detected with Low Fuel Level
• P0420 / P0430: Catalyst System Efficiency Below Threshold (often a consequence of prolonged misfires)

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