This code indicates the engine control unit (ECU) has detected a misfire on Cylinder 3. On Jaguar engines - from the 2.0 Ingenium AJ200D to the AJ126 V6 and AJ133 V8 - this is a specific warning that often points to a faulty ignition coil, spark plug, fuel injector, or compression issue on that cylinder.
P0303 means the ECU has detected a misfire specifically on Cylinder 3. On most Jaguar petrol and diesel engines - including the Ingenium AJ200D, AJ126 V6, and AJ133 V8 - this is typically caused by a faulty ignition coil, a worn spark plug, a clogged fuel injector, or a compression issue on that cylinder. The fix can range from a £100–£300 coil or plug replacement to £400–£800 for injector or compression repairs [PATTERN DATA].
Do not ignore P0303. A misfire on Cylinder 3 can damage the catalytic converter and lead to expensive repairs if left unchecked.
Misfire occurs when the air‑fuel mixture in a cylinder fails to ignite properly, or when combustion is incomplete. The ECU monitors the crankshaft speed to detect misfires - each cylinder firing causes a slight acceleration; if that acceleration is missing or reduced on Cylinder 3, the ECU logs a misfire for that cylinder. On Jaguar engines, P0303 is the specific code for Cylinder 3. On four‑cylinder Ingenium diesels, Cylinder 3 is one of the four; on V6 and V8 engines, Cylinder 3 is typically on Bank 1 (the same bank as Cylinder 1). The most common causes are a faulty ignition coil (on petrol engines), a worn spark plug, a clogged or faulty fuel injector, or a compression issue. Unlike a random misfire code (P0300), P0303 gives you a specific location to start diagnostics, which can save time and money.
If the engine light is flashing, avoid driving and have the car recovered to prevent catalytic converter damage.
Based on real diagnostic patterns from Jaguar specialists. The order reflects both likelihood and typical repair cost.
| Cause | Likelihood | Typical Cost (Fitted) | Notes |
|---|---|---|---|
| Worn or faulty spark plug (Cylinder 3) | High | £100–£250 (set) | Most common on petrol engines; replace as a set |
| Faulty ignition coil (Cylinder 3) | High | £150–£400 | Coil‑on‑plug failure; common on AJ126 and AJ133 |
| Clogged or faulty fuel injector (Cylinder 3) | Medium | £200–£500 | Injector may be stuck or partially blocked; common on diesels and high‑mileage cars |
| Vacuum leak (intake manifold / hoses) | Medium | £100–£300 | Air leak leaning out the mixture on Cylinder 3 or across all cylinders |
| Low compression (Cylinder 3) | Low | £500–£1,500+ | Worn piston rings, valve leakage, or head gasket issue; requires compression test |
| Faulty wiring/connector to injector or coil | Low | £100–£300 | Corrosion or damaged wiring can cause intermittent operation; check harness |
Cost figures [PATTERN DATA] - indicative independent‑specialist pricing, not drawn from our engine replacement dataset.
A competent Jaguar specialist will follow this order to avoid misdiagnosing an expensive repair when a simple coil or spark plug is the real issue.
Read all codes to confirm P0303 and check for others like P0300, P0203 (injector circuit), or P0171 (lean) that could indicate the root cause.
For a quick test, swap the Cylinder 3 ignition coil with Cylinder 1 (or spark plug). If the misfire moves to the other cylinder, the coil or plug is faulty.
If the coil and plug are good, the injector may be at fault. A balance test or swapping injectors can confirm.
Using a smoke tester or propane, check the intake manifold and hoses for leaks that could affect Cylinder 3.
If the above checks are normal, a compression test can reveal mechanical issues like worn piston rings or valve problems.
Our diagnostic wizard walks the same order, or use the registration lookup to confirm your engine.
It depends - but generally, only for a short distance. P0303 indicates a misfire on Cylinder 3 that can cause unburnt fuel to enter the exhaust system, which can overheat and destroy the catalytic converter. If the engine light is flashing, the misfire is severe enough to cause damage - stop driving immediately and have the car recovered. If the light is solid and the engine runs reasonably smoothly, you may be able to drive to a garage, but you should avoid heavy acceleration and extended journeys. Continued driving with a misfire can also lead to further engine damage, especially if the cause is low compression or a timing issue. The safest course is to have the car diagnosed as soon as possible.
It means the ECU has detected a misfire specifically on Cylinder 3, indicating a fuel, ignition, or mechanical issue on that cylinder.
Only for a short distance if the light is solid; if flashing, stop driving immediately to avoid catalytic converter damage.
It can be - a misfire can damage the catalytic converter and lead to expensive repairs. Prompt diagnosis is essential.
The cost ranges from about £100 for spark plugs to £400 for a coil, and up to £1,500+ for compression repairs. Diagnostic time is typically 1–2 hours.
Yes, a clogged or faulty injector on Cylinder 3 can cause a misfire, especially on diesel engines.
Only temporarily - if the underlying issue is not fixed, the code will return. Clearing it without diagnosis is not a solution.
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