F85 Boiler Fault Code: What It Means on Every Brand

The F85 fault code appears on 5 brands — Glow-worm, Heatline, Ravenheat, Saunier Duval and Vaillant — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 5 brands use this code Meaning differs by brand

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 F85 means	Severity	DIY?
GWGlow-worm	The F85 code appears on your Glow Worm boiler when the PCB (printed circuit board) has detected a problem with one or both of the NTC thermistors — the temperature sensors fitted to the… Full guide →	Lockout — engineer likely	No
HHeatline	The F85 fault code appears on Heatline boilers when the control board detects an implausible temperature reading from the flow or return NTC thermistors. Full guide →	Lockout — engineer likely	No
RRavenheat	The F85 fault code on a Ravenheat boiler indicates that one or both of the heating circuit's temperature sensors — the flow thermistor (measuring water leaving the boiler) and the return… Full guide →	Engineer may be needed	No
SDSaunier Duvalshown as F.85	The F.85 code indicates that the boiler's PCB has detected a persistent, abnormal temperature difference between the flow and return NTC thermistors — the two sensors that monitor water… Full guide →	Lockout — engineer likely	No
VVaillantshown as F.85	The F.85 code appears when the boiler's control board detects an implausible temperature relationship between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water… Full guide →	Lockout — engineer likely	No

F85 brand by brand

Glow-worm F85

The F85 code appears on your Glow Worm boiler when the PCB (printed circuit board) has detected a problem with one or both of the NTC thermistors — the temperature sensors fitted to the flow and return heating pipes. The flow thermistor sits on the pipe carrying hot water out of the boiler, while the return thermistor monitors water coming back in after circulating around the heating system. These sensors continuously send resistance readings to the PCB, which uses them to control heat output. When a reading falls outside the expected range — due to a failed sensor, a wiring fault, or in rarer cases a PCB issue — the boiler locks out and displays F85 to prevent unsafe operation. A simple reset is unlikely to clear this fault permanently; the underlying sensor or wiring problem needs to be diagnosed and resolved by a Gas Safe registered engineer.

Causes, fixes & costs for Glow-worm F85 → · All Glow-worm codes

Heatline F85

The F85 fault code appears on Heatline boilers when the control board detects an implausible temperature reading from the flow or return NTC thermistors. In plain terms, the boiler's two temperature sensors — one measuring water leaving the heat exchanger and one measuring water returning from the heating circuit — are either producing conflicting data, appear to be swapped, or are returning values that fall outside the expected operating range. Because the boiler relies on accurate temperature information to fire and regulate safely, it locks itself out until the problem is diagnosed and corrected. F85 sits alongside F83 and F84 in Heatline's temperature-sensor fault family: F83 relates to a sensor dropping below a plausible minimum, F84 flags an implausible difference between flow and return readings, and F85 specifically points to the sensors appearing to be transposed or otherwise incorrectly installed. If you have seen any of those codes recently before arriving at F85, the underlying cause is likely the same sensor or wiring issue.

Causes, fixes & costs for Heatline F85 → · All Heatline codes

Ravenheat F85

The F85 fault code on a Ravenheat boiler indicates that one or both of the heating circuit's temperature sensors — the flow thermistor (measuring water leaving the boiler) and the return thermistor (measuring cooler water coming back) — are sending readings that the PCB considers unreliable or out of range. These sensors are NTC thermistors, small resistive components whose electrical resistance changes predictably with temperature. When the PCB detects erratic or implausible signals from either sensor, it shuts the boiler down to prevent the system from overheating. Typical symptoms include radiators that only warm partially even when the thermostat is turned up high, fluctuating hot water temperatures, or the boiler cycling on and off without reaching the set temperature. Note: Ravenheat Manufacturing Limited was dissolved in 2025, meaning factory support has ended and spare parts are becoming increasingly scarce — this is an important factor when deciding whether to repair or replace the boiler.

Causes, fixes & costs for Ravenheat F85 → · All Ravenheat codes

Saunier Duval F.85

The F.85 code indicates that the boiler's PCB has detected a persistent, abnormal temperature difference between the flow and return NTC thermistors — the two sensors that monitor water temperature as it leaves and re-enters the boiler. This could mean the sensors are fitted to the wrong pipes, are reading incorrect resistance values, have wiring faults, or that heat transfer through the heat exchanger is poor. Because Saunier Duval is part of the Vaillant Group, the F.85 carries the same meaning across Saunier Duval and Vaillant boilers. The boiler will lock out and must be investigated by a Gas Safe registered engineer before normal operation can resume.

Causes, fixes & costs for Saunier Duval F.85 → · All Saunier Duval codes

Vaillant F.85

The F.85 code appears when the boiler's control board detects an implausible temperature relationship between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water coming back). At startup, the PCB expects to see a meaningful temperature difference between these two pipes — if the readings are identical, negligible, or contradictory, the boiler locks out. This typically points to one of three scenarios: the two NTC thermistors have been swapped onto the wrong pipes, both sensors have ended up on the same pipe, or one (or both) sensors is faulty and giving an inaccurate resistance reading. NTC thermistors work by changing their electrical resistance with temperature — at 20°C a healthy sensor reads around 12 kΩ. When the PCB receives conflicting or implausible signals, it raises F.85 rather than risk running the boiler unsafely.

Causes, fixes & costs for Vaillant F.85 → · All Vaillant codes

Frequently asked questions

Is the F85 fault code the same on every boiler?

No. F85 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a F85 fault myself?

Not usually. On every brand listed, F85 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 F85 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.