Where is cylinder 8 on my engine?

On almost every V8 in service today, cylinder 8 is the rearmost cylinder on the passenger side bank. There are exceptions — some BMW V8s and a few European V8 layouts use different numbering conventions — so check your service manual if you're not sure. The general rule on American V8s is that the driver side is bank 1 (cylinders 1, 3, 5, 7) and the passenger side is bank 2 (cylinders 2, 4, 6, 8).

Is P0308 worse than P0307?

Mechanically, no — they're the same problem on a different cylinder. From a labor standpoint, cylinder 8 is usually slightly easier to reach than cylinder 7 because most V8 layouts put cylinder 8 on the passenger side, which tends to be less crowded than the driver side. But the diagnostic process, the parts involved, and the urgency of catalyst protection are identical.

Should I do all the plugs and coils at once on a V8?

If the engine has 75,000+ miles and you don't know the service history, replacing all eight plugs and coils at once is usually the better value. The labor to reach the rear cylinders is the same whether you're replacing one component or four, and the front cylinder ignition parts are typically within a year of failing anyway. Budget $500-$1,000 for parts and another $200-$500 in labor depending on the platform.

How much will P0308 cost to fix?

Best case is a single spark plug or coil replacement at $150-$450. If a fuel injector is the cause, plan for $300-$700. If you're on a GM AFM-equipped V8 and the misfire turns out to be a collapsed lifter, the bill jumps to $1,800-$3,500 because the cylinder head has to come off. A 15-minute coil swap diagnostic tells you which conversation you're having.

OBD-II trouble code

P0308: Cylinder 8 Misfire Detected

Cylinder 8 — the last cylinder in the firing order on a V8 — is missing combustion events. The diagnosis is cylinder-specific, but cylinder 8 sits at the far back of a bank on most V8 platforms, which has real consequences for what the repair actually costs.

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 P0308 mean?

P0308 means the PCM detected enough misfire events on cylinder 8 to set a code. Modern engine controllers measure crankshaft acceleration for every combustion stroke; when cylinder 8's contribution falls below a calibrated threshold for too many cycles in a row, the misfire monitor records the failure against that specific cylinder. Because the code names the cylinder, you skip most of the guesswork — but on a V8, cylinder 8 sits at the rearmost position on one bank, and reaching it usually involves either pulling the intake manifold cover, removing brake booster lines, or working blind from underneath.

The failure pattern on cylinder 8 is dominated by the same three components as any cylinder-specific misfire: the ignition coil, the spark plug, and the fuel injector. On modern coil-on-plug V8s, the rear coils run hotter than the front ones because exhaust heat tends to pool toward the firewall, and so they tend to fail first. Spark plugs at the rear of the engine age slightly faster for the same reason. Fuel injectors fail less often than ignition components but more often than people expect on direct-injection engines, where carbon buildup and high-pressure fuel exposure shortens injector life.

On GM AFM-equipped V8s — the 5.3L, 6.0L, and 6.2L engines that selectively deactivate cylinders to save fuel — cylinder 8 is one of the cylinders that gets deactivated during light cruise. The deactivation hardware uses collapsing lifters, and when those lifters fail to re-extend properly, the result is a persistent misfire on the deactivated cylinder. If your GM truck V8 has 100,000+ miles and is throwing P0308 with no obvious ignition issue, plan for a lifter conversation with your shop.

Common causes

Failing ignition coil on cylinder 8
Worn spark plug at the rear of the engine bay
Clogged or leaking fuel injector
Collapsed AFM/DFM lifter on GM 5.3L / 6.0L / 6.2L V8s
Vacuum leak at the rear intake manifold gasket
Carbon buildup on the intake valve (direct-injection V8s)
Low compression from a worn valve or head gasket failure
Damaged wiring or melted connector at the rear coil
Aftermarket exhaust headers leaking near the O2 sensor or causing reversion

Symptoms

Check engine light flashing during the misfire, solid otherwise
Pronounced rough idle, sometimes worse cold than warm
Vibration felt through the steering wheel or floor
Power loss on acceleration or under load
Slight exhaust smell of unburned fuel
Lower fuel economy than recent tanks
Occasional stumble at light throttle

Diagnostic steps

1.Confirm the misfire counter is climbing on cylinder 8 specifically — if you're seeing P0300 plus several cylinder codes, treat it as a random misfire and chase fuel pressure or vacuum first.
2.Swap the cylinder 8 ignition coil with one from a known-good cylinder (cylinder 2 or 4 is convenient). Clear codes, drive briefly, and see if the misfire follows the coil.
3.If the misfire stays on cylinder 8 after swapping the coil, do the same swap test with the spark plug.
4.Run a compression test on cylinder 8. A reading more than 15% below the average of the other cylinders points to a mechanical failure — worn valve, lifter, or head gasket.
5.On GM AFM/DFM V8s, listen for a lifter tick that's louder under load. A collapsed AFM lifter usually produces both the misfire and an audible noise.
6.Check for carbon buildup on direct-injection V8s — walnut-shell intake valve cleaning is a known fix on Audi and BMW V8s past 60,000 miles.

Repair cost

$80 – $1,200

Single spark plug at the rear of a V8: $80-$250 (the part is cheap, the labor is the variable). Ignition coil replacement: $150-$450. Fuel injector: $300-$700. AFM lifter repair on a GM truck V8 is the worst case at $1,800-$3,500 because the heads need to come off. Most independent shops will quote a 1-hour diagnostic fee on top of any repair.

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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.