P0303 fault code: Cylinder 3 Misfire Detected
Last updated · OBDCode UK editorial
A short, gentle trip to a garage is reasonable if the engine management light is steady and cylinder 3 is not shaking the engine. If the light flashes or the car judders, stop and arrange recovery.
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 We do not publish DIY repair or strip-down steps for this code on this page.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 P0303 mean?
P0303 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 3 Misfire Detected”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.
How to diagnose P0303 yourself
- Confirm P0303 and freeze-frame: Scan with an OBD-II reader while stationary in open air. Note whether P0303 is current or pending, freeze-frame rpm, load and coolant temperature, and any other cylinder or coil codes. Stop if: The engine management light is flashing, the engine judders badly, or you cannot tell which physical cylinder is number 3 on this engine.
Park safely and switch off before locating the socket. Its position varies, so use the vehicle handbook and never force or dismantle trim. - Watch the engine management light outdoors: With the engine running in open air, not a closed garage, watch the engine management light. Note whether it is steady or flashing, and whether the car shakes as if cylinder 3 is dropping out. Do not idle in an enclosed space. Stop if: The light flashes, the engine judders, or exhaust fumes are inside the cabin — stop and arrange recovery.
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. - Read cylinder 3 misfire counts if shown: If your scanner shows per-cylinder misfire counters or similar live data, note the count for cylinder 3 at idle outdoors. Do not clear codes before the garage has seen the list. Do not unplug anything. Stop if: The light starts flashing, you feel dizzy or sick, or the next step in your mind is to swap the coil on cylinder 3.
Will P0303 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. A misfiring cylinder can also fail the exhaust emissions test.
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
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P0303 — quick answers
Is it safe to drive with P0303?
Limit driving. A short, gentle trip to a garage is reasonable if the engine management light is steady and cylinder 3 is not shaking the engine. If the light flashes or the car judders, stop and arrange recovery.
Can I fix P0303 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 P0303 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. A misfiring cylinder can also fail the exhaust emissions test.
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.