The F74 fault code appears on 6 brands — Glow-worm, Heatline, Protherm, 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.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
F74 brand by brand
The F74 code appears on Glow-Worm boilers when the control board detects a problem with the heating circuit's water pressure sensor — either the sensor itself has failed, or there's a wiring issue between the sensor and the PCB. When a heat demand is triggered, the circulation pump starts moving water and the pressure sensor is supposed to detect the resulting pressure change before allowing the burner to fire. If the board doesn't receive a valid signal from the sensor, it blocks ignition as a safety measure and displays F74. Note that F73 and F74 are closely related: both relate to heating circuit pressure sensor faults, with F74 specifically pointing to a sensor or wiring fault rather than a low-pressure reading alone. Because Glow-Worm is part of the Vaillant Group and shares hardware across the two brands, this fault behaves very similarly to the equivalent Vaillant pressure sensor error.
Causes, fixes & costs for Glow-worm F74 → · All Glow-worm codes
The F74 fault code on a Heatline boiler indicates that the PCB has detected an out-of-range signal from the heating circuit water pressure sensor — specifically, a voltage reading that is too high, pointing to a short circuit at 5V or 24V within the sensor wiring, or an internal failure of the sensor itself. Because the boiler relies on this sensor to confirm there is sufficient water pressure before completing its start-up sequence and to verify that the pump is circulating correctly, it cannot safely continue operating without a reliable pressure reading. The boiler therefore shuts down and locks out until the fault is diagnosed and cleared. Heatline boilers — including the Capriz, Advance, and Kalder ranges — share design DNA with Vaillant Group products, so this code and its behaviour are broadly consistent across those related models.
Causes, fixes & costs for Heatline F74 → · All Heatline codes
The F.74 fault code indicates that the boiler's central heating (CH) water pressure sensor is producing an electrical signal that falls outside the expected range. Specifically, the PCB is detecting either a short circuit or an abnormally high voltage returning from the pressure sensor — typically above 4V when a much lower reading is expected. Because the boiler can no longer accurately read system pressure, it shuts down as a precaution to protect internal components. The fault can stem from the sensor itself failing internally, damaged or corroded wiring between the sensor and PCB, or — less commonly — a problem with the PCB misinterpreting an otherwise valid signal. Protherm shares its engineering platform with Vaillant, so F.74 on a Protherm Gepard, Cheetah, or Panther behaves identically to the equivalent fault on Vaillant ecoTEC models.
Causes, fixes & costs for Protherm F.74 → · All Protherm codes
The F74 code on a Ravenheat boiler signals a problem with the heating circuit pressure sensor. The boiler's PCB has detected either genuinely low system pressure or a signal from the pressure sensor that falls outside the expected range. In either case, the boiler will lock out as a safety measure and refuse to fire until the underlying issue is resolved. The closely related F73 code covers a similar pressure sensor fault, so if you have seen both codes on your boiler they point to the same root cause. Ravenheat boilers need system pressure to sit between approximately 1 bar and 1.5 bar during normal operation; once pressure drops below around 0.5–0.6 bar, a lockout is triggered. Important note: Ravenheat Manufacturing Limited was dissolved in 2025, meaning replacement parts are becoming increasingly scarce. Before investing in repairs, it is worth considering the age and broader condition of the boiler.
Causes, fixes & costs for Ravenheat F74 → · All Ravenheat codes
The F.74 code tells you the boiler's PCB is receiving a signal from the water pressure sensor that is outside the expected range — specifically, it is reading too high, which usually points to a short circuit at 5V or 24V in the sensor wiring, or an internal failure within the sensor itself. Because the boiler relies on this sensor to confirm there is enough water pressure in the system before firing, it shuts down as a precaution rather than risk running dry or damaging components. Saunier Duval shares its boiler platform and fault code system with Vaillant (both are part of the Vaillant Group), so this fault behaves identically on both brands and the same diagnostic and repair approach applies to each.
Causes, fixes & costs for Saunier Duval F.74 → · All Saunier Duval codes
The F.74 fault code appears on Vaillant ecoTEC boilers when the water pressure sensor sends a signal to the PCB that falls outside the expected range — specifically, the reading is abnormally high, which the boiler interprets as a potential short circuit at 5V or 24V, or an internal failure within the sensor itself. Because the boiler relies on accurate pressure readings to confirm there is enough water in the system and that the pump is circulating correctly before allowing ignition to proceed, any signal it cannot trust causes an immediate safety shutdown. The boiler will remain locked out and refuse to fire until the root cause is identified and the fault is cleared. In plain terms: the boiler's pressure-measuring circuit has broken down, and the boiler has stopped itself as a precaution.
Causes, fixes & costs for Vaillant F.74 → · All Vaillant codes
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.
