Vaillant and Glow-worm combi boilers frequently display F75 or F73 fault codes. These faults are often misattributed to pump failure. However, in cases where the circulation pump is confirmed to be operational, the primary cause is almost always a malfunctioning pressure sensor.
Both Vaillant and Glow-worm use a common pressure sensor design across a wide range of boiler models.
When a heat demand is initiated, the circulation pump activates and begins moving water through the primary circuit.
The pressure sensor detects the resulting change in pressure and water movement within the system.
This signal is required before the boiler will permit full burner ignition.
If the sensor fails to detect this change accurately, the boiler control board interprets it as a lack of circulation and prevents ignition as a protective measure, resulting in fault code F75 or F73.
The pressure sensor contains an internal sensing channel that monitors changes in system pressure.
Over time, its narrow internal pathway can become partially or fully obstructed due to:
When this occurs, the sensor becomes unable to register water movement correctly during pump operation, leading to repeated ignition failure and system lockout.
In approximately 90% of cases, the pressure sensor does not require replacement.
The correct procedure is to:
1 Remove the sensor
2 Clean and clear its internal sensing channel
3 Refit and test its operation under live conditions
This method restores normal function in the majority of cases and avoids unnecessary replacement of high-cost components such as the circulation pump.
Where a Vaillant or Glow-worm combi boiler displays F75 or F73 fault codes and IF the circulation pump is functioning correctly, the pressure sensor should be considered the primary fault.
Accurate diagnosis and targeted servicing of this component typically resolves the issue without the need for major component replacement.
20th August, 2026
18th March, 2026
