- Boilers
- Gas boiler
F73
Vaillant F.73 on a gas boiler: water pressure sensor signal too low
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Vaillant F.73 means the water pressure sensor gives a signal that is too low. Read the pressure on the display with the system cold: if it is under 0.8 bar, top up as Vaillant describes; if it is correct, call a professional.
Safety first
Stop and check this first
- Always top up with water suitable for heating and close the filling valve afterwards: Vaillant warns that unsuitable water damages seals and components.
- If you smell gas: do not operate switches, sockets, doorbells or phones, no flames or smoking, open doors and windows, close the meter valve and the appliance gas valve if you can, warn neighbours without using the bell, leave the building and call the gas company from outside. That is what Vaillant asks.
Safe checks you can perform
- Fill pressure on the display
- Top up as Vaillant describes
- Visible leaks
- Sensor check by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Real pressure against sensor reading
- Do not attempt this professional task: Earth, cable and connector check
- Do not attempt this professional task: Pressure loss test
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Real pressure against sensor reading
What they doThe technician measures circuit pressure with a gauge and compares it with what the boiler shows.
They must show youthe gauge pressure against the display.
In writingthe difference and any part replaced.
Earth, cable and connector check
What they doThe technician looks for a short to earth in the sensor's cable and connector.
They must show youthe point checked.
In writingthe repair done.
Pressure loss test
What they doIf real pressure was low, the technician checks whether the circuit loses water and the expansion vessel pressure.
They must show youthe pressure at the start and end of the test.
In writingwhere water was lost and what was repaired.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Fill pressure on the display
- Who:
- you.
- How:
- with the system cold, press twice the key the manual names for showing pressure and read the value. Vaillant asks for 1.0 to 2.0 bar.
- Result that matters:
- whether pressure is within that range.
- If it is below 0.8 bar:
- top up with the filling valve as Vaillant describes and bleed the radiators; tell the technician.
- If it is in range:
- lack of water does not explain F.73.
- Evidence to keep:
- a photo of the display with the pressure.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Top up as Vaillant describes
- Who:
- you.
- How:
- ask the professional where the filling valve is, open the radiator valves, open the filling valve slowly to the required pressure, bleed the radiators, check the pressure again and close the valve.
- Result that matters:
- whether F.73 clears and pressure holds.
- If it clears and holds:
- it was lack of water; note the date in case it repeats.
- If pressure drops again or F.73 stays with correct pressure:
- call an authorised professional.
- Evidence to keep:
- photos of the display before and after with the date.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Visible leaks
- Who:
- you.
- How:
- look under the boiler and at radiators and valves for water or drips, without taking anything apart.
- Result that matters:
- whether there is a visible leak.
- If you see water:
- tell the technician; a leak would explain the drop.
- If everything is dry and pressure is correct:
- it points to the sensor or its wiring.
- Evidence to keep:
- photos of stains or drips.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Sensor check by the technician
- Who:
- technician authorised by Vaillant.
- How:
- checks real pressure, the earth, the cable, the connector and the pressure sensor.
- Result that matters:
- whether the sensor tells the truth.
- If the sensor reads lower than real pressure:
- it is replaced or the connection repaired.
- If real pressure is low:
- the water loss is traced.
- Evidence to keep:
- a report with the measured pressure and what was replaced.
Vaillant reserves inspection, maintenance, repair and gas settings for an authorised professional: the user does not open the boiler.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Vaillant
- Product type
- Gas boiler
Known code variants
- ecoTEC F.73
- Error F73 Saunier Duval
- Error F73 Vaillant
- F.73
- Saunier Duval F.73
- ThemaFast Condens F.73
- Vaillant F.73
What it means
In the ecoTEC plus table, F.73 is a water pressure sensor fault, signal too low: the sensor reports water pressure that is too low. Vaillant says to check water pressure, the earth connection, the cable, the connector and the water pressure sensor (short to GND). So the water may really be low or the sensor may have an electrical fault. Correct cold pressure is 1.0 to 2.0 bar; the code does not separate one from the other.
Warnings and stop conditions
- Mediumsource verified
Always top up with water suitable for heating and close the filling valve afterwards: Vaillant warns that unsuitable water damages seals and components.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
- Criticalsource verified
If you smell gas: do not operate switches, sockets, doorbells or phones, no flames or smoking, open doors and windows, close the meter valve and the appliance gas valve if you can, warn neighbours without using the bell, leave the building and call the gas company from outside. That is what Vaillant asks.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Symptoms and causes
Probable causes · in order
- Water pressure really lowPossible
If the display and the technician confirm low pressure and F.73 clears after topping up, lack of water was the cause.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01 · Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
- Pressure sensor shorted to earthPossible
If real pressure is correct and F.73 stays, Vaillant points to the sensor or a short to GND.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Cable, connector or earthPossible
If the sensor tests fine outside the boiler, the fault is in the cable, connector or earth connection.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Virtual technician
Does F73 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Vaillant · 2017 · manual
- 2
Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Vaillant · 2017 · manual
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