Problem Starting Car: Advanced Diagnosis

Problem Starting Car
Problem Starting Car

Problem Starting Car: Mercedes-Benz CLA 45 ; Full Diagnostic Case Study

When a Mercedes-AMG CLA 45 (W118) refuses to start, the cause can range from a simple sensor fault to a major control unit failure. In this real-world case, we’ll explore how a “Problem Starting Car” complaint was diagnosed and resolved in a CLA 45 equipped with the M139 engine one of Mercedes-AMG’s most advanced power units.

Through this case study, you’ll see how systematic diagnostics and ECU analysis led to identifying a defective Engine Control Unit (ME) as the root cause.

Case Study Overview

Vehicle: Mercedes-Benz CLA 45 (W118)
Engine: M139 2.0 L Turbocharged AMG
Complaint:

  • Engine does not start.
  • Oil level indicator appears on the instrument cluster.

This issue rendered the vehicle completely inoperative and required an advanced diagnostic approach using Mercedes STAR/XENTRY tools.

Diagnostic Process: Step-by-Step Investigation

A problem starting car scenario in a modern AMG engine requires methodical electrical and pressure testing before replacing any parts.

Below is the complete diagnostic sequence performed by the workshop.

1. Short Test : Fault Code Retrieval

A complete short test was conducted via the Engine Control Unit (ME) to identify any stored or active fault codes.

Finding:
Fault codes indicated internal malfunction within the ECU (ME-SFI 2.0), suggesting potential communication or processing errors.

Problem Starting Car
Problem Starting Car
Problem Starting Car
Problem Starting Car
Problem Starting Car
Problem Starting Car

2. Fuel Pump Actual Value Check

The low-pressure and high-pressure fuel pumps were tested to confirm delivery capability.

ParameterExpected RangeObserved ValueResult
Low-pressure pump4.0 – 6.0 barOKOK
High-pressure pump40 – 120 bar during crankingFluctuating / unstableIrregular pressure

The high-pressure control values were inconsistent, indicating an electronic control or signal issue rather than mechanical pump failure.

Problem Starting Car
Problem Starting Car

3. Voltage Check : Rail Pressure Sensor (B4/25)

The rail pressure sensor, located on the fuel rail, was tested for correct voltage output.

Test PointExpected VoltageMeasured VoltageObservation
Signal Line0.5 – 4.5 VIrregular / nonlinearAbnormal readings

This suggested that the sensor data reaching the ECU was corrupted or not being interpreted correctly.

Problem Starting Car
Problem Starting Car

4. Continuity Test : ECU to Rail Pressure Sensor

To rule out wiring issues, continuity between the ECU and sensor connector (B4/25) was verified.

Result:

  • Partial continuity loss detected.
  • Electrical resistance higher than specification.
  • Signal interference confirmed on one of the sensor lines.

This proved the issue was not mechanical, but electronic originating from the ECU’s internal circuits.

Workshop Findings

After correlating all results, the workshop established the following key points:

Diagnostic AreaFindingConclusion
ECU fault codeInternal malfunctionECU processing failure
High-pressure controlUnstable readingsFaulty ECU signal output
Sensor voltageIncorrect voltage responseECU reference line affected
Continuity checkInterrupted link ECU ↔ B4/25Internal ECU circuit damage

The Engine Control Unit (ME) was confirmed defective.

Repair & Rectification

Step 1: Replace Engine Control Unit (ME)

A new, genuine Mercedes-Benz Engine Control Unit was installed.
The replacement included:

  • Coding and programming via XENTRY Diagnostic.
  • Initialization of high-pressure fuel control adaptation.
  • Verification of sensor communication integrity.

Step 2: Verification Tests

After installation, all actual values were rechecked.

ParameterBeforeAfter
High-pressure controlUnstableStable and within range
Rail pressure voltageErraticLinear 0.5 – 4.5 V
ECU communicationIntermittentContinuous
Engine start testFailedSuccessful

The vehicle started instantly, idled smoothly, and no fault codes reappeared.

Problem Starting Car
Problem Starting Car

Key Takeaways

InsightExplanation
Thorough diagnostics save time and costSkipping tests can lead to unnecessary part replacements.
ECU is the brain of the engineControls ignition, fuel pressure, and communication with all sensors.
High-pressure fuel systems require precisionEven slight signal corruption prevents proper start.
Always verify wiring and continuityElectrical faults can mimic sensor failures.
Use genuine ECUs onlyAftermarket control units may not meet AMG engine tolerances.

Why the Engine Control Unit Failed

The ECU (ME-SFI 2.0) in the M139 engine operates under high thermal and electrical loads.
Potential contributing factors:

  • Heat exposure near turbocharger assembly.
  • Electrical surges from auxiliary systems.
  • Internal solder joint failure or water ingress.

Because the ECU processes rail pressure and injector timing, even a minor circuit defect can disable the entire start sequence.

Lessons for Technicians

Best PracticeBenefit
Perform short tests before physical inspectionSpeeds up fault isolation
Measure both actual and reference voltagesConfirms sensor logic errors
Use STAR/XENTRY for ECU adaptationPrevents future calibration issues
Document pre- and post-repair valuesBuilds traceability and customer trust

Case Summary

StepActionOutcome
Diagnostic scanFound ECU internal malfunctionConfirmed electronic fault
Voltage & continuity testDetected abnormal valuesVerified communication loss
ECU replacementInstalled & coded new ME unitEngine started successfully

Result: Starting problem resolved.
Root Cause: Defective Engine Control Unit (ME).

If you’re facing similar starting problems, explore our full troubleshooting hub:
Mercedes No Start Issues: Causes, Fixes & Case Studies : a complete guide covering electrical, fuel, and ECU-related no-start scenarios across multiple Mercedes models.

FAQ: Problem Starting Car

1. Why does my Mercedes crank but not start?
It can result from low fuel pressure, sensor faults, or ECU malfunction preventing injector activation.

2. Can an ECU failure cause no-start without warning lights?
Yes, internal ECU defects can block communication while leaving the dashboard functional.

3. How to confirm if my ECU is faulty?
Check for missing reference voltages, abnormal actual values, and loss of communication with sensors via XENTRY.

4. Is ECU replacement plug-and-play?
No. It requires VIN coding, adaptation, and immobilizer synchronization by qualified technicians.

Author Bio

Written by Mercedes Expert
With years of hands-on experience diagnosing and repairing Mercedes-Benz systems, he brings technical depth and practical case studies to help car owners, technicians, and enthusiasts troubleshoot complex automotive issues. His work focuses on clear repair guides, OEM-level procedures, and knowledge-sharing to empower both professionals and drivers.
Last update: October 2025