P1
Vaillant
Indoor Voltage Protection / Supply Voltage Out of Range
P1 code, unit may start then stop, unstable operation
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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
Low or high mains voltage, Loose neutral or live connection, Undersized wiring causing voltage drop, Shared circuit with heavy loads
How to Fix / Troubleshooting
Safety: Electrical measurements should be performed by a qualified electrician.
- Measure supply voltage: Check the voltage at the indoor unit terminals while the unit is running. Compare to the Vaillant nameplate range (typically 220–240 V).
- Inspect connections: Tighten all terminal screws at the indoor unit, outdoor unit, and isolator. Look for signs of overheating.
- Dedicated circuit: Ensure the air conditioner is on a dedicated circuit not shared with high‑load appliances.
- Correct wiring size: Verify that the cable size meets Vaillant installation requirements for the unit’s current draw and cable length.
- If voltage is unstable: Contact your power provider or an electrician to correct supply issues. Persistent P1 with correct voltage may indicate a faulty indoor PCB.
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Repair Difficulty
Hard
4/5
Required Part
Indoor Power Supply Circuit (on PCB)
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