Mercedes-Benz CLS W219 rodent-damaged wiring, interior-temperature sensor and heater-control repair

Sifu White explains what happened

Prices, repair times and warranty terms mentioned in the video applied at the time. Ask the workshop about current arrangements.

Look at these rodent bite marks. The owner wants every function restored to original operation, including heater control, cooling, footwell airflow and all stepper motors. After the main work was completed, we continued checking and found a problem with the interior air-temperature sensor. A new unit is difficult to obtain, while many used units are already broken or damaged.

The original sensor inside the car had broken and gone missing. Sifu sourced another compatible sensor to replace it and provided it to the customer. The second problem was the absence of warm air. Although cooling is normally needed in Malaysia, the owner wants the heater to work when the cabin becomes too cold. One motor cannot return to its original position, and coolant flow through the pump or valve also has to be checked.

The system also has a controller near the coolant pump. When related components fail, the rear vent near the armrest may blow hot air continuously or lose temperature control. Many people simply block the passage because they cannot find the cause. This car has the opposite problem: no warm air at all, so the system must be traced again. The wiring also shows rodent damage.

Why a working refrigerant circuit can still deliver warm air

After the refrigerant circuit cools the air, doors, ducts and temperature controls still have to direct it correctly. Depending on the vehicle, temperature regulation may involve mixing warm and cool air or controlling the heater circuit. One warm side, a hot rear outlet or a change at one particular setting calls for checks of air distribution and heater control as well as the refrigerant side.

A motorised door can stick at a particular position, and a mechanical linkage can become detached. A fault limited to one setting can be missed if testing uses only the default settings. The absence of a code also does not rule out mechanical sticking. Save the temperature, zone and airflow-mode settings that trigger the problem so the technician can reproduce it and compare commanded movement with what actually happens.

Separate component failure from power and control faults

Pressing A/C is a request; the vehicle still has to meet its operating conditions. Controllers, sensors, relays and wiring work together to determine whether a component runs. A compressor that does not start may have an internal fault, or it may not be receiving the required power or command. Before replacement, the technician needs to establish where the failure occurs in the actual circuit rather than treating the visible symptom as the diagnosis.

A fault code helps direct testing, but it does not always identify a part to buy. Actual output, connectors, ground connections and circuit behaviour under load may all need verification. Added relays or bypass wires can also change the original control relationships. After addressing an electrical fault, the air-conditioning needs to be operated again to check the original complaint. A cleared code or newly installed part is not sufficient evidence of a successful repair by itself.

What to tell the workshop about your own car

You do not need to guess a part name first. A clear description of what actually happens is often more useful for reproducing the complaint than requesting a particular replacement.

• Note the affected side or row, temperature setting and airflow mode.
• Mention previous wiring changes, added relays or controller replacements.
• Include the model, year, recent repairs and what changed before or after the symptom began.