Stop if MIL flashing. Steady MIL: repair within 1-2 days. P0305 means the ECM detected a misfire on cylinder 5 based on crankshaft speed irregularity analysis. In most V6 and V8 engines, cylinder 5 is on bank 2. Common causes are a failed ignition coil, worn spark plug, or clogged injector on that cylinder.
What P0305 means
The ECM monitors crankshaft speed deviations every 200 and 1000 revolutions to identify misfiring cylinders. When cylinder-5 misfire events exceed the emission threshold (approximately 1.5 percent misfire rate over 1000 crankshaft revolutions) or the catalyst-damage threshold (77 or more misfires per 200 revolutions), P0305 sets. The emission level uses two-trip detection before the MIL illuminates; the catalyst-damage threshold triggers an immediate MIL flash on the first event. In V6 and V8 engines cylinder 5 sits on bank 2. A bank-2 fuel trim imbalance (Short FT 2 or Long FT 2 outside plus or minus 15 percent) alongside P0305 points to a bank-wide mixture cause rather than a cylinder-5-only ignition or injector fault.
Symptoms
- MIL illuminated steady (emission-level misfire) or MIL flashing (catalyst-damage level reached)
- Rough idle at 600 to 900 rpm in gear, felt as a rhythmic stumble
- Hesitation or surge under acceleration at the rpm and load stored in freeze frame
- Exhaust smells of unburned fuel
- Possible bank-2 long-term fuel trim offset if a vacuum leak is the underlying cause
Common causes
- Worn or fouled spark plug in cylinder 5 -- inspect for cracked porcelain, heavy carbon deposits, or gap beyond specification
- Failed coil-on-plug ignition coil for cylinder 5 -- most efficiently diagnosed by swapping to an adjacent cylinder
- Stuck-closed or leaking fuel injector on cylinder 5 -- a closed injector starves the cylinder; a leaking injector floods it
- Low compression from worn rings, damaged valve, or head gasket failure on cylinder 5
- Vacuum leak on the bank-2 intake runner or manifold gasket -- causes a lean misfire concentrated on bank-2 cylinders
- Engine coolant temperature sensor fault -- incorrect ECT reading during warmup skews cold-start fuel delivery
- VVT oil control valve fault on bank 2 -- retarded intake cam timing reduces effective compression at specific rpm
Severity & driving advice
Severity: high — Flashing MIL = catalyst-damage rate active; stop safely and promptly. Steady MIL: continued driving risks catalytic converter damage within days.
Can I drive? Stop if MIL flashing. Steady MIL: repair within 1-2 days.
Diagnostic approach
Make & model notes
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FAQ
Is P0305 worse than P0300?
P0305 identifies a single misfiring cylinder (cylinder 5) while P0300 indicates simultaneous multi-cylinder misfires. P0305 is generally easier to diagnose because the fault is localized. However, severity depends on misfire rate -- a high-rate P0305 with a flashing MIL is more urgent than a low-rate P0300.
Can a vacuum leak cause only cylinder 5 to misfire?
Yes, if the vacuum leak is at the cylinder-5 intake port gasket or runner seal, it creates a lean condition specific to that cylinder. A leak at the intake manifold valley or throttle body would affect the entire bank or all cylinders.
How do I confirm the cylinder-5 coil is the problem?
Swap the cylinder-5 coil to cylinder 4 (or any adjacent cylinder), clear the code, and test drive under the same conditions. If P0304 now sets instead of P0305, the coil is faulty. If P0305 persists after the swap, the coil is not the cause.