P0308 Code — Cylinder 8 Misfire Detected (Causes, Costs & Fix‑It Guide)

Your check‑engine light just lit up, and the scanner shows P0308 – Cylinder 8 Misfire Detected.

You may notice a shaking idle, a dip in power when you step on the gas, or a sudden increase in fuel consumption. This guide walks you through the most common reasons for a cylinder 8 misfire, how to confirm the culprit, and which repairs you can safely do yourself.

What the Code Means

The ECM has logged a misfire on cylinder 8 because the combustion event did not produce the expected pressure rise. The ECM counts each missed event and stores the code when the count exceeds the manufacturer’s threshold.

Typical Symptoms

If you see any of these, the P0308 code is likely accurate, but you still need to verify the source.

Why Cylinder 8 Misfires Occur

Misfires fall into three broad families:

1. Ignition faults – Bad spark plug, cracked coil boot, or a failing coil prevents a spark.
2. Fuel delivery problems – A clogged injector, low fuel pressure, or a dead injector driver circuit starves the cylinder.
3. Mechanical defects – Low compression from a worn valve, broken piston ring, or a head‑gasket leak disrupts the combustion process.

Identifying the family early saves you from buying unnecessary parts.

Diagnosis Roadmap (Easy → Hard)

Below is the ordered list of actions, from the simplest visual checks to the more involved mechanical tests. Perform each step, compare the “good” reading to the “bad” reading, and decide whether a part needs replacement.

1. Quick Visual Inspection

Safety first – Disconnect the negative battery terminal and wait at least 10 seconds. This prevents accidental sparks while you work around high‑voltage components.

Locate the spark plug, coil, and injector for cylinder 8. On most V‑type engines, cylinder 8 sits on the far side of the bank opposite cylinder 1.

What to look for

• Cracked ignition coil boots or damaged spark‑plug wires.
• A plug that is loose, missing, or covered in oil.
• An injector connector that is not fully seated.

Good vs. Bad

Good: All connections are tight, boots are intact, no oil or coolant leaks near the plug. Bad: A visibly damaged boot, a hanging wire, or a connector that’s half‑out.

If you spot a problem, reseat or replace the offending part. This step often resolves the code in under 15 minutes.

2. Live Data Scan

Plug an OBD‑II scanner into the DLC and select Live Data.

• Watch Misfire Count – Cylinder 8, Short‑Term Fuel Trim (STFT), and Long‑Term Fuel Trim (LTFT) while the engine idles at 750–900 rpm.
• Gently press the accelerator to add load and watch the numbers change.

Good vs. Bad

Good: Cylinder 8 misfire count stays at 0; fuel trims hover around 0 % (±5 %). Bad: Misfire count climbs above 0, especially under load; fuel trims become positive, indicating the ECU is adding fuel to compensate.

Live data confirms that the ECM is actively seeing a misfire, not just a stored code from a past event.

3. Spark Plug & Ignition Coil Test

Spark Plug

• Remove the plug with a 5/8‑inch (or manufacturer‑specified) spark‑plug socket.
• Inspect the tip. A light tan color is normal. Black, oily, or melted plugs point to fuel richness, oil contamination, or overheating.
• Check the gap with a feeler gauge; most gasoline V‑engines call for 0.035–0.045 in (0.9–1.1 mm).

Coil Primary Resistance

• Unplug the coil.
• Set a digital multimeter to the Ω range and measure resistance between the coil’s primary terminals (usually marked + and –).
• Compare to the service‑manual spec, which is typically 0.5–1.5 Ω for many OEM coils.

Good vs. Bad

Good: Plug tip clean, gap within spec, coil resistance inside the specified range. Bad: Fouled plug, gap out of spec, coil resistance infinite or outside the range.

Repair

• Replace the spark plug ($5‑$12).
• Replace the coil if resistance is out of spec ($30‑$80).

4. Fuel Injector Operation Check

Safety note – Keep the engine cool and wear eye protection. The injector can spray fuel under pressure.

• Start the engine and place a mechanic’s stethoscope on the body of the cylinder 8 injector.
• Listen for a rapid “click” that matches the engine’s firing frequency.
• If no click is heard, swap the injector with another cylinder’s injector (e.g., cylinder 6). Re‑scan for the misfire.

