Cummins ISB4.5 CM2150 Fault Code: 3548 PID: SPN: 5392 FMI: 31 Aftertreatment Diesel Exhaust Fluid Dosing Unit Loss of Prime – Condition Exists.

Circuit Description
For an air-assisted SCR aftertreatment system, the aftertreatment diesel exhaust fluid dosing unit requires air pressure from the OEM air tanks in order to prime and help carry the diesel exhuast fluid from the dosing unit to the aftertreatment nozzle. The diesel exhaust fluid dosing unit precisely measures the amount of diesel exhaust fluid to be injected into the aftertreatment system. The diesel exhaust fluid dosing unit has three primary cycles: priming, dosing, and purging. The priming cycle occurs at initial engine start makes sure that diesel exhaust fluid is available at the diesel exhaust fluid dosing unit. During the dosing cycle, the diesel exhaust fluid is being delivered to the aftertreatment nozzle. A purge cycle occurs when the engine is turned off. The purge cycle confirms that all the diesel exhaust fluid is removed from the diesel exhaust fluid lines and aftertreatment nozzle.

Component Location
The aftertreatment diesel exhaust fluid dosing unit location is OEM dependent. Refer to the OEM service manual for more information.

Conditions for Running the Diagnostics
This diagnostic runs when the engine is started and runs while the dosing unit is in the priming state, which could last up to 12 minutes after initial key-on.

Conditions for Setting the Fault Codes
-The dosing unit will attempt to prime every 30 seconds for 20 attempts.
-If the dosing unit has been unable to prime after completing all attempts, this fault code will become active.
-The dosing unit can take up to 12 minutes to complete the priming cycle before a fault code will become active.

Action Taken When the Fault Code is Active
-The ECM illuminates the amber CHECK ENGINE lamp and/or the MIL immediately after the diagnostic runs and fails.
-Engine torque will be reduced.

Conditions for Clearing the Fault Code
-This fault code can not be cleared with INSITE™ electronic service tool.
-To validate the repair, it is necessary to use INSITE™ electronic service tool to perform the Diesel Exhaust Fluid Doser Pump Override Test. Reference Procedure 019-440 in the appropriate service manual.
-Make sure the diesel exhaust fluid dosing unit cycles through all phases successfully (priming, dosing, and purging) and diesel exhaust fluid flows evenly from the aftertreatment nozzle.
-The ECM will turn off the amber CHECK ENGINE lamp and/or MIL immediately after the diagnostic runs and passes.
Use INSITE™ electronic service tool to verify the fault code is inactive.
-The inactive fault code will be cleared from the memory after 9600 hours of engine operation without a re-occurrence.

Shop Talk
The diesel exhaust fluid dosing unit system consists of three phases. These phases occur in the following sequence: Priming, dosing, and purging. This fault code is set when the dosing unit is unable to prime.

Fault Code 3548 is triggered by the following external failure modes:
-Low aftertreatment diesel exhaust fluid level in the diesel exhaust fluid tank
-Low air pressure is detected at the dosing control unit
-Plugged or restricted inline air filter
-Blocked, restricted, frozen, or leaking dosing unit supply line
-Diesel exhaust fluid dosing unit drawing in air through loose or damaged lines or connectors.

Fault Code 3548 is triggered by the following internal diesel exhaust fluid dosing unit failure modes:
-Build up of DEF crystals in the dosing unit restricts air flow to the aftertreatment nozzle
-Internal pressure sensor or poppet valve failure causing the diesel exhaust fluid dosing unit to fail to prime. This will cause diesel exhaust fluid to flow from the aftertreatment nozzle during priming.

If blocked diesel exhaust fluid lines due to build-ups are detected, make sure the dosing unit is purging correctly at key-off. Incomplete purge counts can be viewed using the Data Monitor/Logger menu in the aftertreatment section of INSITE™ electronic service tool. If the vehicle is equipped with a battery isolation switch, it should have a built-in delay device to prevent power down of the dosing unit until the purge sequence is complete.