Engine Locators

The EGR system is very effective at reducing NOx emissions from engine combustion, but current NOx emissions standards require additional measures to further reduce this pollutant. The Selective Catalytic Reduction (SCR) system is used for that purpose. The SCR process starts with the injection of Diesel Exhaust Fluid (DEF) into the exhaust stream. This article discusses DEF, DEF system components, and DEF dosing operation. The chemical reactions that take place are discussed when we cover the SCR system.

Diesel Exhaust Fluid (DEF)

Is a pre-mixed fluid that consists of purified water and Urea at a ratio of 32.5% Urea.
Urea is a nitrogenous compound and water, which readily decomposes into ammonia.
The freezing point of DEF is 12°F (-11°C).
Has a slight ammonia smell
Is nonflammable
Is colorless
Is non-toxic and non-polluting
Is not harmful to handle, but it can be corrosive to certain metals
Readily available from the pump at many fuel stations, or it can be obtained in containers
Filler caps and merchandising often use the color blue as an identifier. (The TITAN XD Diesel DEF fill cap is blue.)

Additional names for DEF:

Urea
AUS 32 (Aqueous Urea Solution 32)
AdBlue
NOx Reduction Agent
Catalyst Solution

NOTE: DEF may be referred to as “Reductant” or “Urea” in the Electronic Service Manual (ESM) and CONSULT-III plus.

DEF Usage

The DEF tank holds 4.5 US gallons (17 L) of Diesel Exhaust Fluid (DEF) and needs to be refilled when the DEF level display indicates the DEF is low.
How often it needs to be refilled is based on vehicle usage such as; driving conditions, loads, speeds, or trip distances.
Nissan recommends that you use a DEF that meets the International Organization for Standardization (ISO) 22241 specification.
DEF may be stored in sealed containers away from direct sunlight in temperatures between 23°F and 77°F (-5°C and 25°C) for up to 18 months. However, for each 9°F (5°C) increment above this recommended temperature, shelf life is reduced by 6 months.
In addition, DEF should not remain in the DEF tank of an unused vehicle for more than 6 months.
Water should never be added to DEF. Adding water to DEF will change the DEF concentration, which will reduce SCR effectiveness. Chemicals introduced by adding unpurified water can also corrode or damage aftertreatment components or damage the DEF dosing valve. In addition, adding water to DEF will raise the freezing point.
DEF concentration can be tested using a refractometer that is calibrated for DEF.

System Components

DEF Tank

The DEF tank is mounted under the vehicle behind the fuel tank. DEF is added to the tank through a filler neck to the right of the diesel fuel-filler neck. The DEF tank houses the following components of the DEF system:

– DEF Controller (Control Unit)

– DEF Quality Sensor

– DEF Dosing Unit (Pump Assembly)

– DEF Fluid Line and Heater

DEF Controller (Control Unit)

The DEF control unit communicates and shares information with the ECM. Using inputs from the ECM and other sensors, the control unit controls the operation of the DEF pump assembly, including the DEF heating systems.

DEF Quality Sensor

A DEF quality sensor is mounted on the side of the DEF tank. This sensor detects the Urea concentration in the DEF to confirm it is acceptable for effective dosing (approximately 32.5% Urea). If the DEF quality sensor detects an incorrect DEF concentration, a DEF System Error - SEE Dealer message will display on the Vehicle Information Display in the combination meter.

NOTE: If the DEF tank is filled with anything other than DEF or has diluted DEF, the aftertreatment system will not operate correctly. A DTC will be set and engine power reductions will follow.

DEF Dosing Unit (Pump Assembly)

The DEF pump assembly is mounted to the bottom of the DEF tank with most of the assembly inside the tank submerged in DEF.

The pump assembly includes:

DEF Pump – The pump draws DEF through its suction port, pressurizes it, filters it, and delivers it to the DEF dosing valve via the pressure outlet port and dosing line.
DEF Tank Heater – The DEF tank heater is used to thaw frozen DEF in the tank. The DEF tank heater is a series of heated strips that are designed to evenly thaw frozen DEF.
DEF Temperature Sensor – The DEF control unit uses this sensor information to determine if tank heater operation is necessary.

DEF Level Sensor – This sensor reports the quantity of DEF in the DEF tank to the DEF control unit and the ECM. This sensor information is also for the DEF level display in the combination meter.
DEF Pressure Sensor – This sensor monitors the pressure in the DEF dosing line to make sure adequate pressure is available for DEF dosing. The DEF pressure sensor also monitors the line pressure after the ignition has been turned OFF to ensure DEF has been purged from the line.
DEF Dosing Line Heater – A DEF dosing line heater is located at the end of the DEF dosing line where it meets the DEF pump. This heater is used to heat the dosing line between the DEF pump and the DEF dosing valve to prevent freezing during vehicle operation.

DEF Connector Line – The DEF connector line carries DEF to the dosing valve during dosing operation and then back to the DEF tank when the system is purged.

Dosing Valve – The DEF dosing valve is located on the decomposition tube. It is a Pulse Width Modulation (PWM) valve that sprays a fine mist of DEF into the exhaust stream when commanded by the ECM. This spray of DEF is sometimes called SCR dosing because the SCR catalyst uses DEF to chemically convert NOx to nitrogen and water.

