- Boilers
- Gas boiler
F71
Saunier Duval F.71 on a gas boiler: flow sensor reading stuck
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Saunier Duval F.71 means the flow temperature sensor reports a constant value. It may be badly placed on its pipe or faulty. Take a photo of the code, press reset once and wait five seconds. If it comes back, call the official service centre.
Safety first
Stop and check this first
- Do not open the boiler to check connectors or cables: Saunier Duval tells you never to do maintenance or repair work yourself and to leave it to a qualified installer. Inside there is mains voltage and gas parts.
- 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
- Recent work on the boiler
- One reset with the fault key
- Sensor position and measurement by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Flow sensor resistance
- Do not attempt this professional task: Flow reading while running
- Do not attempt this professional task: Sensor position on the flow pipe
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.
Flow sensor resistance
What they doMeasures the flow temperature sensor resistance with a multimeter.
They must show youthe measured value against the manufacturer's specification.
In writingthe value and, if the sensor is replaced, its reference and why.
Flow reading while running
What they doAfter the repair, checks in diagnostic code d.40 that the flow temperature changes with the boiler running, and reads the fault memory.
They must show youthe d.40 reading as the boiler heats.
In writingthe test run and the codes found.
Sensor position on the flow pipe
What they doThe technician checks, with the boiler powered off, that the flow temperature sensor is fitted on the flow pipe and sits properly.
They must show youa photo of the sensor in place.
In writingwhether it was badly placed and how it was corrected.
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.71 appears alone or alternates with other codes.
- If it appears alone:
- the fault is in the flow sensor; go on to the next check.
- If it alternates with others:
- note them all; if F.83 or F.85 appear, which also point to badly placed sensors, tell the technician.
- Evidence to keep:
- photos of the display with date and time.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Recent work on the boiler
- Who:
- you.
- How:
- think back to whether anyone opened the boiler recently for a service or a repair.
- Result that matters:
- whether F.71 started right after it.
- If it started after some work:
- tell the technician; a sensor badly placed on its pipe is the first cause Saunier Duval gives.
- If there was no work:
- the sensor may have failed or moved on its own; the technician checks it.
- Evidence to keep:
- date and report of the last service.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · 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.71 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
Sensor position and measurement by the technician
- Who:
- official service centre technician or qualified installer.
- How:
- with the boiler powered off, checks that the flow sensor sits properly on the flow pipe and measures its resistance; then watches the flow reading with the boiler running.
- Result that matters:
- whether the sensor is badly placed or faulty.
- If it was badly placed:
- it is refitted and the reading is checked to change as the boiler heats.
- If it is well placed and does not give a correct value:
- the sensor is replaced.
- Evidence to keep:
- a report with what was found, measured and replaced.
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
- Error F71 Saunier Duval
- F.71
- Saunier Duval F.71
- ThemaFast Condens F.71
What it means
In the ThemaFast Condens table, F.71 is heating flow temperature sensor fault: the flow sensor reports a constant value. Saunier Duval gives two causes: the sensor badly placed on the flow pipe or a faulty flow temperature sensor. It is not an open circuit sensor (F.00) nor a short circuit (F.10): the sensor gives a reading, but it does not change. Other codes in the table, such as F.83 and F.85, also point to sensors badly placed on the pipework.
Warnings and stop conditions
- Mediumsource verified
Do not open the boiler to check connectors or cables: Saunier Duval tells you never to do maintenance or repair work yourself and to leave it to a qualified installer. Inside there is mains voltage and gas parts.
Evidence: 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 badly placed on the flow pipePossible
Saunier Duval lists it first. It fits if F.71 started after work with the boiler open. The technician checks that the sensor sits properly on its pipe.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Faulty flow temperature sensorPossible
If the sensor is well placed, it may be faulty. The technician measures it against the manufacturer's specification before replacing it.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
Virtual technician
Does F71 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
Was this entry useful?