OBD-II Code P0308 — What It Means & How to Fix It

What Does This Error Mean?

P0308 means the engine computer detected a misfire in cylinder 8. This code only applies to V8, V10, or V12 engines. A misfire means cylinder 8 isn't burning its air-fuel mixture properly. Your car will feel rough, shake at idle, and may lack power. Common causes are a worn spark plug, bad coil, or failed injector.

Affected Models

V8 and V10 vehicles 1996+
Common in Chevrolet Silverado and Tahoe with V8 engines
Common in Ford Mustang GT and F-150 V8
Common in Dodge Ram 1500 and 2500 with 5.7L HEMI
Common in Jeep Grand Cherokee with 5.7L V8

Common Causes

Worn, fouled, or damaged spark plug on cylinder 8
Failed ignition coil on cylinder 8 producing weak or no spark
Clogged or failed fuel injector on cylinder 8
Low compression in cylinder 8 from worn rings or valves
Damaged or cracked spark plug wire (if your vehicle uses wires)

How to Fix It

Replace the spark plug on cylinder 8 first. This is the cheapest and most common fix for a single-cylinder misfire. Use the factory-specified plug and gap. Inspect the old plug — a fouled or eroded plug confirms this was the issue.

Cylinder 8 on most GM V8s is at the rear of the passenger side. On Ford V8s, verify the layout with your owner's manual.
If the new plug doesn't help, swap the cylinder 8 ignition coil with a coil from a different cylinder. Clear the code and drive. If the misfire moves to the new cylinder, you've confirmed a bad coil.

Coil-on-plug coils are typically secured with one bolt and have one wiring connector. The swap takes minutes.
Test the fuel injector on cylinder 8 by swapping it with an adjacent cylinder's injector. If the misfire follows the injector, it needs replacement. If it stays on cylinder 8, the injector is not the problem.

Measure the injector's resistance with a multimeter before swapping — this quickly confirms if the coil winding has failed.
Perform a compression test on cylinder 8 if spark, coil, and injector are all normal. Low compression indicates worn piston rings, damaged valves, or a blown head gasket. This is a more serious and expensive repair.

A cylinder with less than 100 PSI, or more than 20% below adjacent cylinders, has a mechanical problem.
Check for vacuum leaks or intake manifold gasket failure on the cylinder 8 side of the engine. A leak allows unmetered air in, causing a lean misfire. Spray carb cleaner carefully around manifold gaskets with the engine running.

A smoke machine is safer for this test and provides more accurate leak detection.

When to Call a Professional

If plug and coil replacement don't resolve the misfire, have a mechanic perform a compression test. A leakdown test can also reveal valve or head gasket problems. Mechanical misfire causes are more expensive to repair but important to catch early. Diagnosis typically costs $100-$150. Spark plug and coil service runs $150-$400 depending on vehicle.

Frequently Asked Questions

My HEMI shows P0308 — is MDS deactivation causing this?

It's possible. Dodge HEMI engines use Multi-Displacement System (MDS) to deactivate cylinders for fuel economy. Cylinders 1, 4, 6, and 7 are typically deactivated. Cylinder 8 is not normally an MDS cylinder, so MDS is probably not the cause. Check the spark plug and coil first.

How serious is a single cylinder misfire?

More serious than people think. A misfiring cylinder sends unburned fuel through the exhaust. This quickly overheats and destroys the catalytic converter. Cat replacement can cost $500-$2,000 or more. Fix single-cylinder misfires promptly — they are usually cheap repairs.

Can a P0308 misfire cause other codes?

Yes. Prolonged misfiring can trigger P0300 (random misfire) as other cylinders are affected by fuel trim corrections. It can also trigger catalyst efficiency codes like P0420 if the cat overheats. Fix P0308 first and the other codes may resolve on their own.