Vaillant aroTHERM fault F.788

What it means

The electronics system of the high-efficiency pump has detected a fault (e.g. dry running, blockage, overvoltage, undervoltage) and has switched off and locked the pump.

Likely causes

1. Dry running Blockage Overvoltage Undervoltage Pump motor failure, pump control failure, electrical connection fault Pump motor failure, electrical connection fault, mechanical blockage Dry running, blockage, overvoltage, undervoltage
2. Dry running
3. Blockage
4. Overvoltage
5. Extended pump inactivity leading to seizing/sticking
6. Prolonged pump inactivity leading to seizing/sticking
7. Pump inactivity leading to sticking/seizure
8. Building power supply switched off
9. Domestic hot water or heating set to off, or temperature set too low
10. Air in the heating installation
11. System control failure
12. Power cut
13. Defective compressor
14. Negative outdoor temperatures Heat pump fault Power cut Defective compressor
15. Extended pump inactivity (23 hours) risking seizure
16. Pump inactivity for extended period Pump stiction after extended idle period Pump inactivity for extended period

Where to start

1. Check the quality of the plug contact on the PCB Check that the pump functions correctly Check that the dirt filter that is in place is permeable Verify pump motor operation, check electrical connections, test pump control signal Verify pump power supply, check motor operation, remove mechanical blockages Switch heat pump off for at least 30 seconds with no current. Check plug contact quality on PCB. Check pump functions correctly. Purge building circuit. Check dirt filter permeability.
2. Switch heat pump off for at least 30 seconds with no current. Check plug contact quality on PCB. Check pump functions correctly. Purge building circuit. Check dirt filter permeability.
3. Switch the heat pump off for at least 30 seconds (no current)