Glow-worm F73 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F73 fault code mean?
The F73 code appears on Glow-Worm boilers when the control board cannot confirm that the circulation pump has become active. When a heating demand is triggered, the pump starts moving water around the primary circuit and the water pressure sensor (WPS) should detect the resulting pressure change almost immediately. The boiler uses this signal as proof of circulation before allowing full burner ignition. If the sensor fails to register that expected change — whether because the sensor itself is faulty, the pump has seized, or there is a wiring problem between the two — the boiler locks out and displays F73 as a protective measure. This fault is common across the Glow-Worm Betacom, Ultracom, and Ultimate ranges, and on many Vaillant models that share the same platform and pressure sensor design.
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
- Faulty water pressure sensor (WPS) Common
Even when the circulation pump is working perfectly, a degraded or failed water pressure sensor will not register the pressure change the pump creates. The boiler then assumes circulation is absent and locks out. Because this is often misread as a pump fault, the WPS should be the first component an engineer tests — checking the supply voltage (typically 1–2 V DC at around 1 bar) and comparing the sensor's output signal against expected values before condemning the pump.
- Seized or failed circulation pump Sometimes
If the pump motor has seized or its impeller is blocked by sludge, water simply does not move around the circuit and the pressure sensor correctly reports no change. Engineers can often feel or hear whether the pump is trying to run, and some diagnostic tools allow them to command the pump directly. A severely sludged system can cause premature pump failure, so power-flushing may be recommended alongside a pump replacement.
- Wiring fault between the WPS and PCB Sometimes
Corroded terminals, a damaged wiring harness, or loose connectors on the pressure sensor circuit can interrupt the signal reaching the control board, triggering F73 even though both the pump and sensor are mechanically sound. An engineer will check continuity, polarity, and connector condition as part of the initial diagnostic.
- Low system water pressure Sometimes
If the boiler's system pressure has dropped below its minimum operating threshold (usually around 0.5–1 bar), the water pressure sensor may not produce a usable output signal, leading to an F73 condition. Topping the system up via the filling loop to approximately 1.0–1.5 bar is a quick homeowner check and can sometimes resolve the fault if low pressure was the sole trigger.
- PCB fault Rare
In a small number of cases the printed circuit board itself fails to interpret the signal from the WPS correctly, or the PCB output to the pump is faulty. This is typically only diagnosed after the sensor, pump, and wiring have been ruled out, as PCB replacement is the most costly intervention.
How to fix it
- Check the system pressure gauge on the boiler DIY safe
Look at the pressure gauge on the front of the boiler. If it reads below 1 bar, the system is under-pressurised. Use the filling loop (usually two inter-connected valves beneath the boiler) to top up to around 1.0–1.5 bar, then close the valves securely. Consult your boiler manual or the label near the filling loop if you are unsure of the procedure.
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or 'R') for a few seconds as described in your boiler manual. If the boiler fires up and runs normally, monitor it over the next day or two. If F73 returns promptly, do not keep resetting — repeated lockouts indicate an underlying fault that needs professional diagnosis.
- Check your gas supply is live DIY safe
Confirm that other gas appliances such as a gas hob or gas fire are working normally. If there is no gas at other appliances, contact your gas supplier rather than a boiler engineer. If other appliances are fine, the gas supply to the boiler is not the issue.
- Do not attempt to access internal components Gas Safe engineer
The water pressure sensor, circulation pump, wiring harness, and PCB are all internal components that must only be inspected or replaced by a Gas Safe registered engineer. Removing the boiler casing and working on these parts yourself is unsafe and will invalidate any remaining warranty.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will remove the boiler casing, check the WPS voltage and output signal, inspect the wiring harness for continuity and correct polarity, and command the pump via diagnostic mode. In most cases they will find either a failed pressure sensor or a seized pump and replace the faulty component. Ask the engineer to check for system sludge at the same time — a dirty system is a common cause of pump wear on these boilers.
Parts you may need
- Water pressure sensor (WPS) · from £35
- Circulation pump (Glow-Worm compatible) · from £120
- Wiring harness / sensor loom · from £25
- PCB (printed circuit board) · from £200
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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow-Worm F73 fault myself?
No. While there are two quick homeowner checks worth trying first — topping the system pressure up to around 1–1.5 bar and attempting a single reset — the underlying cause almost always requires a Gas Safe registered engineer. Accessing the water pressure sensor, circulation pump, or internal wiring without the correct authorisation is both unsafe and illegal on a gas appliance.
Is the F73 usually the pump or the pressure sensor?
It is more often the water pressure sensor (WPS) than the pump itself. F73 is frequently misdiagnosed as a pump failure, but when engineers test the pump directly it is often running fine. The pressure sensor is the first component to test; replacing a faulty WPS is quicker and cheaper than a pump swap, and it restores normal operation in the majority of F73 cases.
How much does it cost to fix an F73 on a Glow-Worm boiler?
Most homeowners pay between £150 and £380 depending on which component is at fault. A pressure sensor replacement typically comes in around £180–£220 including labour, while a circulation pump replacement is usually £280–£380. If the PCB turns out to be the culprit, costs can rise to £400–£600 or more, though PCB failure is rare and is only diagnosed after other components have been ruled out.
Why does my Glow-Worm boiler show F73 after I top up the pressure?
Low pressure can trigger F73 because the WPS cannot produce a valid output signal below a certain pressure threshold. If topping up and resetting clears the fault but it comes back within days or weeks, there is likely a slow leak somewhere in the system — possibly at a radiator valve, a pipe joint, or the pressure relief valve. A Gas Safe engineer can pressure-test the system and find the source of the leak before it causes further damage.