OBD-II Code P0307 — What It Means & How to Fix It

What Does This Error Mean?

P0307 means the engine computer detected a misfire in cylinder 7. A misfire happens when a cylinder fails to ignite its air-fuel mixture properly. This code only applies to V8, V10, or V12 engines — those are the only engines with a cylinder 7. Symptoms include rough running, shaking, reduced power, and poor fuel economy.

Affected Models

V8 and V10 vehicles 1996+
Common in Ford F-150 with 5.0L and 6.2L V8
Common in Chevrolet Silverado with 5.3L and 6.2L V8
Common in Dodge Ram with 5.7L HEMI V8
Common in Ford F-250/F-350 with 6.2L and 7.3L V8

Common Causes

Worn or fouled spark plug on cylinder 7 no longer producing a strong spark
Faulty ignition coil on cylinder 7 with weak or intermittent spark
Clogged or failed fuel injector not delivering fuel to cylinder 7
Low compression in cylinder 7 from worn rings, valves, or head gasket
Vacuum leak or intake manifold gasket failure affecting cylinder 7

How to Fix It

Start with the simplest fix — replace the spark plug on cylinder 7. V8 engines have the plug on the same side and position as the cylinder number. Use the factory-specified plug type and gap setting.

On some V8 engines, cylinder 7 is on the passenger side. Consult your vehicle's cylinder diagram to confirm location.
Swap the ignition coil from cylinder 7 to a different cylinder. Clear the code and drive. If the misfire follows the coil to its new location, the coil is bad. If the misfire stays on cylinder 7, the coil is fine.

Coil packs on modern engines are individual coil-on-plug units. Swapping takes about 5 minutes per coil.
Similarly, swap the fuel injector from cylinder 7 to another cylinder if you suspect an injector issue. A misfire that follows the injector confirms an injector fault. A misfire that stays on cylinder 7 points elsewhere.

If both the coil and injector test fine after swapping, perform a compression test next.
Perform a compression test on cylinder 7 and compare to adjacent cylinders. A reading 20% or more below the others indicates internal engine wear — rings, valves, or head gasket. This requires professional evaluation.

Compression test kits are available for about $30 at auto parts stores. It's a straightforward DIY test.
Check for vacuum leaks near the intake manifold on cylinder 7. Use a can of carburetor cleaner sprayed carefully around intake manifold gaskets with the engine running. A change in idle speed indicates a leak.

Be cautious with flammable spray near hot engine components. Some mechanics use a smoke machine instead for safety.

When to Call a Professional

If spark plug and coil replacement don't fix the misfire, suspect a fuel or mechanical issue. A compression test and leak-down test will determine if cylinder 7 has an internal problem. Head gasket failures are expensive — $1,000-$2,500 — but early detection prevents further damage. Diagnosis typically costs $100-$150. Spark plug and coil replacement runs $150-$400 depending on the vehicle.

Frequently Asked Questions

What engines have a cylinder 7?

Only engines with 7 or more cylinders. This means V8, V10, and V12 engines. On a standard GM V8, cylinder 7 is on the driver's side, third from the front. On Ford V8s, the numbering is different — always check your specific vehicle's layout.

Can I drive with a P0307 misfire?

Short distances are okay in a pinch, but don't ignore it. A misfiring cylinder dumps raw fuel into the exhaust. This can overheat and destroy the catalytic converter — a $500-$2,000 repair. Fix it as soon as possible.

I replaced the spark plug but the misfire is still there — what next?

Move on to the ignition coil — it's the next most common cause. Do the coil swap test described above. If the coil is fine, test the injector. If both are fine, perform a compression test to check for internal engine wear.