What Is SPN 5394?

SPN 5394 covers Aftertreatment DEF dosing system faults. The DEF dosing system includes the DEF pump, dosing injector, supply lines, and control module. When any component fails to deliver the correct amount of DEF to the SCR catalyst, the ECM sets this fault.

DEF Dosing System Components

1. 1

   DEF pump — pressurizes DEF from the tank to the dosing injector (typically 8–10 bar)

2. 2

   DEF dosing injector — injects DEF into the exhaust stream upstream of the SCR catalyst

3. 3

   DEF supply lines — carry DEF from tank to pump to injector

4. 4

   DEF filter — filters DEF before it reaches the pump

5. 5

   DEF control module (DCU) — controls dosing rate based on NOx sensor feedback

Step-by-Step Diagnostic Procedure

Start with the simplest checks first.

1. 1

   Check DEF level and quality. Low DEF or bad DEF quality causes dosing system faults.

2. 2

   Inspect the DEF dosing injector for crystallization. Remove the injector and inspect the tip. White crystalline deposits indicate the injector is clogged.

3. 3

   Check DEF pump pressure with a scan tool. Commanded pressure vs actual pressure should match within 1 bar.

4. 4

   Inspect DEF supply lines for kinks, cracks, or crystallization blockages.

5. 5

   Check the DEF filter. A clogged filter restricts flow and causes low pump pressure.

6. 6

   Test the dosing injector electrically. Check resistance between injector terminals — typically 2–5 ohms. Open circuit means failed injector.

7. 7

   If pump pressure is low with a clean filter and good supply lines, the pump is worn. Replace the DEF pump.

Dosing Injector Cleaning Procedure

A clogged dosing injector can often be cleaned rather than replaced.

1. 1

   Remove the dosing injector from the exhaust pipe.

2. 2

   Soak the injector tip in warm water for 30 minutes to dissolve urea crystallization.

3. 3

   Use a soft brush to remove remaining deposits.

4. 4

   Test injector flow by connecting to a DEF supply and measuring flow rate.

5. 5

   If flow is still restricted after cleaning, replace the injector.