- Boilers
- Gas boiler
F72
Saunier Duval F72 on a gas boiler: flow/return NTC fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
F72 (shown as F.72) on Saunier Duval ThemaFast Condens boilers means an excessive temperature difference between the flow and return NTC sensors, caused by a poorly seated or faulty probe. Press Reset once and wait 5 seconds; if F.72 returns, contact Official Saunier Duval Service.
Safety first
Stop and check this first
- Always unplug the boiler from the 230 V mains supply before touching internal plug connections or temperature sensors: never open the front casing or handle internal wiring yourself.
- If F.72 returns after pressing Reset once and waiting five seconds, stop resetting and contact Official Saunier Duval Service; if you smell gas, close the building gas shut-off valve, ventilate without using electrical switches, and call from outside.
Safe checks you can perform
- Watch whether F.72 shows alone or alternates every two seconds
- Check system water pressure and radiator valves
- Press the Reset key once and wait 5 seconds
- Compare diagnostic values d.40 and d.41 and inspect NTC probes 11 and 12
Leave these tasks to a qualified professional
- Do not attempt this professional task: Read flow temperature d.40 and return temperature d.41 and check fault memory
- Do not attempt this professional task: Refit the flow and return NTC sensors on their pipes and check wiring plugs
- Do not attempt this professional task: Replace any faulty NTC probe with an original part and test heating and DHW
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.
Read flow temperature d.40 and return temperature d.41 and check fault memory
What they doThe technician enters the diagnostic menu (holding mode for 7 seconds with access code 96) to read flow temperature (d.40) and return temperature (d.41), checks status codes (such as S.53 or S.98), and reviews the last 10 fault codes stored in memory.
They must show youthe live temperature readings in °C displayed under diagnostic codes d.40 and d.41.
In writingcode F.72, the temperatures recorded in d.40 and d.41, the fault memory entries, and the date.
Refit the flow and return NTC sensors on their pipes and check wiring plugs
What they doWith the boiler unplugged from the electrical supply, the technician checks the physical mounting of the heating flow sensor (item 11) and return sensor (item 12) on their pipes, and inspects their plug connections and harness to the PCB.
They must show youboth NTC sensors properly clipped in place on the flow and return pipes and their seated connectors.
In writingthe reseating or wiring correction carried out on the NTC probes and the electrical check result.
Replace any faulty NTC probe with an original part and test heating and DHW
What they doIf either NTC probe remains out of specification, the technician replaces it with an authorized original spare part, clears the fault memory (d.94 = 1), and runs a heating test (status S.04 for at least 15 minutes) and hot water test (status S.14).
They must show youthe replaced NTC sensor, matching d.40 and d.41 readings, and status code S.04 during the heating test.
In writingthe original NTC part replaced, fault memory reset d.94, and the completed S.04 and S.14 test results.
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.
Watch whether F.72 shows alone or alternates every two seconds
- Who:
- you.
- How:
- observe the boiler display for at least ten seconds to see whether F.72 stays on screen alone or alternates every two seconds with another fault code (such as F.71, F.83, F.84, or F.85).
- Result that matters:
- knowing whether F.72 is paired with another flow or return sensor fault before calling a technician.
- If only F.72 is shown steadily:
- note the code and check that the water pressure bar graph on the display is normal.
- If it alternates every two seconds with another F.xx code:
- write down every code that cycles on the screen.
- Evidence to keep:
- a photo or short clip of the screen showing F.72 and any second fault code alternating with it.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Check system water pressure and radiator valves
- Who:
- you.
- How:
- make sure the pressure indicator is not flashing below 0.5 bar and verify that thermostatic radiator valves have not all closed at the same time during a heating demand.
- Result that matters:
- ruling out low water pressure or blocked radiator circulation alongside the temperature difference alert.
- If the pressure reading flashes below 0.5 bar:
- top up the heating circuit as described in the user manual to 1.0-1.5 bar.
- If water pressure is normal and radiators are open:
- proceed to the single reset check.
- Evidence to keep:
- the water pressure in bar shown on the display when F.72 appeared.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Press the Reset key once and wait 5 seconds
- Who:
- you.
- How:
- press the fault clearance button (Reset) once and wait five seconds, following the troubleshooting table in the user manual.
- Result that matters:
- testing whether F.72 clears or whether the mismatch between the flow and return NTC probes is persistent.
- If the fault code clears and the boiler resumes normal operation:
- keep an eye on the display during heating.
- If F.72 comes back after the reset attempt:
- do not press Reset repeatedly, contact Official Saunier Duval Technical Service.
- Evidence to keep:
- whether F.72 cleared after waiting five seconds or reappeared on the screen.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Compare diagnostic values d.40 and d.41 and inspect NTC probes 11 and 12
- Who:
- authorized installer or Official Saunier Duval Technical Service.
- How:
- enter diagnostic mode (hold mode for 7 seconds, enter installer code 96) to compare live flow temperature d.40 and return temperature d.41, then unplug the boiler and inspect both NTC sensors and their connectors.
- Result that matters:
- identifying which NTC probe (flow item 11 or return item 12) is unseated, loose, or reading out of range.
- If a probe was poorly seated on the pipe or had a loose plug:
- refit it firmly, secure the connector, and recheck d.40 and d.41.
- If the flow or return NTC probe is defective:
- replace it with an original spare part, clear fault list d.94, and verify S.04/S.14.
- Evidence to keep:
- the temperatures read in d.40 and d.41, the 10-fault history log, and the seating condition of both probes.
Inspecting the NTC sensors and PCB harness requires unplugging the boiler from the 230 V mains supply: reserved for the F.72 service engineer.
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
- caldera Saunier Duval F72
- F 72
- F.72
- F72
- Saunier Duval boiler F72
- Saunier Duval F.72
- Saunier Duval F72
- ThemaFast Condens F.72
- ThemaFast F72
What it means
In the installation and maintenance manual for the Saunier Duval ThemaFast Condens condensing boiler (Appendix E, code F.72), this fault indicates a flow and return temperature sensor error: the control board detects an excessive temperature difference between the flow and return NTC probes because the flow sensor and/or return sensor is poorly seated on its pipe or defective. Fault codes take priority on the display and alternate every two seconds if multiple faults occur at once. Seeing F.72 does not by itself prove a sensor is broken before checking its physical seating and wiring.
Warnings and stop conditions
- Highsource verified
Always unplug the boiler from the 230 V mains supply before touching internal plug connections or temperature sensors: never open the front casing or handle internal wiring yourself.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Highsource verified
If F.72 returns after pressing Reset once and waiting five seconds, stop resetting and contact Official Saunier Duval Service; if you smell gas, close the building gas shut-off valve, ventilate without using electrical switches, and call from outside.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Symptoms and causes
Probable causes · in order
- Flow or return NTC sensor poorly seated on its pipeCommon
The installation manual explicitly lists a poorly fitted flow temperature probe and/or return temperature probe as a direct cause of the excessive temperature difference behind F.72.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Defective flow or return NTC temperature sensorCommon
An internal drift or fault in the heating flow temperature sensor (item 11 on the wiring diagram) or return temperature sensor (item 12) causes an implausible gap between the two readings.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Loose plug connection or wiring harness contactPossible
A poorly inserted sensor plug or multi-pin connector on the printed circuit board alters the resistance signal read by the electronics and triggers sensor discrepancy faults.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
Virtual technician
Does F72 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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