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Autopapyrus · Mercedes-Benz B-Class W247 1.5 CDI (OM608, 2019)
https://autopapyrus.com/guides/mercedes-benz/b-class/engine-management-system-diagnostics/
Compiled automatically from service literature and owner experience. Specs are for the typical configuration — check them against your exact car before you start.

Mercedes-Benz B-Class W247 1.5 CDI (OM608, 2019): Engine Management System Diagnostics

The OM608 engine management system includes the electronic control unit (ECU), sensors (mass airflow, temperature, boost pressure, oxygen), actuators (Common Rail injectors, variable geometry turbocharger, EGR valve), and the exhaust aftertreatment system (SCR with AdBlue, diesel particulate filter (DPF)). Diagnostics start by reading fault codes through the OBD-II port, then checking live data and the condition of components.

Difficulty: Medium Time: 30-60 minutes Steps: 10 Tools: OBD-II scanner (generic or Mercedes-specific), laptop with diagnostic software (optional, for advanced diagnostics), flashlight

Part

  • 🔩 None required (diagnostic procedure)
There's a full repair manual for this car Every system, wiring diagrams and torque specs — not just this job. Haynes AutoFix (online) and 2 more. See manuals →

Step by step

10 steps
  1. Locate the OBD-II diagnostic port under the steering column, near the driver's pedals. It is a black 16-pin port and may be covered by a protective cap.
  2. Connect the OBD-II scanner to the port and turn the ignition on without starting the engine. The warning lights on the instrument cluster will come on.
  3. On the scanner, select the function for reading fault codes (Read Codes). Write down all stored and pending codes in the P####, U####, B#### and C#### formats (Mercedes-Benz five-character manufacturer-specific codes are also possible).
  4. Common codes for the OM608 and its aftertreatment system: P20EE (low SCR catalyst efficiency; check the AdBlue level and the NOx sensors), P2002 (reduced DPF efficiency; a regeneration or filter replacement is needed), P0401 (insufficient EGR flow; clean or replace the valve), P0171/P0172 (lean/rich mixture; check the MAF sensor and the intake for leaks), P030X (misfire in cylinder X; on a diesel this points to an injector or compression problem).
  5. Switch to real-time parameter view (Live Data) and monitor: coolant temperature (operating range 85-95 °C), mass airflow (2-4 g/s at idle), boost pressure (up to 1.8 bar under load), oxygen sensor voltage (lambda sensor fluctuating between 0.1 and 0.9 V), throttle position, and engine speed.
  6. Run the readiness monitors test (Readiness Monitors). All monitors (catalyst, catalyst heating, EVAP, oxygen sensor, EGR) should show Ready. If they show Not Ready, the system has not yet finished its self-diagnostics after a code reset or repair; a drive cycle of 50-100 km in mixed driving is required.
  7. Check the AdBlue level in the tank (at least 10% is needed for the SCR to work). A low level will prevent the engine from starting after several warnings. Top up with genuine ISO 22241 urea solution.
  8. Inspect the engine bay: the condition of the intake hoses (cracks let in unmetered air and cause lean-mixture codes), the sensor connectors (corroded contacts), and any coolant or oil leaks onto the ECU.
  9. If your scanner supports component activation (Actuator Test), check the operation of the EGR valve, radiator fan and glow plugs. The response should be immediate.
  10. After fixing the cause of the fault, clear the codes (Clear Codes) and repeat the test. If the Check Engine light does not come on after 10-15 minutes of driving, the problem is resolved.

Common mistakes and warnings

  • A steady Check Engine light indicates a non-urgent fault. A flashing light indicates a critical problem (misfires, catalyst overheating) and requires you to stop driving immediately. For advanced diagnostics (injector adaptation, DPF regeneration, sensor calibration), you need a Mercedes-specific scan tool with access to all 47 control units. A basic OBD-II scanner reads only engine and transmission codes. If you get SCR or DPF codes and lack experience, take the car to a repair shop, because an incorrect regeneration can damage the turbocharger or the catalyst.