Saunier Duval F.84 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.84 fault code mean?
The F.84 fault code indicates that the Saunier Duval boiler's control board has detected a sustained or implausible temperature discrepancy between the flow NTC thermistor (the sensor on the pipe carrying hot water out of the boiler) and the return NTC thermistor (the sensor on the pipe carrying cooler water back in). Because this temperature difference is either impossible under normal operating conditions or has persisted without a logical explanation, the boiler triggers a safety lockout and shuts down. NTC thermistors work by changing their electrical resistance as temperature changes — the PCB reads this resistance to monitor heat levels in real time. When the two sensors report readings that simply don't make sense together, the boiler refuses to operate until the issue is resolved. This fault requires investigation by a Gas Safe registered engineer.
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
- Flow and return sensors swapped or incorrectly positioned Common
If the flow and return NTC thermistors have been fitted to the wrong pipes — either during original installation or after a recent service — the PCB will receive temperature readings that are inverted or otherwise implausible. This is one of the most straightforward causes to identify and correct, but it requires a Gas Safe engineer to inspect and reposition the probes correctly.
- Faulty NTC thermistor(s) Common
One or both of the temperature sensors may have failed or drifted out of calibration, sending inaccurate resistance readings to the PCB. A correctly functioning thermistor should read approximately 12 kΩ at 20°C. An engineer can test each sensor individually with a multimeter — a reading that deviates significantly from the expected value at a known temperature points to a failed sensor. Importantly, just because one thermistor is faulty does not mean the other is fine, so both should be tested.
- Wiring fault or poor connection to sensors Sometimes
Damaged wiring, corroded terminals, or a loose connector between the NTC thermistors and the PCB can cause erratic or inaccurate resistance readings, mimicking the symptoms of a failed sensor. An engineer will need to test each section of the wiring loom and all connections, not just the sensors themselves, to rule this out.
- Faulty PCB Rare
If both thermistors and their wiring test within the correct parameters, the problem may lie with the PCB itself. The control board may be misreading the sensor signals due to an internal fault. PCB failure is the least common cause of F.84 and typically the most expensive to resolve, so it should only be diagnosed once sensor and wiring issues have been properly excluded.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Saunier Duval boiler (usually marked with a flame symbol or the word 'Reset') and press it once. If the boiler restarts and runs normally, monitor it closely over the next hour. If the F.84 code returns, do not reset again repeatedly — further resets will not fix the underlying fault and can place additional stress on the PCB. Switch the boiler off and move on to calling an engineer.
- Check whether a recent service has taken place DIY safe
If your boiler has recently been serviced or had parts replaced, make a note of this for the engineer. The F.84 fault can appear after a service where the flow and return sensors were disturbed and then refitted incorrectly. Letting the engineer know saves diagnostic time.
- Engineer inspection — verify correct sensor positioning Gas Safe engineer
A Gas Safe registered engineer will inspect the physical location of both the flow and return NTC thermistors to confirm they are mounted on the correct pipes and seated properly in their pockets. If they have been swapped or are not making proper thermal contact with the pipework, the engineer will reposition them and clear the fault.
- Engineer testing — multimeter check of both thermistors and wiring Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of each NTC thermistor at a known temperature and compare the result to the manufacturer's specification (approximately 12 kΩ at 20°C). They will also check the continuity and integrity of the wiring between each sensor and the PCB. Both sensors must be tested independently, as it is possible for both to have drifted or failed simultaneously.
- Engineer repair — replace any faulty thermistor(s) Gas Safe engineer
If one or both thermistors test outside the acceptable range, the engineer will replace the affected sensor(s). The parts themselves are inexpensive, and this is typically the most cost-effective resolution for F.84. After replacement, the boiler should be tested through a full heating cycle to confirm the fault has cleared.
- Engineer repair — inspect and test the PCB if fault persists Gas Safe engineer
If both sensors and all wiring test correctly but the F.84 fault continues to appear, the PCB is the likely culprit. The engineer will assess whether the board can be repaired or must be replaced. Given the cost of PCB replacement on an older boiler, the engineer may also discuss whether a full boiler replacement would be a better investment at this stage.
- Call a Gas Safe registered engineer Gas Safe engineer
If the fault has not cleared after a single reset attempt, contact a Gas Safe registered engineer to carry out a full diagnosis. All work on the gas boiler's internal components — including sensors, wiring, and the PCB — must be performed by a Gas Safe registered professional. You can verify an engineer's registration at GasSafeRegister.co.uk.
Parts you may need
- Flow NTC thermistor (Saunier Duval compatible) · from £18
- Return NTC thermistor (Saunier Duval compatible) · from £18
- Thermistor wiring harness / connector · from £25
- PCB (Saunier Duval model-specific) · from £320
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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.84 fault myself?
The only homeowner-safe action is a single reset attempt. If the fault returns, all further work — including testing the sensors, checking wiring, and replacing parts — must be carried out by a Gas Safe registered engineer. This is both a legal requirement and a safety necessity for gas appliances in the UK.
How much does it cost to fix a Saunier Duval F.84 fault?
For most people, the repair involves replacing one or both NTC thermistors, which typically costs between £120 and £320 including the engineer's call-out, diagnosis, parts, and labour. If the problem turns out to be the PCB rather than the sensors, costs can rise significantly — PCB replacement on a Saunier Duval boiler can reach £450–£700 depending on the model and whether the board can be repaired rather than replaced outright. For that reason, engineers will always test and rule out sensor faults before diagnosing a PCB issue.
Why has the F.84 code appeared after my boiler was serviced?
This is a known trigger for F.84. During a service, the flow and return NTC thermistors are sometimes removed and refitted — if they are placed back on the wrong pipes, or not seated correctly in their pockets, the PCB detects an implausible temperature difference and locks out. Contact the engineer who carried out the service, as this should be covered under their workmanship.
Is F.84 a dangerous fault?
The F.84 lockout is a safety response by the boiler — it has shut itself down because it detected something that doesn't make sense, rather than continuing to operate in an unknown state. The fault itself is not directly dangerous, but you should not attempt to bypass the lockout or repeatedly reset the boiler. Leave it off and arrange for a Gas Safe engineer to investigate. Your home will still be safe; you simply won't have heating or hot water until the fault is resolved.