System Operation

Heating State

Since DEF freezes at 12°F (-11°C), at ignition ON, the DEF temperature sensor is used by the DEF control unit to determine if DEF tank heating is needed. The DEF control unit will not command the pump to prime until the DEF is warmed to a sufficient temperature. DEF heating, if needed, will activate and continue to heat the DEF based on the thawing time chart.
The DEF system also has a heater in the pump and a heater for the DEF dosing line. Based on ambient temperatures, the DEF control unit will also operate these heaters to thaw frozen DEF before priming and to prevent DEF from freezing during vehicle operation.

Priming State

After engine start, exhaust flow begins to heat the Selective Catalytic Reduction (SCR). When the SCR reaches 302°F (150°C) and the DEF temperature is in an acceptable range, the DEF control unit will command the DEF pump to start its priming process. The DEF pump draws DEF from the tank, pressurizes it, filters it, and then sends it through the DEF dosing line to the DEF dosing valve. The ECM commands the DEF dosing valve to open and close to purge any air from the system. After the system builds sufficient pressure, it is ready to dose (dosing state).

Dosing State

The DEF pump runs continuously to maintain DEF dosing line pressure when the system is in the dosing state. The pump runs whether or not the dosing valve is spraying DEF.
The DEF dosing valve is directly controlled by the ECM. When the ECM determines that DEF dosing should occur, it sends a PWM signal to energize the DEF dosing valve, spraying DEF into the decomposition tube. Dosing is not necessarily consistent; the ECM commands dosing frequency and quantity based on several sensor inputs and engine operating conditions.
The following conditions are required for effective dosing to occur:

– No current DTCs for the SCR or DEF system

– DEF tank level must be above 6% filled with DEF

– DEF quality must be verified by the quality sensor

– DEF pressure must be sufficient for dosing

– The SCR intake temperature must be at least 302°F (150°C) and the SCR outlet temperature must be at least 392°F (200°C)

– Both NOx sensors must be reading NOx levels

– DEF temperature must be above 27°F (-3°C)

NOTE: The ECM compares the inlet and outlet NOx sensor values to determine if the aftertreatment system is efficiently removing NOx. If NOx levels remain too high at the outlet NOx sensor, the ECM may command additional EGR or additional DEF dosing to lower these levels.

Purging State

When the engine/ignition is turned OFF, the DEF system enters the purging stage. An audible click and a pumping sound are heard from the DEF pump assembly when it is in a purge cycle. The DEF pump will slide its internal reverting valve and cause a change in the flow direction of the DEF. The DEF pump pulls all of the DEF out of the DEF dosing valve and DEF dosing line, and then returns the unused DEF to the fluid tank. During the purging process, the DEF dosing valve opens, eliminating any vacuum created in the line, allowing for a more complete purge process.
Purging/removing DEF from the lines, when not in operation, will guard against damage due to freezing in cold climates.
If power to the DEF control unit is removed, such as disconnecting the battery before the purging process is complete, a DTC will be set. The incomplete purge counter can be viewed in CONSULT-III plus.

DEF Fill Level Warning Lamp and Warning Messages

The instrument cluster contains a DEF warning lamp that operates in conjunction with DEF warning messages in the vehicle information display. If the DEF level is 25-100% full, no warnings will be present. If the DEF level is below 25%, warnings will provide system status and describe performance decrease as follows:

DEF Fill Level Warnings

Full – If 25-100% full, no fill level warnings are present.
DEF Low – XX% Refill Soon – Message displays when DEF level reads 15-25% (decrements of 5%).

– DEF Lamp State: Solid DEF Lamp

– DEF Lamp State: Flashing DEF Lamp

DEF Low – Limited Engine Power in XX Miles – Message displays when DEF driving range is between 0-249 miles (below 5%). Engine torque will be reduced. Approximately 25% torque reduction.

– DEF Lamp State: Flashing DEF Lamp

DEF Empty – 5 MPH Limit Soon – Refill DEF

– Engine torque will be reduced. Approximately 40% torque reduction.

– DEF Lamp State: Flashing DEF Lamp

The following conditions will limit the vehicle speed to a maximum of 5 MPH:

– Engine shutdown and restart

– Engine is idled for extended period of time (approximately 1 hour)

– ECM detects fuel level in the tank has increased

DEF Empty Refill DEF – See Owner’s Manual – Vehicle speed limited to a maximum of 5 MPH.

– DEF Lamp State: Flashing DEF Lamp

DEF System Error Warning Messages

When the ECM detects errors in the operation of the DEF system, a warning message will display in the vehicle information display.

Types of DEF System Errors:

When any errors are detected, the DEF System Error Warnings will be given as follows:

DEF System Error – See Dealer – Warning 1 of 4. Warnings progress from 1-4 and increase in severity.
DEF System Error – 5 MPH Limit in XX Miles – Warning 2 of 4. Message displays when driving range is 249 miles until 5 MPH limit.
DEF System Error – 5 MPH Limit Soon – Warning 3 of 4. The following conditions will limit the vehicle speed to a maximum of 5 MPH:

– Engine shutdown and restart

– Engine is idled for extended period of time (approximately 1 hour)

– ECM detects fuel level in the tank has increased

Speed Limited to 5 MPH – See Dealer – Warning 4 of 4. Vehicle speed limited to a maximum of 5 MPH.