P0304 fault code: Cylinder 4 Misfire Detected
Last updated · OBDCode UK editorial
You may nurse the car a short way to a garage if the engine management light stays steady and cylinder 4 is not knocking the engine about. Flashing light or hard judder: stop, do not keep driving to ‘clear it’.
Can I DIY?
Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage.
Some checks may be possible for a careful DIY driver with the right tools and a safe place to work. Stop if anything feels beyond your skill or kit.
Never open a hot coolant cap — pressure can scald you. Exhaust and catalytic converter get hot enough to burn and ignite debris. Stay clear of electric fans; they can run after shut-off.
Never idle in a closed garage to read codes — an open door is not enough. CO is odourless and can kill. Run diagnostics in open air. Headache or nausea with exhaust smell: get fresh air and stop.
What does P0304 mean?
P0304 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P03xx block, which covers ignition system and misfire. The stored description is “Cylinder 4 Misfire Detected”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.
How to diagnose P0304 yourself
- Confirm P0304 and freeze-frame: Plug in an OBD-II reader with the car parked outdoors. Record whether P0304 is current, the freeze-frame snapshot (rpm, load, temperature), and every companion code from that same scan. Stop if: You cannot identify cylinder 4 on this engine layout, the lamp is flashing, or you are about to guess at a coil pack.
Park safely and switch off before locating the socket. Its position varies, so use the vehicle handbook and never force or dismantle trim. - Decide steady light versus flashing light: Start the engine in the open. Watch the engine management light for several seconds. Write down ‘steady’ or ‘flashing’ and whether the shake feels like one cylinder dropping. Never do this in a closed garage. Stop if: The lamp flashes, power collapses, or anyone in the car feels unwell from exhaust smell.
A flashing engine management light means stop safely and arrange recovery, whether or not you feel judder. A steady light still needs prompt diagnosis, and becomes a stop if it starts to flash, judder or lose power. - Note cylinder 4 live misfire tally: If the scanner lists misfire counts by cylinder, watch cylinder 4 at idle in open air. Photograph or write the numbers. Leave the codes stored. Do not disconnect the coil on cylinder 4. Stop if: The lamp begins to flash, you feel dizzy, or you are tempted to swap the cylinder 4 coil to ‘prove’ it.
Will P0304 fail an MOT?
An illuminated engine management light is a Major defect at MOT (automatic fail) until the fault is repaired and the light stays off. Cylinder 4 misfire can also push the car over the emissions limit.
Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.
What the MOT data says
These figures are copied from the MOT fail cohort for the certificate wording below. They are not a diagnosis of this car.
Certificate wording: “Malfunction indicator lamp Engine MIL illuminated indicating a malfunction”.
Of 391,384 Class 4 vehicles that failed a 2024 MOT on that wording, 14.8% (95% CI 14.7–14.9) failed on the same wording again in 2025. The not-tested gap versus control was 11.5 points.
Certificate wording: “Catalyst emissions Lambda coefficient outside the default limits or the range specified by the manufacturer”.
Of 166,607 Class 4 vehicles that failed a 2024 MOT on that wording, 7.5% (95% CI 7.4–7.6) failed on the same wording again in 2025. The not-tested gap versus control was 13.4 points.
Engine management light MOT outcomes
Related guides
Related fault codes
P0304 — quick answers
Is it safe to drive with P0304?
Limit driving. You may nurse the car a short way to a garage if the engine management light stays steady and cylinder 4 is not knocking the engine about. Flashing light or hard judder: stop, do not keep driving to ‘clear it’.
Can I fix P0304 myself?
Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage. Basic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.
Will P0304 fail my MOT?
An illuminated engine management light is a Major defect at MOT (automatic fail) until the fault is repaired and the light stays off. Cylinder 4 misfire can also push the car over the emissions limit.
Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.