Cooling System Fault Mercedes EQ: Repair Guide

Mercedes-Benz EQ: How to Fix a Cooling System Fault (Case Study & Step-by-Step Repair)

Electric vehicles like the Mercedes-Benz EQ rely heavily on a well-regulated thermal management system. So, when the yellow warning message “Cooling system fault. Operating energy input reduced. Service required.” appears on the instrument cluster, it’s more than just an annoyance it signals a potential problem affecting battery performance and safety.

In this case study, we’ll explore a real-life diagnosis and repair process for a Cooling System Fault in a Mercedes-Benz EQ, highlighting a common failure point: the battery cooling system expansion valve (component Y140/1). By the end, you’ll understand how to identify the issue, verify the cause, and resolve it step-by-step.

Vehicle Overview

Model: Mercedes-Benz EQ (applicable across various EQ series such as Mercedes EQS, EQS SUV, EQE or EQE SUV depending on platform and part number compatibility)

Complaint: Yellow instrument cluster warning: “Cooling system fault. Operating energy input reduced. Service required.”

System Affected: Battery cooling system

Fault Code: B3A0077

Component of Interest: Expansion valve Y140/1 in the refrigerant circuit

Step 1: Understanding the Cooling System Fault

The Mercedes EQ platform uses a complex cooling system not just for the motor and power electronics, but also for the high-voltage battery. Proper cooling ensures optimal performance and battery longevity. A failure in this system can lead to restricted power output or even vehicle shutdown in extreme cases.

In our case, a diagnostic scan revealed the fault code B3A0077, which points to the regulation valve in the refrigerant circuit for battery cooling. The description indicates that the valve is stuck open and cannot achieve the commanded position. This component coded Y140/1 is responsible for regulating the flow of refrigerant through the battery cooling circuit.

Step 2: Root Cause – Faulty Y140/1 Expansion Valve

Upon deeper investigation, the culprit was the expansion valve Y140/1, a critical component that modulates the refrigerant flow based on thermal demand. This valve is electrically actuated and prone to mechanical sticking over time.

The issue in our scenario stems from an outdated part version. Mercedes-Benz issued an updated part with improved reliability.

Compare the Part Number:

• – Older Version (Needs Replacement): A 297 830 54 03
• – Updated Version (Reliable): A 297 830 89 03

If the vehicle has the older version installed, you must replace Y140/1 to eliminate the Cooling System Fault. If the updated part is already present, further diagnostics may be necessary.

Step 3: Repair Preparation

Tools and Equipment Required:

• – Diagnostic tool (e.g., Mercedes-Benz XENTRY)
• – Refrigerant recovery machine
• – Coolant draining system
• – Torque wrench
• – Safety gloves and eye protection
• – New expansion valve Y140/1 (P/N: A 297 830 89 03)
• – OEM coolant

Step 4: Cooling System Fault – Component Replacement Procedure

To replace the faulty expansion valve and fix the Cooling System Fault in your Mercedes EQ, follow these steps carefully:

1. Drain the Air Conditioner

Before working on the refrigerant circuit:

• – Connect a certified recovery machine.
• – Recover all refrigerant from the A/C system in compliance with local environmental regulations.

2. Drain the Coolant

• – Safely open the coolant drain plug.
• – Collect the coolant in a clean container for reuse if not contaminated.
• – Be sure the coolant circuit is fully depressurized to avoid injury.

3. Remove the Faulty Y140/1 Expansion Valve

• – Locate the Y140/1 valve in the battery cooling loop.
• – Disconnect the electrical connector.
• – Use appropriate tools to remove the refrigerant and mounting connections.
• – Carefully remove the expansion valve.

4. Install the New Expansion Valve

• – Fit the updated A 297 830 89 03 valve.
• – Torque all fittings to OEM specifications.
• – Reconnect the electrical harness.

Step 5: Refill and Bleed the Cooling System

After replacing the expansion valve, it’s essential to restore and purge the coolant circuit properly to avoid residual air that could cause future cooling problems.

Steps to Follow:

1. Activate the Coolant Circuit Filling Mode

• – Using the diagnostic tool, enter the special coolant fill mode to allow proper circulation and venting.

2. Pour in the Coolant

• – Add Mercedes-Benz approved coolant into the reservoir.
• – Observe the fill level and ensure it stays between MIN and MAX.

3. Disable the Coolant Circuit Fill Mode

• – Exit the fill mode via the scan tool once the system has been adequately filled.

4. Start the Coolant Circuit Purge Routine

• – Launch the purge routine using your scan tool.
• – This process eliminates trapped air and ensures proper coolant flow through all components.

5. Check Coolant Level

• – Once the purge routine finishes, verify the coolant level again.
• – Top off if necessary to meet system specifications.

6. Install the Lower Engine Compartment Trim

• – Reattach all panels and trims removed for access.
• – Ensure all clips and fasteners are secured to avoid rattles.

Step 6: Confirming the Repair

Post-repair Checks:

• – Clear fault codes using XENTRY.
• – Re-scan to verify no new or pending codes exist.
• – Start the vehicle and monitor system operation for 10–15 minutes.
• – Confirm the Cooling System Fault message no longer appears.
• – Perform a short road test if needed.

Why This Cooling System Fault Is Common in EQ Models

This type of Cooling System Fault is relatively common in early models of the Mercedes-Benz EQ lineup due to the originally fitted expansion valve (A 297 830 54 03) not meeting long-term durability standards.

Mercedes-Benz issued a silent revision to address this issue, but many vehicles still on the road have the older valve. If you’re an owner or technician working on Mercedes EQ models, this case highlights the importance of checking part revisions when dealing with battery or thermal system faults.

Conclusion

The Cooling System Fault warning in a Mercedes-Benz EQ shouldn’t be ignored. It directly impacts the performance and longevity of the high-voltage battery. In our case study, we identified a faulty expansion valve Y140/1 as the root cause, verified by fault code B3A0077. Replacing the outdated part with the newer revision and properly restoring the system resolved the issue completely.

By following the outlined steps from diagnostics to component replacement and system purge you can confidently address this common cooling fault and keep your Mercedes EQ performing as it should.

What does “Cooling System Fault” mean on a Mercedes EQ?

This warning means the thermal management system for the high-voltage battery isn’t functioning correctly. It can reduce power input and affect driving performance. The issue is often linked to a stuck or malfunctioning component like the expansion valve (Y140/1).

What is the fault code B3A0077?

B3A0077 indicates the regulation valve in the refrigerant circuit for the battery cooling is stuck open and can’t reach the desired position. This usually points to a faulty expansion valve, requiring inspection or replacement.

Which part is responsible for this fault in the Mercedes EQ?

In many cases, the expansion valve Y140/1 is the cause. Vehicles with the older part number A 297 830 79 03 are more prone to this issue and should be updated to A 297 830 89 03.

Can I drive with a Cooling System Fault?

Driving with this fault is not recommended for extended periods. While the vehicle may enter a reduced power mode, prolonged operation could stress the battery system and affect long-term performance.

How is the Cooling System Fault fixed?

The repair involves:

• – Diagnosing with a scan tool.
• – Checking the part number of Y140/1.
• – Replacing it if outdated.
• – Draining and refilling the A/C and coolant circuits.
• – Running coolant purge and rechecking fault codes.