P0305: Combustion irregularity detected in cylinder 5

What does this code mean?

A cylinder is one of the engine chambers where fuel burns to produce power. P0305 means cylinder 5 did not contribute that power consistently. Its number helps locate it but does not establish whether ignition, fuel or a mechanical issue caused the misfire.

Locate the reported problem

Confirm that the engine has a fifth cylinder and identify its position from service information. A scanner label that conflicts with the engine needs verification.

Read it with the other codes

Compare cylinder 5 with the other cylinders under the same conditions. P0300 or fuel-pressure codes may add shared-system context to a localized report.

Possible causes to investigate

These are possibilities, not a diagnosis or a ranked list of failed parts.

Ignition problems, including a plug, coil, connection or related circuit.
Fuel delivery or injector operation that produces uneven combustion.
Air leaks, compression or other mechanical conditions that require testing.

What to do first

Record the codes and when the shaking or hesitation occurs: first start, idle, acceleration or another condition.
Report recent plug or coil work, refueling and any other warning lights. Do not keep revving the engine to reproduce a severe misfire.
If the check engine light flashes or the vehicle shakes strongly, reduce load, stop safely and seek assistance. Follow the vehicle handbook; a code lookup cannot establish whether driving is safe.

What the workshop can check

A workshop can use available misfire information, ignition and injector checks, and mechanical testing when indicated. Any component substitution test must suit the engine and be done safely. A changed fault pattern is evidence to interpret, not permission to replace every coil.

A useful question to ask

Is cylinder 5 correctly identified, and what separates an ignition, fueling or mechanical problem?

Your vehicle changes the next step

Year, make, model, engine and the complete code list matter. Cylinder layout, test conditions and repair procedures can differ. Use service information for the exact vehicle; a generic code does not establish an exact failed part.

Can I keep driving?

This page cannot decide whether your car is safe to drive. A flashing engine light, severe shaking, overheating, strong fuel smell or major power loss calls for prompt attention. Stop safely when needed, follow your handbook and seek assistance rather than continuing a test drive.

Terms in this guide

These words appear in the explanation. Select an underlined term in the text to reach its definition.

Cylinder and piston: A cylinder is the chamber where fuel burns. A piston moves inside it and receives the force from combustion. Cylinder numbering depends on the engine.
Misfire: A cylinder does not produce its normal power contribution consistently. Ignition, fuel or mechanical condition may be involved, not just a spark plug.
Monitor: A check the car performs when particular conditions are met. Here it does not mean a display screen.
Spark plug, ignition coil and ignition: In a gasoline engine, the coil generates the voltage for the spark plug to ignite the mixture. Diesel engines do not use this spark-ignition system.
Injector: A component that meters and sprays fuel so the engine can burn it.
Circuit, supply and ground: A circuit includes the wiring and connections carrying electricity. Supply powers a component; ground is its electrical return. A fault in any of these can change a reading.
Compression: Pressure created as a piston squeezes the contents of a cylinder. Checking it helps assess the engine mechanical sealing.
Idle and rpm: Idle is the engine running without accelerator input. Revolutions per minute, or rpm, describe how fast it turns.
Engine load: How much work is demanded from the engine, such as on a hill. It is not just passenger or luggage weight.

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Sources and scope

Definitions and general OBD context are cross-checked against the references below. Manufacturer documents describe specific vehicles and systems; their procedures and thresholds are not instructions for every make. This is educational guidance, not a vehicle-specific repair procedure.