Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
SCR control unit
The SCR control unit performs the following functions:
- PT-CAN bus communication, network master
- Activation of the 3 pumps for AdBlue
- Activation of the SCR metering module for AdBlue
- Activation of the two heating units for AdBlue
- Evaluation of both the fuel level sensors and temperature sensors
- Monitoring functions
- On-board diagnosis monitoring
The DDE control unit performs the following functions:
- Evaluation of the nitrogen oxide sensors
- Calculation of the injection rate of AdBlue and transmission to the SCR control unit via PT-CAN
- Control over SCR system deactivation
The SCR control unit has to ensure that the input parameters for metering via the DDE are within the permitted range:
Attention!
Fluid level in active tank and passive tank: do not use the vehicle without AdBlue.
The engine can no longer be started.
- Temperature of AdBlue in the active tank and passive tank and the pressure line: AdBlue freezing point at -11 °C
- Pressure line pressure build-up: ensures the injection rate
Faults in the SCR system are indicated by the emissions warning light in the instrument cluster. Corresponding fault code entries are stored in the SCR control unit.
The SCR control unit is connected via a 94-pin plug connection. The SCR control unit is a partner on the PT-CAN.
Attention!
The plug connection must not be connected or disconnected while under voltage!
The SCR control unit has 5 pins for the voltage supply and 1 pin as a ground connection.
| Item | Explanation |
|---|---|
| 1 | SCR control unit |
| 2 | 94-pin plug connection |
The SCR control unit has various installation locations depending on the series (see examples).
- F06, F07, F10, F11, F12: spare wheel well
- F30: luggage compartment, left side wall
- F15: outer engine support
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.