F89 Boiler Fault Code: What It Means on Every Brand

The F89 fault code appears on 2 boiler brands — Glow-worm 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 2 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 F89 means	Severity	DIY?
GWGlow-worm	The F89 code appears on Glow Worm boilers when the appliance detects a problem with the circulation pump or the system's ability to move water around the heating circuit. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.89	The F.89 code appears when the boiler loses a reliable signal from the heating flow NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for monitoring the… Full guide →	Lockout — engineer likely	No

F89 brand by brand

Glow-worm F89

The F89 code appears on Glow Worm boilers when the appliance detects a problem with the circulation pump or the system's ability to move water around the heating circuit. On startup, the boiler's control board checks that the pump is running and that water is circulating correctly — if it cannot confirm this within an expected timeframe, it triggers F89 and locks out as a precaution. This protects the heat exchanger and other internal components from overheating due to stagnant water. The fault sits alongside related pump and pressure codes F73 and F25 on Glow Worm's diagnostic system. Because Glow Worm is part of the Vaillant Group, engineers familiar with Vaillant pump diagnostics will generally recognise this fault behaviour.

Causes, fixes & costs for Glow-worm F89 → · All Glow-worm codes

Vaillant F.89

The F.89 code appears when the boiler loses a reliable signal from the heating flow NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for monitoring the temperature of water as it leaves the boiler into your heating system. Without accurate flow temperature data, the boiler cannot regulate heat output safely, so it shuts down and locks out as a precaution. The fault can mean the sensor has physically failed, its wiring has come loose or broken, or — less commonly — the PCB has stopped reading the sensor correctly.

Causes, fixes & costs for Vaillant F.89 → · All Vaillant codes

Frequently asked questions

Is the F89 fault code the same on every boiler?

Broadly, yes — on Glow-worm and Vaillant the F89 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 F89 fault myself?

Not usually. On every brand listed, F89 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 F89 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.