How to Fix a Glow-worm F9 Fault

A Glow-worm F9 fault normally indicates a problem with the boiler's water pressure sensor. The sensor monitors the heating system pressure and helps the boiler operate safely. If it detects incorrect readings or fails completely, the boiler locks out and displays the F9 fault code.

Although low pressure is a common cause, the fault can also result from a failed sensor, leaks within the heating system or an internal boiler problem. Identifying the underlying cause is the key to a lasting repair. If your boiler is showing a different error, see our guide to Glow-worm boiler fault codes for explanations of other common fault codes.

What Does the Glow-worm F9 Fault Mean?

The F9 error tells you the boiler has detected a pressure sensor fault or an abnormal pressure reading. The boiler prevents normal operation until the issue is resolved to protect the appliance and central heating system.

Common Causes of a Glow-worm F9 Fault

Low or high system pressure

Most sealed heating systems should operate at around 1 to 1.5 bar when cold. Pressure below or above this range can trigger the F9 fault.

Pressure problems are commonly caused by:

• Small system leaks
• Air released while bleeding radiators
• Gradual pressure loss over time
• Blockages or trapped air affecting circulation

Faulty pressure sensor

Pressure sensors wear out over time. A faulty sensor may send incorrect readings even when the system pressure is normal.

Loose electrical connections or corrosion can also cause inaccurate readings.

Internal boiler faults

In some cases, debris, limescale or an electronic fault such as a PCB issue may contribute to the F9 error. These faults usually require professional diagnosis.

How to Fix a Glow-worm F9 Fault

Check the boiler pressure

Look at the pressure gauge first.

If the pressure is below the recommended level, top up the system using the filling loop, following the instructions in your boiler manual.

If the pressure is too high, bleeding a radiator may reduce it.

Check for leaks

Inspect visible pipework, radiator valves and the area beneath the boiler for signs of water leaks.

Even a slow leak can cause pressure loss and repeatedly trigger the F9 fault.

Reset the boiler

Once you've corrected the pressure, reset the boiler using the manufacturer's instructions.

If the F9 code returns immediately, there's likely another underlying fault.

Inspect the pressure sensor

If the pressure is correct but the boiler still shows F9, the pressure sensor may need cleaning or replacing.

This involves opening the boiler casing and should be left to a Gas Safe registered engineer — not attempted as a homeowner repair.

When Should You Call a Gas Safe Engineer?

Call an engineer if:

• The F9 fault keeps returning.
• The boiler loses pressure repeatedly.
• You suspect a faulty pressure sensor.
• You notice leaks inside the boiler.
• The boiler still won't operate after repressurising and resetting.

A Gas Safe engineer can accurately diagnose the fault, replace defective components and check the boiler is operating safely. Regular servicing also helps reduce the chance of pressure-related faults developing. If the F9 fault keeps returning after repressurising or resetting the boiler, book a Glow-worm boiler repair in London and we'll diagnose the underlying problem.

Frequently Asked Questions - (FAQs)

Yes, a single reset is safe to try after checking the pressure. If the fault returns immediately, avoid repeating the reset and arrange an inspection instead.

Not usually — the boiler locks out as a safety precaution rather than because of an immediate hazard. It shouldn't be ignored, though, since the underlying cause needs fixing.

No. Low pressure is the most common cause, but a faulty sensor or another internal fault can trigger the same code even when pressure is normal.

This depends on the boiler model and whether other components are also affected. A Gas Safe engineer can confirm the exact cost once the fault has been diagnosed.

Need Help with a Glow-worm F9 Fault?

If you've checked the pressure and the F9 fault remains, professional diagnosis is usually the quickest solution.

Serviceteam's Gas Safe engineers repair Glow-worm boilers across London and can identify whether the issue is the pressure sensor, a leak or another internal component before carrying out the appropriate repair. Get in touch today to arrange a boiler repair anywhere across London.