What does Siemens G120 A03520 Fault - Causes & Fix mean?

A03520 is a temperature sensor fault in the G120 Control Unit. Most often caused by poor airflow or a failed CU sensor circuit.

How do I fix Siemens G120 A03520 Fault - Causes & Fix?

Follow the step-by-step diagnosis below. The most likely fix is identified by checking power, sensor resistance, and control-board signals in order — using a multimeter and the part numbers documented in the parts reference table.

Can I keep running the equipment with G120 active?

Generally no. Faults on Vfd units are emitted when a sensor, safety, or control circuit is outside its operating envelope. Running through an active fault risks compressor, heater, or control-board damage. Diagnose and clear the fault first.

What part is most commonly replaced for Siemens G120 A03520 Fault - Causes & Fix?

The most frequently replaced parts for Siemens G120 A03520 Fault - Causes & Fix are listed in the Parts Reference section of this guide with OEM part numbers and typical price ranges. Check the table for direct replacements.

Is Siemens G120 A03520 Fault - Causes & Fix a DIY repair or do I need a technician?

Sensor swaps, fan motor replacements, and condenser cleaning are typical DIY-level repairs. Anything involving refrigerant, gas, or high-voltage (>240V) work requires a licensed technician for both safety and code-compliance reasons.

Siemens G120 A03520 Fault - Causes & Fix

Siemens G120 A03520 Fault — What It Means

A03520 on a Siemens SINAMICS G120 is a temperature sensor fault in the Control Unit (CU), not a motor overtemperature alarm. The ‘A’ prefix means this is an alarm rather than a hard trip fault, so the drive may continue to run while the alarm is active. The code indicates the CU’s internal temperature sensing circuit has failed, disconnected, or detected an out-of-range condition. In most cases the drive logs the alarm and continues operation, but the condition should be addressed to prevent future issues or progression to a fault that stops the drive.

Jump to Fix

Common Causes

Faulty or disconnected CU temperature sensor circuit The Control Unit’s internal temperature sensor or its wiring has failed or become disconnected, triggering the alarm even when ambient temperature is normal.
Blocked air intake or poor ventilation around the Control Unit Dust, debris, or obstructed cooling vents prevent adequate airflow over the CU, causing elevated temperatures that can trigger the sensor alarm.
Failed or stopped Control Unit cooling fan If your G120 uses a dedicated CU fan, a fan failure will reduce cooling and may cause the temperature sensor to report a fault condition.
Internal CU electronics degradation Age, heat stress, or component wear inside the Control Unit can cause the temperature sensing circuit to malfunction and report false alarms.
Firmware or configuration issue Outdated firmware or corrupted CU configuration can occasionally produce spurious temperature sensor alarms that persist across resets.
High ambient temperature in the enclosure Enclosure temperatures exceeding the Control Unit’s rated operating range will cause legitimate temperature sensor alarms even if the sensor itself is functioning correctly.

Step-by-Step Fix

Record the alarm details from the drive display or parameter buffer (check the fault/alarm history) before clearing or resetting anything, so you have a reference if the alarm recurs.
Power down completely by switching off all incoming power to the G120 and waiting at least 30 seconds, then power back up. A full power cycle clears transient faults and allows the CU to re-initialize its sensor circuits.
Inspect and clean all cooling paths around the Control Unit. Remove dust and debris from air intake slots, verify ventilation openings are not blocked, and confirm the CU fan (if fitted) is spinning and free of obstructions.
Allow the drive to cool for 15 to 30 minutes if the enclosure feels warm, then check whether the alarm clears automatically. Siemens temperature alarms often reset once the CU temperature drops below the threshold.
Check ambient enclosure temperature with a thermometer. If the enclosure exceeds the G120’s rated ambient range, improve ventilation, add external cooling, or relocate the drive to a cooler environment.
Update or reload CU firmware if the alarm persists after cooling and cleaning. Consult your G120 documentation or Siemens support for the correct firmware version and update procedure for your specific Control Unit model.
Replace the Control Unit if the A03520 alarm returns immediately after power-up or does not clear with normal airflow and cooling. Match the replacement CU to your G120 Power Module type code and make sure firmware compatibility before installation.

Parts Often Needed

Part	Notes
Siemens G120 Control Unit (CU)	Amazon \| Match the CU model and firmware version to your existing Power Module. Common CU variants include CU240E-2, CU250S-2, and others specific to your G120 configuration.
Control Unit cooling fan	Amazon \| If your G120 CU uses a dedicated fan, verify the part number from the drive nameplate or service manual before ordering a replacement.

When to Call a Pro

Call a qualified drive technician or Siemens-certified service provider if the A03520 alarm persists after you have cleaned cooling paths, verified airflow, and performed a full power cycle. Control Unit replacement requires matching the correct CU model to your Power Module and may involve firmware updates or parameter cloning, which are best handled by a technician with Siemens drive experience and access to commissioning tools. Also call a pro if you are unfamiliar with high-voltage industrial equipment, if the drive is part of a critical process, or if your facility requires documented service records for safety and compliance.