– 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

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.

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.