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REMANUFACTURED INJECTORDiesel Injectors & Fuel Pumps
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How to Test Diesel Injectors

Injector Compatibility Checker

Quick Answer

The most reliable way to test a diesel injector is with a leak-off (back-leak) test using clear hoses connected to the injector return lines. Excessive fuel in the return line indicates a worn injector. Electrical resistance tests, scan tool live data, and spray pattern analysis are also effective diagnostic methods.

Symptoms Indicating Injector Failure

Rough or uneven idle

A worn injector delivers inconsistent fuel, causing the engine to shake or misfire at idle.

Black or white exhaust smoke

Over-fuelling produces black smoke; a leaking injector can cause white or blue-grey smoke on startup.

Hard starting or extended cranking

Fuel draining back through a leaking injector means the rail loses pressure overnight.

Loss of power under load

A stuck or partially blocked injector cannot deliver enough fuel at high demand.

Increased fuel consumption

A leaking or over-fuelling injector wastes fuel, reducing MPG noticeably.

Engine knocking or detonation

Poor spray atomisation or incorrect timing from a faulty injector causes combustion knock.

Step-by-Step Testing Methods

Leak-Off (Back-Leak) Test

Easy
  1. 1Connect clear hoses to each injector return line.
  2. 2Route hoses into identical graduated containers.
  3. 3Crank or idle the engine for 30 seconds.
  4. 4Compare fuel volumes — a significantly higher volume indicates a leaking injector.
  5. 5Replace any injector returning more than double the lowest reading.

Tools needed: Clear hoses, containers, basic spanners

Electrical Resistance Test

Easy
  1. 1Disconnect the injector electrical connector.
  2. 2Set a multimeter to ohms (Ω).
  3. 3Measure resistance across the injector terminals.
  4. 4Compare to the OE specification (typically 0.5–1.5 Ω for solenoid, 180k+ Ω for piezo).
  5. 5An out-of-range reading confirms a faulty injector.

Tools needed: Digital multimeter

Scan Tool Live Data

Moderate
  1. 1Connect a diagnostic scan tool to the OBD-II port.
  2. 2Navigate to injector correction values (IMA/pilot quantity).
  3. 3Idle the engine and observe values.
  4. 4Correction values beyond ±3 mg/stroke typically indicate a worn injector.
  5. 5Identify which cylinder is out of range.

Tools needed: OBD-II diagnostic scan tool

Spray Pattern Test (Workshop)

Advanced
  1. 1Remove the injector from the engine.
  2. 2Mount on an injector test bench.
  3. 3Activate the injector at rated pressure.
  4. 4Observe the spray pattern — it should be a fine, even cone.
  5. 5Dripping, uneven spray, or poor atomisation means replacement.

Tools needed: Injector test bench, safety equipment

Tools Required

Clear fuel hose (1m lengths)
Graduated measuring containers
Digital multimeter
OBD-II diagnostic scan tooloptional
Basic spanner set (10mm–19mm)
Injector removal tool (if seized)optional

When Replacement Is Necessary

Replace an injector when: the leak-off test shows excessive return fuel (more than double the best cylinder), correction values exceed ±4 mg/stroke, resistance is out of specification, or the spray pattern is poor. If the vehicle has over 150,000 miles and one injector has failed, consider replacing the full set — the remaining injectors are likely near end of life.

Compatible Injector Parts

0987654567
0987654567Bosch
£99.883450
3450Bosch
£99.903415
3415Bosch
£99.903407
3407Bosch
£99.903533
3533Bosch
£99.903467
3467Bosch
£99.903392
3392Bosch
£99.903390
3390Bosch
£99.903413
3413Bosch
£99.9045584736378
45584736378Bosch
£99.904343454321
4343454321Bosch
£99.903538
3538Bosch
£99.90

Affected Engines

N47D20 OM651 CAYC DW10C CRBC

Vehicle & Engine Combinations

AUDI A6CAYC
AUDI A6CRBC
BMW 1 SERIESN47D20

Related Fault Codes

P0201P0202P0203P0204P0263P0300

Related Injector Problem Guides

Black SmokeRough IdleHard Start

Frequently Asked Questions

Can I test diesel injectors without removing them?
Yes. The leak-off test, electrical resistance check, and scan tool live data can all be performed with injectors installed in the engine. Only the spray pattern test requires removal.
How much fuel return is too much in a leak-off test?
There is no universal threshold — compare cylinders relative to each other. If one injector returns more than double the fuel of the lowest, it is likely worn and should be replaced.
What resistance should a diesel injector have?
Solenoid injectors typically read 0.5–1.5 Ω. Piezo injectors read 180 kΩ or higher. Always check the manufacturer specification for your exact injector.
Do I need to replace all injectors if one fails?
Not strictly, but it is recommended on high-mileage vehicles. If one injector has worn out, the others are likely approaching end of life as well.

Other Testing Guides

How to Test a Diesel Injector – Step-by-Step MethodsDiesel Injector Leak-Off Test – How to Diagnose a Leaking InjectorCommon Rail Injector Testing – Diagnosis & Test Procedures

Need a Replacement Injector?

All our remanufactured injectors are fully tested on a dedicated bench, calibrated to OE specifications, and supplied with a 12-month warranty.

View All InjectorsReplacement Cost Guide Call 020 3355 6581

Explore Diesel Injector Guides

Browse by VehicleBrowse by EngineInjector Part NumbersCross Reference NumbersInjector ProblemsReplacement Cost Guide

More common questions from UK installers

Specific testing, packaging and report questions our customers ask before posting injectors to us.

What does an injector test report actually show?

Every Remanufactured Injector test report shows four numbers per injector: idle delivery (mg/stroke), full-load delivery (mg/stroke), back-leakage (ml/min) and a spray-pattern grade. Anything outside OEM tolerance is flagged red. The report is a single A4 PDF emailed within 48 hours of receiving your injectors — no phone-call interpretation needed.

Do I need to send all four injectors for testing?

We recommend sending all injectors from the affected engine even if you only suspect one. Back-leakage is a relative measurement — a single injector tested in isolation cannot be compared against the others on the same engine. Testing the full set costs £80 and rules out borderline units that would otherwise come back as a workshop return weeks later.

How should I package injectors for testing?

Wrap each injector individually in bubble wrap, keep the cylinder number marked on the side (e.g. 'Cyl 3'), and place upright in a small cardboard box with packing material. Royal Mail Special Delivery or DPD Tracked is sufficient. Our address is on the confirmation email — please do not include fuel; pump the injector dry first or bag it separately.

Related testing & diagnostic searches

Decide between testing, repair and replacement with our cost & symptom guides.

Testing deep dives from the Knowledge Centre

Workshop-grade guides from the team that bench-tests every injector we sell.

Diesel Injector Leak-Back Test: Complete Workshop GuideThe on-vehicle comparison test that names the suspect cylinder.How Injector Test Benches WorkWhat happens inside professional injector testing.Reading an Injector Test ReportDelivery, back-leak and spray results explained.The Injector Cut-Out TestIsolating a faulty cylinder with a scan tool.