P0304 on Jaguar: How to Diagnose & Fix
Cylinder 4 Misfire Detected
1 Jaguar model · 1 documented issue · $300-$1,000 repair
P0304 on Jaguar by model: symptoms, diagnosis and fix
The AJ-V8 uses individual coil-on-plug ignition coils that fail with age and heat, producing single- or multi-cylinder misfires and a check-engine light. A frequent contributor is oil pooling in the spark-plug wells from a leaking cam-cover gasket, which fouls the coil boots. Coils typically last 70k-100k miles but wear faster with heavy idling and heat cycling.
Common Symptoms
- Check engine light with misfire code
- Rough idle / stumble
- Hesitation or loss of power under acceleration
- Occasional backfire
- Oil in spark-plug wells fouling coil boots
How to Fix
Read the misfire code to identify the cylinder, then swap the suspect coil to another cylinder to confirm the fault moves. Replace the failed coil (and spark plug); when an older high-mileage car drops one coil it is common to replace the full set and plugs together. If wells are oily, also renew the cam-cover gasket to stop recurring fouling.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Bremi/STI AJ810445 ignition coil for the 4.2L branchOEMAJ810445 / Bremi 5121
Swap-confirmed failed ignition coil on a 2007-2009 XK 4.2L branch Fitment/availability: AJ810445 is explicitly cataloged for the 2007-2009 XK Coupe 4.2L and eight are used. Confirm body/trim/VIN and the four-pin connector; it is not the …
- NGK 7866 IFR5N10 iridium spark plug for the 4.2L branchOEMNGK 7866 / IFR5N10 / AJ84575
Sold individually; confirm VIN/current specification, required quantity and factory gap/torque before installation.
- Bremi/STI AJ811378 ignition coil for the early AJ133 5.0L branchOEMAJ811378 / Bremi 5158
Swap-confirmed failed ignition coil on a compatible 2010-2015 XK/XKR/XKR-S 5.0L AJ133 Fitment/availability: AJ811378/Bremi 5158 is cataloged for XK, XKR and XKR-S 5.0L applications, quantity eight. Confirm VIN and complete engineering re…
- NGK 94940 ILKAR7C10 set of eight AJ133 spark plugsOEMNGK 94940 / ILKAR7C10 / C2P23089
Complete plug service or confirmed worn/fouled plugs on a VIN-compatible 2010-2015 XK 5.0L branch Fitment/availability: The kit contains eight NGK ILKAR7C10/94940 plugs and is listed for XK/XKR-S applications, cross-referencing C2P23089.…
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Sources for this issue (2)
P0304 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0304 mean on a Jaguar?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On Jaguar it is documented across 1 model: XK.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a Jaguar?
Published repair estimates on Jaguar range from $300 to $1,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0304 on a different make?
View P0304 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.