Why does my engine have a cylinder 7?

Cylinder 7 is the seventh cylinder in the firing order layout, which only exists on engines with eight or more cylinders. If you're seeing P0307, you almost certainly have a V8 — common examples include the GM 5.3L and 6.2L truck V8s, Ford 5.0L Coyote and modular V8s, Dodge Hemi engines, and Toyota's V8 truck engines. On most layouts, cylinder 7 sits at the rear of one bank, which is why it's a bit of a pain to reach.

Can I keep driving with a P0307?

Drive only as far as you need to get the car somewhere safe. A misfire dumps unburned fuel into your exhaust, and that raw fuel will overheat your catalytic converter — which is the single most expensive component a misfire can damage. If the check engine light is flashing rather than solid, that's the PCM warning you that catalyst damage is actively happening. Get it diagnosed within days, not weeks.

Should I just replace all 8 coils and plugs at once?

On a high-mileage V8 where you don't know the service history, doing all 8 at once often makes sense — you save labor on rear-cylinder access, and the other coils are usually within months of failing anyway. Expect to spend $400-$900 for the parts and another $200-$500 in labor depending on how buried the rear plugs are. If the vehicle is newer or the service history is documented, replacing just the one failed component is fine.

How much does it cost to fix P0307?

Best case is a single spark plug at $50-$200 if the misfire is purely ignition wear. A coil replacement runs $150-$450. If it turns out to be a fuel injector, plan for $300-$700. Worst case on GM truck V8s is an AFM lifter failure, which can run $1,500-$3,000 or more. The diagnostic test of swapping the coil between cylinders takes 15 minutes and tells you which path you're on.

OBD-II trouble code

P0307: Cylinder 7 Misfire Detected

Cylinder 7 is failing to fire cleanly on a measurable share of combustion events. Because only V8 (and a handful of V10/V12) engines have a cylinder 7, this code points you directly at a specific coil, plug, or injector on a known side of the engine.

Quick facts

System: Powertrain
Category: Ignition / Misfire
Severity: High severity
Drivable: Usually safe to drive short-term
Repair cost range: $80 – $1,200
DIY difficulty: Intermediate DIY

What does P0307 mean?

P0307 sets when the powertrain control module sees cylinder 7's crankshaft acceleration pattern fall short of what a clean combustion event should produce. The PCM watches the rotational speed of the crank as each cylinder fires, and when one cylinder isn't pushing on the crank hard enough — or isn't pushing at all — the slowdown shows up in the data and a misfire counter increments for that specific cylinder.

Because P0307 is cylinder-specific, you skip most of the diagnostic work involved in a generic P0300. You already know which cylinder, which means you know which coil pack, which spark plug, which fuel injector, and which side of the intake manifold to look at. On almost every V8 layout in service today, cylinder 7 sits at the rear of one bank — the firewall end — so it's also one of the more annoying cylinders to actually reach. Plan for some labor time even on what looks like a $50 spark plug job.

The most common cause is an aging ignition coil. Coil-on-plug coils run hot, sit right above an exhaust manifold, and start to break down internally well before they fail outright. Carbon-fouled or worn spark plugs are the next most common cause — V8 plugs on modern engines are spec'd for 60,000-100,000 miles, but the heat cycling in the rear cylinders tends to age the back plugs faster than the front ones. Fuel injector problems, vacuum leaks at the rear of the intake manifold, low compression from a bad valve, and (on GM AFM/DFM-equipped engines) collapsed lifters round out the realistic causes.

Common causes

Failing ignition coil on cylinder 7
Worn or carbon-fouled spark plug
Clogged or leaking fuel injector
Vacuum leak at the rear of the intake manifold
Low compression on cylinder 7 (worn valve, head gasket, or rings)
Collapsed AFM/DFM lifter on GM 5.3L / 6.2L V8s
Damaged spark plug wire (on older distributor-style V8s)
Carbon buildup on the intake valve (direct-injection engines)
Wiring or connector fault at the coil or injector

Symptoms

Check engine light flashing during the misfire event, solid otherwise
Noticeable rough idle, especially when warm
Shaking or vibration felt through the seat or steering wheel
Loss of power, particularly on acceleration
Slight gasoline smell from the exhaust
Fuel economy 1-3 MPG worse than usual
Hesitation when pulling away from a stop

Diagnostic steps

1.Confirm with a scan tool that the misfire counter is climbing specifically on cylinder 7. Random misfires across multiple cylinders usually indicate a different root cause.
2.Swap the cylinder 7 ignition coil with a known-good coil from another cylinder (say cylinder 1). Clear the code and drive briefly. If the misfire follows the coil to the new cylinder, you've found the coil.
3.If the misfire stays on cylinder 7, swap the spark plug the same way. Worn plugs and bad coils together cause about 80% of cylinder-specific misfires on V8s.
4.If both coil and plug check out, run a compression test on cylinder 7. A reading 15% or more below the other cylinders points to an internal mechanical issue.
5.On GM AFM-equipped V8s with high mileage, listen for lifter tick on cylinder 7 — a collapsed lifter is a known failure mode that shows up as a persistent misfire on one cylinder.
6.If compression is good and ignition checks out, test the fuel injector with a scan tool's injector balance test or an inductive amp clamp.

Repair cost

$80 – $1,200

A single spark plug replacement on an accessible V8 is $50-$200 — most shops will charge a minimum diagnostic fee on top. An ignition coil replacement runs $150-$450. A fuel injector replacement is $300-$700. If the misfire is from a collapsed AFM lifter on a GM truck V8, you're looking at $1,500-$3,000+ for the lifter and pushrod job, which is why catching it early matters.

Estimate your repair

Run the numbers for your vehicle

Open the Repair Cost Estimator with ignition coil replacement preselected. Adjust labor rate and vehicle category to fit your situation.

Estimate your repair cost

DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.