What F73 means
Sealed Vaillant ecoTEC boilers use an electronic water pressure sensor rather than relying only on a mechanical gauge. The sensor sends the board a signal that rises and falls with the pressure. F73 means that signal has fallen below the range the board expects.
Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) list F73 as "Fault: Water press. sensor (signal too weak)", with the cause that the sensor "reports that the water pressure is too low". The LPG version (document 0020239015) describes it as the signal being "in the wrong range (too low)". Vaillant's tables also list an interruption or short circuit in the sensor or its supply line, or a defective sensor, as causes. So F73 has two quite different explanations, and the first job is to work out which one you have.
The boiler relies on this reading to decide whether it is safe to fire, which is why a weak signal stops it even when the radiators are full of water.
Which Vaillant boilers use F73
- ecoTEC plus VU and VUW ../5-5 system and combi models (combi 825 to 838), document 0020244997.
- ecoTEC pro 24 and 28 combis, document 0020244995.
- ecoTEC plus VU 246/6-5 OVZ LPG boiler, document 0020239015.
F73 does not appear in the 2021 fault table for the ecoTEC plus ../6-5 OVZ open-vent regular boilers (document 0020238195). Open-vent systems are fed from a tank rather than a filling loop, so if your open-vent boiler shows something that looks like F73, check the exact code against its own manual.
F73 compared with other Vaillant pressure codes
Vaillant uses several codes around water pressure, and they mean different things:
| Code | Vaillant's description | What it points to |
|---|---|---|
| F22 | Low water pressure | The pressure has genuinely dropped. On the ecoTEC pro and ../5-5, the boiler switches off below 0.5 bar. |
| F73 | Water pressure sensor signal too weak | Pressure very low, or a sensor, cable or plug fault |
| F74 | Water pressure sensor signal too strong | Pressure too high, or a sensor fault in the other direction |
| F75 | Pump or water shortage | No pressure jump detected when the pump starts |
If you have seen F22 and F73 in the same week, low pressure is the more likely story. If the pressure looks normal and you still get F73, the sensor circuit becomes the main suspect.
Safe checks you can do
- Read the pressure with the heating off and the system cold. Vaillant's ecoTEC pro and ecoTEC plus ../5-5 operating instructions give 1.0 to 2.0 bar as the cold range and say to top up below 0.8 bar.
- If it is low, top up slowly through the filling loop as your installer showed you, close the valves fully and recheck. Our Vaillant boiler pressure guide walks through this.
- Press reset once as the manual describes and see whether the boiler restarts.
- Look under the boiler and along visible pipework for drips, damp or staining, without removing any covers. If you have to top up often, there is water going somewhere.
- If the display shows a pressure figure that jumps around, reads zero when you know the system has water in it, or does not change while you top up, the sensor itself is suspect. Stop topping up and call an engineer, so you do not overfill the system.
- If F73 returns after two or three resets with the pressure correct, leave the boiler off.
When you top up, watch the reading as it rises. A healthy sensor shows the pressure climbing smoothly. A reading that sticks or leaps is a useful detail to pass on to the engineer.
What the engineer will check
Vaillant's measures for F73 in the ecoTEC plus instructions are specific: check the water pressure, the earth connection, the cable and plug, and the water pressure sensor itself for a short circuit to ground. In practice the engineer will confirm the real system pressure, then work along the sensor circuit until they find the break or short. If the wiring is sound, the sensor is replaced. Afterwards the engineer should bring the system back to the correct pressure and confirm the display reading rises and falls as it should.
If the pressure was genuinely low, they should also look for why. A leak, a pressure relief valve that is letting by or an expansion vessel that has lost its charge can all cause repeated pressure loss. Our guide on a boiler losing pressure covers those causes.
Never remove the casing or unplug the pressure sensor. It sits on the water circuit inside the boiler, and disturbing it can cause a leak onto live electrics.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Water pressure sensor replacement | Typically £100 to £220 |
| Plug, earth connection or cable repair | Typically £100 to £180 |
| Internal leak or pipe joint repair (if pressure keeps dropping) | Typically £100 to £300 |
| Expansion vessel recharge (if that is behind the pressure loss) | Typically £80 to £150 |
What to tell the engineer
Say how often you have been topping up, what the pressure reads cold and hot, whether F22 or F75 have appeared too, and whether the display reading behaves oddly. Those four details usually separate a real pressure problem from a sensor fault before the casing comes off. A photo of the display taken before the fault appeared is useful too. If the boiler is under guarantee, contact Vaillant or your installer first. Other ecoTEC codes are on our Vaillant fault codes page.