What it means
Used by the Baxi 200, 400, 600 and 800 Combi 2, the Assure 500 Combi 2 and the 600, 800 and Assure 500 System 2. The display shows H.01 alternating with 18.
What to check, in order
- Check the pressure gauge
Baxi says the pressure should be between 1 and 2 bar.
- Top up if it's low
Baxi says that when the correct pressure is restored the boiler will reset automatically.
When to call a Gas Safe engineer
If it doesn't clear, Baxi says an installer is needed for most checks. Its user guide lists the installation pressure, air in the system (a venting cycle) and the pump.
Quick answers
What does the Baxi H.01.18 fault code mean?
Water isn't moving through the boiler properly. Baxi treats it as a temporary fault that clears once the cause is fixed. Used by the Baxi 200, 400, 600 and 800 Combi 2, the Assure 500 Combi 2 and the 600, 800 and Assure 500 System 2. The display shows H.01 alternating with 18.
Can I fix Baxi H.01.18 myself?
Easy (about 15 minutes). Start with: check the pressure gauge; top up if it's low. If it doesn't clear, Baxi says an installer is needed for most checks. Its user guide lists the installation pressure, air in the system (a venting cycle) and the pump.
Is Baxi H.01.18 worth repairing?
Usually free. If it was low pressure, topping up costs nothing. If not, an engineer needs to check the pump and the system.
Checked against
- Baxi Professional: boiler error codes
- Baxi 400 Combi 2 user guide (PDF)
- Baxi: how to repressurise your boiler
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.