Short, gentle trips are usually okay with a steady light — but if the check-engine light is FLASHING, stop driving and fix it, as that means catalyst-damaging misfire. P0308 means the engine computer has detected that cylinder 8 is misfiring — repeatedly failing to burn its fuel cleanly. It is a V8-only code (only eight-cylinder engines have a cylinder 8), and the most common causes are a worn spark plug or a failed ignition coil on that cylinder. A steady check-engine light means an ongoing miss; a flashing light means the misfire is bad enough to damage the catalytic converter, so stop driving.
What P0308 means
The engine computer watches tiny changes in how fast the crankshaft spins as each cylinder fires. When cylinder 8 burns weakly or not at all, the crank slows slightly on that cylinder's power stroke, and once the missed-fire rate crosses a set threshold the computer sets P0308. Cylinder 8 exists only on V8 engines, such as the Toyota 5.7L, Ford 5.0L/5.4L, GM 5.3/6.2L and RAM 5.7L HEMI.
Symptoms
- Check-engine light on, or flashing during a hard misfire
- A shake or rough running, worst at idle and light load
- Hesitation, stumble, or a flat spot on acceleration
- Down-on-power feeling, especially under load or climbing
- Higher fuel use and an occasional raw-fuel or sulphur smell
- Possible hard start or brief stalling when the miss is bad
Common causes
- A worn or fouled spark plug on cylinder 8
- A failed ignition coil (coil-on-plug) or, on older engines, a bad plug wire
- A clogged, leaking or electrically failed injector for cylinder 8
- A vacuum or intake leak affecting that cylinder or bank (lean misfire)
- Low compression on cylinder 8 — a burned valve, worn rings or a head-gasket fault
- A wiring or connector fault to the coil or injector
- Less common: contaminated fuel, a carbon-tracked plug, or a valvetrain problem
Severity & driving advice
Severity: Moderate-High — A single mild misfire won't strand you, but a sustained one pumps unburned fuel into the exhaust and can destroy the catalytic converter.
Can I drive? Short, gentle trips are usually okay with a steady light — but if the check-engine light is FLASHING, stop driving and fix it, as that means catalyst-damaging misfire.
Diagnostic approach
- Read all codes and freeze-frame first — Note whether other misfire codes are present and the RPM, load and temperature at which P0308 set; a cold-only or load-only pattern narrows the cause.
- Swap-test the ignition parts — Move cylinder 8's coil (and plug) to a neighbouring cylinder. If the misfire follows the part, the coil or plug is the fault; if it stays on cylinder 8, the problem is fuel, compression or wiring.
- Inspect the plug and check the injector — Pull the plug on cylinder 8 and judge the gap, wear and fouling. Confirm the injector is pulsing and measure its resistance against spec and against the other injectors.
- Verify the electrical circuit — Check the coil and injector connectors and harness for power, ground and the computer's control signal before condemning a part.
- Test cylinder sealing — Run a compression and, if needed, a leak-down test on cylinder 8. A low reading points to valves, rings or a head gasket rather than ignition or fuel.
- Look for a lean cause — Check for intake or vacuum leaks and review the fuel trims — a cylinder- or bank-specific lean condition can cause a miss even with good ignition parts.
- Clear, road-test and recheck — After the repair, clear the code and complete a drive cycle so the misfire monitor re-runs and confirms the count stays below threshold.
Make & model notes
V8 engines: P0308 can only occur on eight-cylinder engines — Toyota Tundra/Sequoia 5.7L, Ford F-150 5.0/5.4L, GM 5.3/6.2L, RAM 5.7L HEMI and similar. Locating cylinder 8 depends on the engine's cylinder-numbering, so check a firing-order diagram for your specific V8.
FAQ
Is P0308 always a spark plug or coil?
No. Those are the most common and cheapest to rule out, but injector, wiring, vacuum-leak and low-compression problems produce the same code, so confirm with a swap test before replacing parts blindly.
Can I just move the coil to see if it follows?
Yes — that is the fastest real diagnostic. If the miss jumps to the cylinder you moved the coil to, replace the coil; if it stays on cylinder 8, look at fuel or compression.
Why only cylinder 8 and not the whole engine?
The computer tracks each cylinder individually, so a per-cylinder code means the fault is isolated to that cylinder's ignition, fuel or mechanical hardware.
Will it hurt the engine to keep driving?
A steady light with a light miss is lower-risk short-term, but ignoring it risks catalytic-converter damage and, if the cause is mechanical, further wear.