F84 Boiler Fault Code: What It Means on Every Brand

The F84 fault code appears on 5 boiler brands — Glow-worm, Heatline, Ravenheat, Saunier Duval and Vaillant — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 5 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F84 means	Severity	DIY?
GWGlow-worm	The F84 code appears when the boiler's control board detects a persistent, irreconcilable difference between the readings from the heating flow NTC thermistor and the heating return NTC… Full guide →	Lockout — engineer likely	No
HHeatline	The F84 fault code on a Heatline boiler indicates that the control board has detected an implausible difference between the temperatures being reported by the flow and return sensors. Full guide →	Lockout — engineer likely	No
RRavenheat	The F84 code appears when the Ravenheat boiler detects a persistent, abnormal difference between the temperatures recorded by the flow NTC thermistor (the sensor monitoring water leaving… Full guide →	Lockout — engineer likely	No
SDSaunier Duvalshown as F.84	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… Full guide →	Lockout — engineer likely	No
VVaillantshown as F.84	The F.84 code appears on Vaillant boilers when the control board detects a persistent, unresolved temperature difference between the flow and return NTC thermistors. Full guide →	Lockout — engineer likely	No

F84 brand by brand

Glow-worm F84

The F84 code appears when the boiler's control board detects a persistent, irreconcilable difference between the readings from the heating flow NTC thermistor and the heating return NTC thermistor. Under normal operation, these two sensors work in tandem — the flow thermistor monitors water leaving the heat exchanger, while the return thermistor reads the cooler water coming back from the radiators. When the gap between their readings is permanently outside acceptable limits, the boiler concludes something is wrong with the sensing arrangement itself and locks out to prevent unsafe operation. This typically points to a faulty, swapped, or disconnected thermistor, damaged wiring between the sensors and the PCB, or — less commonly — a circulation problem that is preventing normal heat distribution around the system.

Causes, fixes & costs for Glow-worm F84 → · All Glow-worm codes

Heatline F84

The F84 fault code on a Heatline boiler indicates that the control board has detected an implausible difference between the temperatures being reported by the flow and return sensors. In plain terms, the two readings don't add up — the boiler cannot make sense of what the water temperature actually is, so it shuts down as a precaution to prevent overheating or damage to internal components. Until the sensors are sending believable, consistent data, the boiler will stay locked out. Note that related codes F83 and F85 are also associated with implausible temperature sensor readings on Heatline boilers: F83 typically points to the return sensor reading being too low relative to the flow, while F85 indicates the flow temperature has risen excessively — all three faults share similar diagnostic and repair paths.

Causes, fixes & costs for Heatline F84 → · All Heatline codes

Ravenheat F84

The F84 code appears when the Ravenheat boiler detects a persistent, abnormal difference between the temperatures recorded by the flow NTC thermistor (the sensor monitoring water leaving the boiler) and the return NTC thermistor (monitoring cooler water coming back from the heating circuit). A temperature gap between flow and return is perfectly normal during operation — heat is transferred to your radiators and the returning water is cooler. The fault triggers when that difference becomes excessive or stays fixed in a way the boiler's control logic cannot account for, suggesting that one or both sensors are giving inaccurate readings, there is a wiring problem between the sensors and the PCB, or something physical such as limescale or a heat exchanger fault is causing genuine abnormal heating conditions. The boiler shuts down as a safety measure to prevent overheating.

Causes, fixes & costs for Ravenheat F84 → · All Ravenheat codes

Saunier Duval F.84

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.

Causes, fixes & costs for Saunier Duval F.84 → · All Saunier Duval codes

Vaillant F.84

The F.84 code appears on Vaillant boilers when the control board detects a persistent, unresolved temperature difference between the flow and return NTC thermistors. Under normal operation, the flow pipe (carrying hot water away from the heat exchanger) will always be warmer than the return pipe (carrying cooler water back), but that gap should be within a predictable range. When the boiler's electronics see a temperature differential that stays outside acceptable limits — whether too large, too small, or contradictory — it locks out and displays F.84 to protect itself from running under abnormal conditions. The fault most commonly points to a faulty NTC sensor, sensors fitted to the wrong pipes, or damaged wiring between the sensors and the PCB. It cannot be cleared by repressurising the system or bleeding radiators, and almost always needs a Gas Safe engineer to diagnose and resolve.

Causes, fixes & costs for Vaillant F.84 → · All Vaillant codes

Frequently asked questions

Is the F84 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Ravenheat, Saunier Duval and Vaillant the F84 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F84 fault myself?

Not usually. On every brand listed, F84 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F84 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.