P0304 Cylinder 4 Misfire Detected — Diesel Causes & Fix
Updated for 2026 diagnostics & repair guidance
P0304 at a Glance
What it means
The ECU has detected repeated combustion failures in cylinder 4.
Most common cause
On diesels: a worn or electrically failed injector; also compression loss or glow plug faults when cold.
Severity
Serious if persistent — raw fuel can damage the DPF and catalyst.
Can you drive?
Only short distances. A hard misfire should be diagnosed promptly.
Typical repair cost
£100-£350 for injector replacement; less for wiring or glow plug fixes.
What Does P0304 Mean?
P0304 is the cylinder-specific version of the P0300 random-misfire code: the crankshaft sensor shows cylinder 4 repeatedly failing to contribute torque. Diesels do not have ignition coils, so the usual petrol causes do not apply — the injector is the first suspect, followed by compression and, on cold starts, the glow plug.
Common Causes
Injector-Related Causes
Failed or worn injector
Electrical failure or internal wear stops cylinder 4 receiving the right fuel charge.
Injector circuit fault
Wiring or connector faults interrupt the injector signal intermittently.
Other Possible Causes
Low compression
Valve, ring or head-gasket faults prevent the cylinder firing even with correct fuelling.
Glow plug failure (cold misfires)
A dead glow plug causes cold-start misfires on the affected cylinder that fade as the engine warms.
Symptoms Drivers Notice
- Shaking or vibration, worst at idle
- Flashing or steady engine management light
- Loss of power and increased smoke
- Diesel knock from one cylinder
Vehicles Commonly Affected
Can You Still Drive with P0304?
A flashing warning light means active misfire — keep driving to a minimum. Unburned fuel overheats the DPF and catalyst and can cause expensive secondary damage.
How Mechanics Diagnose P0304
- 1
Confirm the misfiring cylinder
Use live misfire counters to verify which cylinder is dropping out.
- 2
Test the injector
Run a leak-off comparison and check coil resistance against specification.
- 3
Swap-test the injector
Move the cylinder 4 injector to a neighbour — if the misfire follows, the injector is condemned.
- 4
Check compression and glow plug
Compression-test the cylinder; test the glow plug if misfires are cold-only.
Repair Solutions & Costs
Replace the failed injector
Fit a bench-tested remanufactured unit and code it to the ECU.
Repair wiring or connector faults
Repair faults found on inspection of the injector circuit.
Address compression or glow plug faults
Investigate mechanical or glow plug causes where the fuel system tests clean.
Replacing injectors? Browse our remanufactured diesel injectors, reconditioned injectors or refurbished diesel injectors — all bench-tested with a 12-month warranty.
When Injector Replacement Is Required
If the misfire follows the injector in a swap test, or leak-off/resistance readings are out of specification, replace the injector — internal wear and coil failure are not serviceable.
Compatible Remanufactured Injectors
OEM bench-tested injectors for engines affected by P0304 — verified pricing, 12-month warranty, free UK delivery.
Delphi A6510704987
Remanufactured Delphi 28272472DNR diesel injector for Mercedes-Benz 2.1 CDI OM651 engines. Direct-fit replacement for C-Class, E-Class, CLA, GLA, GLK, Vito, V-Class, Sprinter and SLK applications. Cross-references A6510704987, A6510702387, HRD351, 28272472. OEM-tested and calibrated with full flow and leak-back verification. Includes 12-month unlimited-mileage warranty. Supplied as an exchange unit — return your old injector within 30 days.
Fits: OM651, OM651.911, OM651.924, OM651.925
View injector
Bosch 0445110613
BMW 2.0d B47 Remanufactured Common Rail Injector
Fits: B47, B47C20A, B47D20, B47D20A
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Bosch 13538514148
2.0L Diesel Remanufactured Solenoid Injector (BMW B47)
Fits: B47, B47C20A, B47D20, B47D20A
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Delphi 28307309
Delphi remanufactured diesel injector for Mercedes OM651 applications. OEM tested and calibrated on CR318 PRO test bench. 12-month warranty included. Free UK delivery. Suitable for professional garage use. Exchange unit — return your old injector within 30 days.
Fits: OM651.955, OM651.956, OM651.957, OM651.958
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Delphi 28388960
Remanufactured Delphi 28388960 diesel injector for Ford 2.0 TDCi and Peugeot/Citroen 2.0 HDi/BlueHDi DW10 engines. Direct replacement for Ford Mondeo, Kuga, S-Max, Galaxy, Focus, C-Max, Edge and Peugeot 308, 508, 3008, 5008, Expert, Citroen C4, C5, DS4, DS5, Jumpy, Jumper applications. Cross-references 28602948, 9674984080, DS7Q9F593BA, HRD360. OEM-tested and calibrated with full flow and leak-back verification. Includes 12-month unlimited-mileage warranty. Supplied as an exchange unit — return your old injector within 30 days.
Fits: DW10C, DW10CB, DW10CT, DW10CTED4
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Bosch 0445110601
BMW 2.0d N47 Remanufactured Common Rail Injector
Fits: N47C20A, N47D20A, N47D20C
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Bosch 0445110616
BMW 2.0L / 3.0L Diesel Remanufactured Common-Rail Injector (N47/N57)
Fits: N47D20B, N47D20C, N47D20O1
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Siemens VDO / Continental 5WS40156Z
DW10 / 2.0 HDi / 2.0 TDCi / D4204T Remanufactured Common Rail Injector
Fits: DW10CB, DW10CTED4
View injectorCompatible Injector Part Numbers
6 injectors compatible with engines affected by P0304.
Cross Reference Numbers
OEM and alternative part numbers for injectors affected by P0304.
Affected Vehicle & Engine Combinations
Need a Replacement Injector?
UK-tested remanufactured injectors with 12-month warranty. Search by engine code or vehicle registration.
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Engine-Specific Guides
Frequently Asked Questions
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Why It Happens
Common-rail injectors live at extreme pressures with clearances measured in microns. Fuel quality, heat cycles and mileage wear the internal control valve and nozzle: back-leak rises, spray patterns deteriorate and delivery drifts from calibration. Most 'engine' complaints on high-mileage diesels trace back to this gradual injector wear.
Professional Diagnosis
Workshops follow a fixed order: scan for codes and freeze-frame data, compare requested vs actual rail pressure on live data, read cylinder balance corrections, then run a comparative leak-off test. That sequence identifies the faulty injector without guesswork — and prevents needlessly replacing pumps, sensors or the wrong injector.
Typical Workshop Findings
The most frequent findings are: one injector returning several times the leak-off volume of its neighbours; carbon build-up from a failed copper washer; corroded injector connectors; and missing injector coding after a previous repair. Genuine high-pressure pump failures are rarer than back-leak faults that mimic them.
When to Replace Injectors
Replace when testing proves internal wear: leak-off clearly above the other injectors, balance corrections at their limits, or a failed bench test. Cleaning and additives cannot restore worn internal clearances. Bench-tested remanufactured injectors restore original calibration at a fraction of the new price — always coded to the ECU where the engine requires it.
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