What it means
Vaillant's VWZ AI fault table calls this 'Building circuit: Pump fault'. The building circuit is the water circuit that carries heat into your home.
What to check, in order
- Switch off for 30 seconds
Vaillant's first remedy is to switch the heat pump off for at least 30 seconds so it has no power. Use the heat pump's circuit breaker, then switch it back on.
- Check the water pressure
Running dry is one of the causes Vaillant lists. If the pressure on the display is low, see F.022 and ask your installer to top up.
- Call your installer if it returns
If the fault comes back after the power reset, the pump and the heating circuit need checking.
When to call your installer
Vaillant's table has the engineer check the pump's plug contact on the circuit board, test the pump, purge air from the heating circuit and check that the dirt filter is not blocked. Vaillant's fault code page gives the same causes: dry running, a blockage, or over or under voltage.
Quick answers
What does the Vaillant aroTHERM F788 error code mean?
The heating pump's own electronics have found a problem, such as running dry, a blockage or a voltage fault, and have switched the pump off and locked it. Vaillant's VWZ AI fault table calls this 'Building circuit: Pump fault'. The building circuit is the water circuit that carries heat into your home.
Can I fix Vaillant aroTHERM F788 myself?
Easy (about 5 minutes). Start with: switch off for 30 seconds; check the water pressure; call your installer if it returns. Vaillant's table has the engineer check the pump's plug contact on the circuit board, test the pump, purge air from the heating circuit and check that the dirt filter is not blocked. Vaillant's fault code page gives the same causes: dry running, a blockage, or over or under voltage.
Is Vaillant aroTHERM F788 worth repairing?
Worth fixing. A power reset sometimes clears it. If it comes back, an engineer needs to check for air, a blocked filter or a failing pump.
Checked against
- Vaillant heat pump appliance interface VWZ AI (for aroTHERM plus VWL ..5/6 A): operating and installation instructions (PDF, 0020291573_01)
- Vaillant: heat pump fault codes
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.