The F73 fault code appears on 6 boiler brands — Glow-worm, Heatline, Protherm, 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.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
F73 brand by brand
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.
Causes, fixes & costs for Glow-worm F73 → · All Glow-worm codes
The F73 code appears on Heatline boilers when the heating circuit water pressure sensor is sending a signal that falls outside the expected range — or no signal at all. Because Heatline is part of the Vaillant Group, this code carries the same meaning across both brands. The sensor's job is to confirm there is sufficient water pressure in the system before the boiler fires and to verify the pump is circulating water adequately. When the PCB loses reliable communication with that sensor — whether because the sensor itself has failed, its wiring has been damaged, or system pressure has genuinely dropped too low — the boiler locks out as a precaution to prevent it running dry. The fault may cause a complete lockout straight away, or the boiler may work intermittently if a wiring connection is intermittently making and breaking contact.
Causes, fixes & costs for Heatline F73 → · All Heatline codes
The F.73 fault code on a Protherm boiler indicates that the central heating water pressure sensor is either not communicating correctly with the control board, or its signal has fallen outside the accepted range. In practical terms, the boiler's electronics are seeing a reading that suggests the sensor cable has broken, developed a short-circuit to 0V, or the sensor itself has failed completely. Because the boiler cannot confirm safe water pressure, it shuts down as a precautionary measure. Protherm is part of the Vaillant Group, and this fault code carries the same meaning across both brands — so if you own a Vaillant and see F.73, the diagnosis and remedy are essentially identical.
Causes, fixes & costs for Protherm F.73 → · All Protherm codes
The F73 code appears on Ravenheat boilers when the control board detects a problem with the heating circuit pressure sensor. Either the system pressure has genuinely dropped to an unsafe level (typically below 0.6 bar), or the sensor itself has developed a fault and is feeding the PCB misleading readings — causing the boiler to believe there is a pressure problem when there may not be one. In both scenarios the boiler enters a lockout state, cutting off both heating and hot water until the fault is resolved. The closely related F74 code shares the same pressure sensor circuit and is often seen alongside F73 on the same models; if you see F74, the diagnostic approach is identical. Code 08 can also appear in conjunction with these codes when low pressure is the root cause.
Causes, fixes & costs for Ravenheat F73 → · All Ravenheat codes
The F.73 code appears when the boiler's water pressure sensor sends a signal that falls outside the expected range, or when the sensor connection to the control board is interrupted or short-circuited. In plain terms, the boiler can no longer trust the reading it's getting from the pressure sensor — so it shuts down as a precaution. You'll typically see the F.73 code flashing on the display, with no heating or hot water available, and the fault may return immediately after a reset if the underlying cause hasn't been addressed. Saunier Duval boilers are part of the Vaillant Group, and this fault code carries the same meaning across compatible models in both ranges.
Causes, fixes & costs for Saunier Duval F.73 → · All Saunier Duval codes
The F.73 fault code indicates that the boiler's printed circuit board (PCB) is receiving no usable signal from the water pressure sensor — the reading is either absent or so low it falls outside the accepted range. This typically points to a break or short-circuit in the wiring between the sensor and the PCB, or to the sensor itself having failed. Because the pressure sensor plays a critical role in confirming there is sufficient water pressure before the boiler can fire safely, the boiler immediately shuts down and enters lockout mode when this signal is lost. In most cases the boiler will refuse to restart via a reset until the underlying fault is resolved, though intermittent wiring problems can occasionally allow the boiler to run sporadically before the fault reappears.
Causes, fixes & costs for Vaillant F.73 → · All Vaillant codes
Frequently asked questions
Is the F73 fault code the same on every boiler?
Broadly, yes — on Glow-worm, Heatline, Protherm, Ravenheat, Saunier Duval and Vaillant the F73 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 F73 fault myself?
On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.
Who should I call about a F73 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.
