Component Description For Nox Sensor Control Unit - GF14.40-P-3024MRD

Engine 274.9 in model 213 (except 213.050/053/153), 238 up to model year 2021 

with code 920 (Direct petrol injection with stratified charging) 

View of exhaust system 

Location 

In every exhaust system there is a NOx sensor with permanently attached NOx sensor control unit. The NOx sensor is screwed into the exhaust pipe. The NOx sensor control unit it located on the underfloor.

Task 

The NOx sensor records the NO_x and O₂ concentration in the exhaust gas downstream of the NOx storage catalytic converter. The NOx sensor control unit reads in the signals from the NOx sensor directly and transmits them via the drive train sensor CAN (CAN I) to the ME-SFI [ME] control unit (N3/10).

Input and output signals 

The following input signals are evaluated by the NOx sensor control unit and appropriate output signals are emitted:

• Direct input signals 
• CAN input signals from the ME-SFI [ME] control unit 
• CAN output signals to the ME-SFI [ME] control unit 

Direct input signals 

• Circuit 87

CAN input signals from the ME-SFI [ME] control unit 

• Diagnosis requests

CAN output signals to the ME-SFI [ME] control unit 

• NO_x concentration in the exhaust
• O₂ concentration in exhaust
• Diagnostic data

Structure 

Courtesy of MERCEDES-BENZ USA

The NOx sensor (double chamber sensor with evaluation circuit) is inseparably connected via an 8-pin signal line to the NOx sensor control unit.

An integrated sensor heater which is controlled by the NOx sensor control unit ensures rapid operational readiness of the NOx sensor.

Function 

The active ceramic probe bodies consist of many layers with two reaction chambers. The oxygen (O₂ ) in the exhaust is detected in the first chamber, as in an oxygen sensor.

To do this, a pump voltage is applied to the electrodes in the first chamber. This causes the O₂ molecules to be split into double charged oxygen ions and then pumped out of or into the first chamber (depending on whether rich or lean exhaust gas is present) until a 450 mV voltage difference at the electrodes is reached. The oxygen concentration is calculated from the size of the required pump current "Ip1".

The nitrogen oxide is split at the ground electrode into nitrogen and oxygen in the second chamber if present. The oxygen is pumped out if the chamber again. The concentration of the nitrogen oxides in the exhaust is calculated from the resulting pump current "Ip2". The voltage signals from the NOx sensor are sent to the NOx sensor control unit and processed there. The NOx sensor control unit sends the NO_x and O₂ concentration levels via the drive train sensor CAN to the ME-SFI [ME] control unit.

Using this information, the exhaust treatment can be monitored in stratified operation (lean-burn operation) of the engine and thus the NOx storage catalytic converter purging can be controlled. The NOx sensor also recognizes damage and "sulfur poisoning" of the NO_x storage catalytic converter. A desulfuring process can then be initiated by the ME-SFI [ME] control unit by increasing the temperature of the NO_x storage catalytic converter.

Schematic function diagram of the NOx sensor (double chamber sensors) 

Courtesy of MERCEDES-BENZ USA

	Wiring diagram for gasoline-DI control unit	Engine 274.9 in model 213 (except 213.050/053/153) up to model year 2021 with code 920 (Direct petrol injection with stratified charging)	PE07.08-P-2101-97DBB
		Engine 274.9 in model 238 up to model year 2021 with code 920 (Direct petrol injection with stratified charging)	PE07.08-P-2101-97DCB