What it means
In cooling mode the shortage is on the water side instead, and Vaillant's table says to check that any thermostatic valves in the heating circuit are suitable for cooling.
What to check, in order
- Check the outdoor unit is clear
Look for leaves, snow or anything else blocking the grilles. Switch off at the breaker before clearing anything. Vaillant's operating instructions say to keep the air inlet and outlet grilles clear.
- Reset once
Press the reset button on the control module. If the fault keeps coming back, Vaillant says to contact Customer Service.
When to call your installer
If the unit is clear and the fault returns, call your installer. Vaillant's table has them check the fan unit for dirt, the electronic expansion valve, the compressor inlet sensor and the refrigerant charge. Refrigerant work needs an F-gas certified engineer.
Quick answers
What does the Vaillant aroTHERM F733 error code mean?
The refrigerant in the outdoor unit is getting too cold. Vaillant gives two main causes: not enough air passing through the outdoor unit's heat exchanger, or too little refrigerant. In cooling mode the shortage is on the water side instead, and Vaillant's table says to check that any thermostatic valves in the heating circuit are suitable for cooling.
Can I fix Vaillant aroTHERM F733 myself?
Easy (about 15 minutes). Start with: check the outdoor unit is clear; reset once. If the unit is clear and the fault returns, call your installer. Vaillant's table has them check the fan unit for dirt, the electronic expansion valve, the compressor inlet sensor and the refrigerant charge. Refrigerant work needs an F-gas certified engineer.
Is Vaillant aroTHERM F733 worth repairing?
Worth fixing. A blocked outdoor unit is an easy fix. If the refrigerant level is low, an F-gas engineer needs to find out why.
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
- Vaillant aroTHERM plus VWL 35/6 to 125/6 A: operating, installation and maintenance instructions, Nordic edition (PDF, 0020297933_03)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.