Saunier Duval F.72 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.72 fault code mean?
The F.72 code appears when your Saunier Duval boiler detects a persistent and excessive difference between the water temperature leaving the heat exchanger (flow) and the water temperature returning from the heating system (return). Two NTC (Negative Temperature Coefficient) thermistors monitor these temperatures continuously — CTN2 on the flow pipe and CTN1 on the return pipe. When the gap between their readings stays outside acceptable limits for too long, the boiler treats it as a safety concern and shuts down, displaying F.72 on the panel. The display will flash, alternating between the two digits that make up the code. The fault is a lockout, meaning the boiler will not restart without a manual reset — and if the underlying problem has not been fixed, the code will return.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or degraded NTC thermistor (flow or return) Common
Over time, NTC thermistors age, corrode, or suffer thermal stress, causing their electrical resistance to drift away from the expected value. The PCB then receives conflicting temperature data from the two sensors and interprets this as a real thermal mismatch — even if the heating system itself is working fine. Replacing the faulty sensor (flow or return) resolves the fault in the majority of cases, without any need for PCB work.
- Loose or corroded wiring connectors Common
The connectors linking CTN1 and CTN2 to the control board can work loose over time due to burner vibration, or oxidise in the damp environment inside a boiler cabinet. A poor connection introduces extra resistance into the measurement circuit, distorting the temperature readings sent to the PCB and mimicking an abnormal flow-to-return differential — even when both sensors are physically intact.
- Poor circulation causing a genuine temperature differential Sometimes
If water is not circulating efficiently through the system, heat can build up in the heat exchanger and create a real, rather than a measured, temperature gap between flow and return. This can stem from a sluggish or seized circulation pump, a clogged system filter, closed or stuck radiator valves, or air locks in the pipework. In these cases, the sensors are reporting accurately — it is the system hydraulics that need attention.
- Limescale on thermistor sensors Sometimes
In hard-water areas, mineral deposits can build up on the surface of NTC thermistors, acting as an insulating layer that slows the sensor's response to actual water temperature. This causes one sensor to lag behind the other, producing an apparent temperature difference that trips the F.72 fault even when flow and return temperatures are not genuinely mismatched.
- Internal condensation or minor water leak onto wiring Sometimes
Small internal leaks — sometimes too minor to notice as a visible drip — or condensation settling on wiring looms can cause moisture ingress to the sensor cables. This introduces intermittent resistance faults in the signal path between the thermistors and the PCB, resulting in sporadic or recurring F.72 lockouts.
- Sensors fitted to incorrect pipes at installation Rare
In rare cases, a previous installation error may have placed the flow and return thermistors on the wrong pipes, creating a permanent reading inversion that the boiler flags as F.72. This is uncommon because a competent installer would normally commission and test the boiler immediately after fitting, catching this kind of error at the time.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button on the boiler's control panel as described in your user manual. Wait for the boiler to attempt to restart. If it fires up and runs normally, monitor it closely over the next few hours. A single successful reset after a one-off glitch is a reasonable outcome, but if F.72 returns — especially within the same heating cycle — the underlying cause has not been resolved and you should move on to the next steps rather than resetting repeatedly.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply to the property, contact your gas supplier. Although F.72 is not primarily a gas-supply fault, it is worth ruling this out before calling an engineer.
- Check your system pressure DIY safe
Look at the pressure gauge on your boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top up via the filling loop following your boiler's user guide. Very low pressure can impair circulation and contribute to abnormal flow-return temperature differentials.
- Do not attempt to test or replace the NTC thermistors yourself Gas Safe engineer
Although NTC thermistors are relatively low-cost components, safely accessing, testing with a multimeter, and replacing them requires draining relevant parts of the circuit, handling electrical connections on the PCB, and re-commissioning the boiler afterwards. This work must be carried out by a Gas Safe registered engineer.
- Do not attempt to inspect or repair wiring connections yourself Gas Safe engineer
Checking and re-securing the connectors between the thermistors and the PCB involves working directly on the boiler's electrical system. This is not DIY-safe and must be left to a qualified engineer, who will inspect for oxidation, pin damage, and continuity issues as part of their diagnosis.
- Do not attempt to inspect or service the circulation pump yourself Gas Safe engineer
If poor circulation is suspected, an engineer will need to test pump output and pressure drop across the system. Hydraulic disassembly and pump replacement on a sealed system require professional tools and competence; attempting this without training risks water damage and incorrect reassembly.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will use a multimeter to test the resistance of both NTC thermistors against manufacturer specifications, check connector integrity, assess pump performance and system flow rate, and look for signs of limescale or internal leaks. In most cases they will resolve the fault by replacing one or both sensors or reseating the wiring connectors. If the fault persists after sensor replacement, they may investigate whether the PCB itself is misreading the sensor signals, though this is a less common outcome.
Parts you may need
- NTC flow thermistor (CTN2) · from £18
- NTC return thermistor (CTN1) · from £18
- Thermistor wiring harness / connector set · from £25
- Circulation pump (compatible replacement) · from £75
- PCB (control board) — last resort · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the F.72 fault myself by just resetting the boiler?
A reset is always worth trying first — occasionally F.72 is triggered by a brief sensor spike and clears after a restart. However, if the code comes back after one or two resets, the boiler has identified a recurring problem that will not go away on its own. At that point, resetting repeatedly will not help and a Gas Safe registered engineer is needed to find and fix the root cause.
How much does it cost to repair a Saunier Duval F.72 fault in the UK?
For the most common repair — replacing one or both NTC thermistors — you should budget roughly £100 to £200 including parts and a standard call-out. If the fault is traced to wiring connectors alone, a single visit to reseat and clean the connections can cost less. If the circulation pump needs attention or a system flush is required, costs can rise to around £250 to £280. A PCB replacement, which is rarely the first step, typically costs more and would be discussed with you as a separate decision. If your boiler is old and the total repair cost starts to approach the price of a new boiler, your engineer may recommend replacement instead.
Is F.72 dangerous — should I stop using my boiler immediately?
The boiler shuts itself down when F.72 is displayed, so it will not continue to operate unsafely on its own. You do not need to treat this as an immediate emergency in the same way as a gas smell or carbon monoxide alarm. However, you should arrange for a Gas Safe engineer to inspect it promptly rather than leaving it indefinitely, both to restore your heating and hot water and to ensure the underlying fault is properly diagnosed.
Saunier Duval and Vaillant both show an F.72 — are they the same fault?
Yes. Saunier Duval is part of the Vaillant Group and the two brands share significant engineering and components, including their diagnostic fault codes. The F.72 fault means the same thing on both brands: a persistent mismatch detected between the flow and return NTC thermistor readings. An engineer familiar with either brand will be able to diagnose and repair this fault on both.