- Boilers
- Gas boiler
F73
Saunier Duval F.73 on a gas boiler: pressure sensor too low
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Saunier Duval F.73 means the water pressure sensor signal is below its range: the sensor or its cable has failed. Read the pressure on the display; if it is low, top up slowly to 1 to 1.5 bar. If F.73 stays, call the official service centre.
Safety first
Stop and check this first
- With F.73 the pressure on the display may not be real: do not top up blindly. If the figure reaches 2.5 bar or more and the indicator flashes, Saunier Duval says to call the official service centre.
- If you smell gas: Saunier Duval says to keep away from the area, open doors and windows, light no flames and do not smoke, do not touch switches, sockets, doorbells or phones, close the meter and appliance gas valves, warn neighbours without the bell, leave the building and call the gas company from a phone outside.
Safe checks you can perform
- Code on the display and other codes
- Pressure on the display, without blind topping up
- One reset with the fault key
- Pressure sensor and cable checked by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Sensor supply and signal
- Do not attempt this professional task: Cable continuity and insulation
- Do not attempt this professional task: Real pressure after the repair
- Do not attempt this professional task: Fault memory
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.
Sensor supply and signal
What they doThe technician measures with a multimeter the supply voltage and the signal at the water pressure sensor connector.
They must show youthe measured values against the manufacturer's specification.
In writingthe values and the conclusion.
Cable continuity and insulation
What they doChecks the sensor power supply cable between board and sensor for breaks or earth faults.
They must show youwhere the break or leak was.
In writingwhat was repaired and, if the sensor is replaced, the reference and why.
Real pressure after the repair
What they doAfter the repair, checks that the displayed pressure matches the system filling pressure and leaves the circuit between 1 and 1.5 bar.
They must show youthe pressure on the display with the boiler running without F.73.
In writingthe final pressure.
Fault memory
What they doThe technician reads the last 10 codes stored in the fault memory.
They must show youthe list of codes.
In writingthe codes found before they are cleared.
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.
Code on the display and other codes
- Who:
- you.
- How:
- take a photo of the display. If several faults are present, Saunier Duval shows them alternately every two seconds: wait a few seconds and note them all.
- Result that matters:
- whether F.73 appears alone or alternates with other codes.
- If it appears alone:
- the fault is in what F.73 monitors; go on to the next check.
- If it alternates with others:
- note them all; the technician needs them for the diagnosis.
- Evidence to keep:
- photos of the display with date and time.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Pressure on the display, without blind topping up
- Who:
- you.
- How:
- with no heat demand, read the pressure on the display. If it shows less than 1 bar, top up slowly with the filling valve to 1 to 1.5 bar, as Saunier Duval describes, and close the valve.
- Result that matters:
- whether the figure rises as you add water and F.73 clears.
- If it rises and F.73 clears:
- note the date and watch the pressure for a few days.
- If the figure does not change as you add water:
- close the valve and stop; the reading may come from the faulty sensor.
- Evidence to keep:
- photos of the display before and after.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
One reset with the fault key
- Who:
- you.
- How:
- press the fault clearing (reset) key and wait five seconds, as Saunier Duval describes. Do it once only.
- Result that matters:
- whether F.73 comes back.
- If it clears and stays away:
- note the date and tell the technician at the next service.
- If it persists or returns:
- do not keep resetting; call the official service centre.
- Evidence to keep:
- a photo of the display with the code and the time.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Pressure sensor and cable checked by the technician
- Who:
- official service centre technician or qualified installer.
- How:
- measures the supply and signal at the pressure sensor connector and the cable continuity to the board.
- Result that matters:
- whether the cable or the sensor has failed.
- If the cable was broken or earthed:
- it is repaired and the sensor does not need replacing.
- If the supply arrives correctly and the signal stays out of range:
- the sensor is replaced.
- Evidence to keep:
- a report with the measured values.
Saunier Duval reserves inspection, maintenance, repair and gas adjustments for a qualified installer: do not open the casing.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
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
- Saunier Duval
- 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 ThemaFast Condens table, F.73 is water pressure sensor signal outside the correct range (too low). Saunier Duval gives as causes an open or short circuit in the water pressure sensor, an open circuit or earth fault in the sensor power supply cable, or a faulty sensor. All of them are electrical: this is not the lack of water shutdown of F.22. Without a valid signal the boiler cannot monitor pressure. The code does not separate the sensor from its cable.
Warnings and stop conditions
- Mediumsource verified
With F.73 the pressure on the display may not be real: do not top up blindly. If the figure reaches 2.5 bar or more and the indicator flashes, Saunier Duval says to call the official service centre.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
- Criticalsource verified
If you smell gas: Saunier Duval says to keep away from the area, open doors and windows, light no flames and do not smoke, do not touch switches, sockets, doorbells or phones, close the meter and appliance gas valves, warn neighbours without the bell, leave the building and call the gas company from a phone outside.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Symptoms and causes
Probable causes · in order
- Sensor open or short circuitedPossible
The signal is cut or shorted in the sensor itself or its connection. The technician separates it from the cable by measuring at the sensor connector.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Sensor supply cable broken or earthedPossible
The sensor gets no power or leaks it to earth. It is told apart by measuring the supply cable between board and sensor.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Faulty pressure sensorPossible
If the supply arrives correctly and the signal stays out of range, the fault is inside the sensor. It is confirmed after ruling out the cable.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
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
Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
Saunier Duval · 2016 · manual
- 2
Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Saunier Duval · 2016 · manual
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