P4
Vaillant
Inverter Module Overheat Protection
P4 code, unit stops or runs intermittently, especially in high ambient temperatures
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Blocked airflow around outdoor control box, Failed inverter cooling fan (if fitted), Poor thermal contact to heat sink, High ambient temperature combined with heavy load
How to Fix / Troubleshooting
Safety: Turn off power before opening the outdoor control box.
- Check ventilation: Ensure the outdoor unit has adequate clearance and is not enclosed in a tight space.
- Inspect heat sink: Open the control box and check that the inverter heat sink is clean and free of dust buildup.
- Cooling fan (if present): Some models have a small fan cooling the PCB. Verify it runs when the compressor is operating; replace if not.
- Ambient conditions: In extreme heat, consider shading the outdoor unit while maintaining airflow.
- If P4 persists: The inverter module may be failing and should be evaluated by a Vaillant technician.
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Repair Difficulty
Hard
4/5
Required Part
Outdoor Inverter PCB Heat Sink / Cooling Fan
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