Does P0305 mean my engine has six cylinders?

Yes. Cylinder numbering follows the engine layout, and cylinder 5 only exists on six-cylinder engines such as V6s and straight-sixes, so P0305 will not appear on a four-cylinder car.

What is the cheapest thing to check first?

The spark plug and coil for cylinder 5. They are the most common and least expensive causes of a single-cylinder petrol misfire, and swapping them to another cylinder quickly proves the point.

Can I drive with P0305?

Briefly, to reach help. A steady light is less urgent than a flashing one, but any misfire risks the catalytic converter and should not be left, particularly under load.

Why is the rear bank harder to fix?

On a transverse V6 the rear bank sits against the bulkhead with very little room, so reaching cylinder 5's plug, coil, and injector takes more dismantling than the front bank.

Could one tank of bad fuel cause P0305?

It can contribute, especially with water or contamination, but a fault that keeps returning after fresh fuel points to a worn plug, coil, injector, or a mechanical issue on cylinder 5.

Will P0305 fail an MOT?

Yes. A lit or flashing engine management light is an MOT failure, and the raised emissions from a misfire can fail the test on their own.

P0305 - Cylinder 5 Misfire Detected

What Is P0305?

P0305 means the engine ECU has detected a misfire on cylinder 5. It counts the tiny speed fluctuations of the crankshaft, and when cylinder 5 repeatedly fails to fire cleanly it logs this code. As cylinder 5 only exists on six-cylinder engines, this is a V6 or straight-six fault.

A misfire is a combustion problem, not a single component, so the cause can be ignition, fuel, or mechanical. On a V6, cylinder 5 usually sits on the rear bank against the bulkhead, which makes it one of the more awkward cylinders to reach and a favourite for neglected, hard-to-access ignition parts.

Common Symptoms

Rough idle and vibration
Flashing engine management light under load
Hesitation or stumble on acceleration
Loss of power
Increased fuel consumption
Smell of unburnt fuel at the tailpipe

Common Causes

Worn spark plug - a fouled or worn cylinder 5 plug, the cheapest and most common petrol cause.

Failed coil or lead - a breaking-down coil-on-plug or HT lead on cylinder 5.

Injector fault - a blocked or failing cylinder 5 injector starving it of fuel.

Vacuum leak - an intake or manifold leak affecting the rear bank.

Low compression - a burnt valve, worn rings, or head gasket issue on cylinder 5.

Wiring fault - a poor connection to the cylinder 5 coil or injector.

How to Diagnose P0305

Locate Cylinder 5

On a V6, cylinder 5 is usually on the rear bank toward the bulkhead; on a straight-six it is the fifth from the front. Confirm the layout for your engine before testing.

Swap the Coil and Plug

Move cylinder 5's coil and plug to a neighbouring cylinder. If the misfire follows them you have found a faulty ignition part; if it stays on cylinder 5, look deeper.

Check the Injector

Listen for the injector clicking and check its resistance and wiring. A blocked or dead cylinder 5 injector will misfire just as an ignition fault would.

Look for Vacuum Leaks

Inspect the intake and manifold gaskets on the rear bank. A leak here leans out cylinder 5 and is easy to miss because of the poor access.

Test Compression

If ignition and fuel check out, run a compression or leak-down test on cylinder 5. Low compression points to valves, rings, or a head gasket and needs mechanical repair.

Read Misfire Counts

Use live data to watch the per-cylinder misfire counters. Confirming the count is climbing on cylinder 5, and whether it worsens hot, cold, or under load, narrows the cause.

Catalyst and Cold MisfiresA persistent misfire dumps unburnt fuel into the catalytic converter and can overheat it, especially with a flashing light. On the rear bank of a V6 a neglected misfire is easy to ignore until it gets expensive, so deal with it promptly.

Verdict

On a petrol six, start cheap: swap the cylinder 5 coil and plug to prove the ignition side, then check the injector. Only move on to vacuum leaks and a compression test if those come back clear, as mechanical faults are the least common cause.

Want the full picture? The OBD Fault Code Plain English Guide (PDF) covers the most common UK fault codes in one plain-English download.