Water pressure sensor signal in the wrong range, it is too low
The F73 error code on Vaillant boilers indicates water pressure sensor signal in the wrong range, it is too low.
Vaillant F73 means: water pressure sensor signal in the wrong range, it is too low. The usual causes are (too low), interruption/short circuit of water pressure sensor, interruption/short cir or sor defective. You can safely reset the boiler once — if F73 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does F73 Mean
The F73 code is shown by the boiler’s control board when it detects water pressure sensor signal in the wrong range, it is too low. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows F73 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 8 technical sources.
| Cause |
|---|
| (too low) |
| Interruption/short circuit of water pressure sensor, interruption/short cir |
| sor defective |
Safety Warning
F73 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Check the boiler pressure gauge — normal operating pressure is 1.0-1.5 bar (15–22 psi) when cold. 2. Inspect all visible pipework, radiators, and valves for signs of leakage. 3. Check the pressure relief valve discharge pipe — if water is dripping, the PRV may be faulty or the expansion vessel has failed. 4. Test the expansion vessel — with the system cold and drained, check the Schrader valve on the vessel
air should escape at the correct pre-charge pressure (typically 0.75-1.0 bar, 11–15 psi). 5. Inspect the pressure sensor connections. 6. Check automatic air vents for signs of weeping
How to Fix It
Repressurise the system via the filling loop to 1.0-1.5 bar (15–22 psi)
repair any identified leaks
if the expansion vessel has lost its charge, re-pressurise it with a pump (or replace if the diaphragm has failed)
replace the pressure relief valve if it is passing water
replace the pressure sensor if it is giving incorrect readings. Note: frequent pressure loss indicates an underlying leak that must be found and repaired
Refer to the Vaillant service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Vaillant models that can display fault code F73:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Vaillant models | — | Possible |
Frequently Asked Questions
Repressurise the system via the filling loop to 1.0-1.5 bar (15–22 psi). According to the technical documentation, this is the first check on the list for F73.
F73 is not classified as a high-severity fault, so the boiler can be reset once. If F73 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.