Good vs. Bad

Good: Consistent clicking; after swapping, the misfire count drops, confirming the original injector was fine. Bad: No click, or the misfire follows the injector after swapping, confirming a faulty injector.

Repair

• Clean the injector with a professional cleaning kit ($30‑$50).
• Replace the injector if cleaning does not restore the click ($80‑$150 per unit).

5. Compression & Leak‑Down Test

Compression Test

• Remove all spark plugs.
• Screw the compression gauge into cylinder 8, crank the engine for 4–6 revolutions, and record the pressure.
• Compare to the other cylinders; a difference greater than 10‑15 psi is significant.

Leak‑Down Test (if compression is low)

• Attach a leak‑down tester to cylinder 8, pressurize to ≈100 psi, and listen for air escaping from the intake, exhaust, or coolant passages.

Good vs. Bad

Good: Compression matches the rest of the engine (typically 150‑200 psi for most gasoline V‑engines). Bad: Low pressure; audible hissing from a valve, exhaust, or coolant indicates a mechanical fault.

Repair

• Low compression usually means a valve‑seat, piston‑ring, or head‑gasket issue. These repairs require disassembly of the engine block and are best performed by a professional shop.

Vehicle‑Specific Notes

Knowing the typical failure point for your make can save you time and money.

Cost Breakdown (2026 USD)

Bottom line – Ignition‑related fixes are usually under $150 total if you do the work yourself. Mechanical repairs can quickly exceed $1,000, so a professional diagnosis is the safest route once low compression is confirmed.

Emissions Impact

A cylinder 8 misfire throws unburned fuel into the exhaust. This raises hydrocarbon (HC) and carbon monoxide (CO) levels, often causing a failed state emissions inspection. The excess fuel can also overheat the downstream catalytic converter, leading to premature catalyst failure. Fixing the misfire restores proper combustion, reduces HC/CO output, and protects the catalyst.

Driving Safety Assessment

• Short trips – You can drive 5‑10 miles at light throttle without immediate danger.
• High load – Hill climbs, rapid acceleration, or towing can overheat the misfiring cylinder and damage the catalytic converter.
• Noise – A popping or back‑fire sound indicates severe unburned fuel ignition and should be addressed immediately.

Limit use to gentle city driving until the code is cleared and the engine runs smoothly.

Frequently Asked Questions

How do I fix code P0308? Start with the visual inspection, then move to live data, spark plug/coil testing, injector verification, and finally a compression test if earlier steps are clean.

How long can I drive with a P0308 code? A few days of light‑load driving are acceptable, but avoid aggressive acceleration or long highway trips.

Will this code cause a failed emissions test? Yes. Unburned hydrocarbons typically push HC readings above the legal limit, resulting in a fail.

Is the misfire fixable? In 80‑90 % of cases, replacing a spark plug, coil, or injector resolves the issue.

What should I watch for on a Chevy Silverado with P0308? The 5.3 L V8 often suffers coil‑pack failures on the far bank. Swapping the coil pack is usually the quickest fix.

Related DTCs You Might Encounter

• P0300 – Random/Multiple Cylinder Misfire Detected – Indicates misfires on more than one cylinder.
• P0307 – Cylinder 7 Misfire Detected – Same troubleshooting steps, just a different cylinder.
• P0420 – Catalyst System Efficiency Below Threshold – Often appears after a prolonged misfire because the catalyst gets overloaded.
• P0402 – Exhaust Gas Recirculation (EGR) Flow Insufficient – Can be triggered by high HC levels from a misfire.

Bottom Line

P0308 – Cylinder 8 Misfire Detected is most often an ignition or fuel‑delivery issue that you can diagnose and repair with basic tools. Begin with a quick visual check, verify live data, then test the spark plug, coil, and injector. If those components are healthy, a compression test will reveal whether the problem lies deeper in the engine. By following this roadmap, you’ll avoid unnecessary parts purchases, keep emissions within legal limits, and protect your vehicle’s catalytic converter.

Happy wrenching, and drive safely!

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