Description:
A Diesel Exhaust Fluid (DEF) Dosing Unit A0001405539 is a component in modern diesel vehicles that injects Diesel Exhaust Fluid into the exhaust system to reduce harmful emissions.

Part Number: A0001405539 
Condition: New
Manufacture/Brand: Detroit Diesel
Related Part Numbers: 0001405539, DDE-A0001405539, DDEA0001405539, RA0001405539, EA0001405539

Fits A0001405539 For

 Detroit Diesel / Freightliner heavy trucks with SCR emissions systems

– Especially on GHG17 / EPA10 emission spec engines
– Used on models like Freightliner Cascadia with Detroit engines
(DEF metering unit in the aftertreatment)

Mercedes-Benz heavy trucks such as the Actros MP4 / similar commercial trucks

Trucks and engines that require a DEF dosing system under EPA emissions regulations


Purpose of using A0001405539:

 1.  Inject DEF into the Exhaust


The dosing unit:

  • Receives signals from the engine control module (ECM)

  • Sprays a precise amount of DEF into the hot exhaust stream

Accuracy is critical — too much or too little affects emissions performance.

2. Reduce NOx Emissions

When DEF is injected:

  • It turns into ammonia in the exhaust

  • Inside the SCR catalyst, ammonia reacts with NOx gases

  • Harmful NOx is converted into harmless nitrogen and water vapor

This allows the engine to meet strict environmental regulations.

3. Protect the SCR System

The dosing unit ensures:

  • Proper DEF quantity

  • Correct spray pattern

  • Even distribution in exhaust

This prevents:

  • DEF crystallization

  • Catalyst damage

  • Inefficient emissions conversion

4. Prevent Engine Derate

If the dosing unit fails:

  • The truck may enter power reduction mode

  • Emissions faults appear

  • The vehicle may limit speed

The dosing unit keeps the emissions system functioning correctly so the engine can operate at full power.


Signs of a Bad Diesel Exhaust Fluid Dosing Unit A0001405539:

1. Check Engine Light / SCR Warning Light

  • Emissions system warning appears

  • “SCR System Fault” message

  • DEF system warning

This is usually the first sign.

2. Engine Power Derate

  • Reduced engine power

  • Limited speed (sometimes 5–55 mph depending on severity)

  • “Engine Derate” message

If the system can’t control NOx emissions, the truck protects itself by limiting power.

3. DEF Crystallization Buildup

  • White, chalky deposits around the injector

  • DEF residue on exhaust piping

This may indicate:

  • Leaking injector

  • Poor spray pattern

  • Stuck dosing valve

4. Excessive NOx Fault Codes

Diagnostic trouble codes related to:

  • NOx efficiency

  • SCR performance

  • DEF dosing rate

The system detects improper injection amounts.

5. White Smoke from Exhaust

  • Visible white vapor under load

  • Strong ammonia smell

This can happen if:

  • Too much DEF is injected

  • DEF is not atomizing properly

6. Poor Fuel Economy

Improper emissions control can cause the engine to compensate, slightly affecting efficiency.

7. DEF Consumption Issues

  • DEF level dropping too fast

  • DEF not being used at all

Either can indicate a dosing malfunction.