How do I know if a misfire is caused by the injector?

An OBD scan will identify the misfiring cylinder. A cylinder contribution test and leak-off test confirm whether the injector is at fault versus a wiring, compression, or fuel supply issue.

Can a misfire damage my engine?

Yes. Unburnt fuel washes oil from the cylinder walls, increasing bore wear. It also contaminates the DPF and catalytic converter. Fix misfires promptly to avoid secondary damage.

Do I need to code a new injector?

Yes. Modern common-rail diesel injectors must be coded (IMA coding) to the ECU after replacement. The code is printed on the injector body and entered using a diagnostic tool. Running an uncoded injector causes poor performance.

Can injectors cause intermittent misfires?

Yes. A partially worn injector may work normally when warm but misfire when cold, or work at idle but misfire under load. An extended leak-off test at both idle and 2,000 RPM reveals these marginal faults.

Should I replace one injector or all four?

If one injector has failed and the vehicle has high mileage (80,000+ miles), the others are likely to fail soon. Replacing the full set gives balanced performance and avoids repeat repairs.

Diesel Engine Misfire From Injector Fault

A diesel engine misfire is one of the most common — and most disruptive — symptoms of injector failure. Unlike petrol engines, diesel misfires are almost always fuel-related because there are no spark plugs. A dead or weak injector starves its cylinder of fuel, causing the engine to run on three (or fewer) cylinders. This guide explains how to identify, diagnose, and fix injector-related misfires.

Fix this issue

Most diesel engine misfire from injector fault issues in diesel engines are caused by failing fuel injectors.

✔Replace worn or leaking injectors
✔Restore performance and fuel economy
✔Prevent further engine damage

View Compatible Injectors

OEM Tested & Calibrated
12-Month Warranty
Free UK Delivery

Symptoms

Engine shaking at idle

A misfiring cylinder produces no power stroke, causing the engine to vibrate noticeably. The shaking is rhythmic and worsens when the engine is cold.

Loss of power

Running on fewer cylinders reduces total power output. The vehicle feels sluggish, struggles on hills, and may not reach motorway speeds safely.

Rough running with fault code

The ECU detects the misfire and logs codes P0300 (random misfire) or P0301-P0304 (cylinder-specific misfire). The engine management light illuminates.

Excessive cranking before starting

A completely dead injector means one cylinder contributes nothing during cranking. The remaining cylinders must overcome the compression of the dead cylinder, extending crank time.

Uneven exhaust note

A healthy diesel has a smooth, even tick at idle. A misfiring engine has an uneven, lumpy exhaust note. You can often hear which cylinder is missing.

Increased fuel consumption

The ECU compensates for the missing cylinder by enriching the working cylinders. This increases overall fuel consumption by 15-25%.

Diagnostic Tests

OBD fault code scan

Codes P0201-P0204 identify injector circuit faults for specific cylinders. P0300 indicates random misfires. P0087 indicates low fuel rail pressure that can cause misfires on all cylinders.

Cylinder contribution test

Using a diagnostic scanner, command each injector off in sequence. If switching off a cylinder makes no difference to RPM, that cylinder was already not contributing — confirming the fault.

Leak-off test

A worn injector may misfire because it cannot build enough fuel pressure internally. The leak-off test reveals which injector is returning excessive fuel to the tank.

Guide

Injector resistance test

Measure the electrical resistance of each injector coil with a multimeter. Compare against manufacturer spec (typically 0.2-2.0 ohms for solenoid injectors). Out-of-spec resistance indicates electrical failure.

Common Causes

Complete injector electrical failure

The solenoid or piezo actuator fails completely, preventing the injector from opening. No fuel enters the cylinder, causing a complete misfire on that cylinder.

Blocked injector nozzle

Severe carbon buildup or foreign particles can block the nozzle orifices entirely. The injector fires electrically but no fuel passes through. Ultrasonically cleaning or replacing the injector resolves this.

Internal mechanical seizure

The injector needle can seize in the closed position due to fuel contamination or corrosion. This is more common after the vehicle has stood unused for extended periods.

Wiring or connector fault

A damaged wire, corroded connector, or loose pin in the injector harness prevents the ECU signal from reaching the injector. This mimics a dead injector but is a wiring fault, not an injector fault.

Low fuel rail pressure

If the high-pressure pump is weak or the fuel filter is blocked, rail pressure drops below the threshold needed for injection. All cylinders may misfire intermittently under load.

Repair Solution

For a confirmed injector misfire (verified by leak-off test or cylinder contribution test), replacement with a remanufactured injector is the standard fix. Each remanufactured unit is electrically tested, flow-calibrated, and supplied with a new IMA code for ECU coding. If the misfire is on all cylinders, check the fuel filter and high-pressure pump before replacing injectors.

Browse Remanufactured Injectors by Brand

Bosch Delphi Siemens Denso Continental