Vauxhall P0303

Cylinder 3 misfire on Vauxhall — A14NET coil pack and Z19DTH injector loom chafe.

Vauxhall P0303 — Cylinder 3 misfire on Vauxhall — A14NET coil pack and Z19DTH injector loom chafe. Typical UK independent garage cost: £70–£500.

What actually causes P0303 on a Vauxhall

Z19DTH P0303 with intermittent rough running is usually injector loom chafe on the rocker cover stud — easy to miss visually, drops out on heat-soak. On A14NET it's a coil pack 90% of the time.

Vauxhall engines that log P0303

• 1.4 Turbo A14NET (Astra J/Corsa E)
• 1.6 CDTi B16DTH (Astra J/Insignia)
• 1.9 CDTi Z19DTH (Vectra/Zafira)

How we diagnose P0303 on a Vauxhall

1. Visual inspection of injector loom along rocker cover
2. Wiggle test injector connectors with engine running
3. Read injector correction values
4. Compression test if cold-misfire only

The common misdiagnosis to avoid

Aftermarket injectors on Z19DTH cause new fault codes within 1,000 miles — only OEM Bosch reman with new copper washers.

Vauxhall P0303 repair cost in the UK

Expect £70–£500 including parts and labour at an independent garage. Main-dealer quotes on Vauxhall are usually higher for the same job, and the spread exists because the cheap cause and the expensive cause throw the same code — pay for the diagnosis before you pay for parts.

Bob's workshop tip

Z19DTH P0303 — check the loom before the injector. A £15 repair pin saves a £350 injector job.

Codes usually seen alongside P0303 on Vauxhall

• P0301
• P0302
• P0304
• P0300

Symptoms drivers report with P0303

• The engine 'kangaroos' or hesitates when you’re pulling away from a roundabout.
• The car feels like it’s 'chugging' or running on three cylinders when idling at traffic lights.
• The EML (Engine Management Light) starts flashing when you’re pushing it up a steep hill or overtaking.
• A heavy smell of unburnt petrol coming from the exhaust pipe at cold start.
• The car struggles to 'pick up' from 2,000 RPM in 4th or 5th gear.

Repairs that clear P0303

• Plug & coil replacement

For the generic, all-makes explanation of this code — meaning, every known cause and UK cost bands — see our P0303 guide.

Reading P0303 on a Vauxhall properly

A generic reader will show P0303 and nothing else. What matters is the freeze-frame data stored with it — engine speed, load, coolant temperature and fuel trims at the moment the fault was logged — plus whether any other module on the Vauxhall has logged something at the same time. Codes rarely arrive alone, and the order they were stored in usually points at which one is the cause and which are the consequences.

Clearing P0303 without fixing it

Erasing the code resets the Vauxhall's readiness monitors as well as the light. The fault returns on the next drive cycle if the cause is still there, and in the meantime the incomplete monitors can stop the car passing an emissions check. After a genuine repair, the car is driven until the relevant monitor completes before it is handed back.

Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P0303 is showing on your Vauxhall, bring any scan results with you — we will read the live data ourselves, confirm which cause it actually is, and price the repair before any work starts.