Glow-worm F74 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F74 fault code mean?
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.
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 or blocked water pressure sensor Common
The water pressure sensor (WPS) itself is the most frequent culprit. Over time the sensor can fail electrically, or its sensing port can become clogged with sludge, limescale, or corrosion debris — particularly in hard-water areas such as the South East of England. When the sensor port is blocked it cannot detect pressure changes accurately, so the boiler's control board never receives the confirmation it needs to allow ignition.
- Loose, damaged, or damp wiring connection Common
The wiring harness that runs between the pressure sensor and the main PCB is exposed to vibration and the damp environment inside the boiler casing. Connectors can work loose, pins can corrode, or the cable insulation can chafe and cause intermittent contact. Engineers are trained to check polarity and confirm the correct voltage at the sensor connector (typically around 1 bar reference voltage) before replacing any parts.
- Low system pressure Sometimes
If the boiler's system pressure has dropped below roughly 1.0 bar, the pressure sensor may be operating outside its normal working range and report an error rather than a straightforward low-pressure warning. Topping up via the filling loop is a simple homeowner check that should be carried out before anything else.
- Sludge build-up and poor water quality Sometimes
In systems without adequate corrosion inhibitor, magnetite sludge and scale can accumulate throughout the primary circuit. This debris can partially block the sensor tube, affect pump performance, and cause erratic pressure readings that trigger F74 even when all components are otherwise sound.
- PCB fault Rare
If the sensor, its wiring, and system pressure all check out correctly, the main PCB itself may be misreading the signal or failing to process it. PCB faults are the least common explanation for F74 but should be investigated by an engineer once all other causes have been ruled out.
How to fix it
- Check the system pressure gauge DIY safe
Find the pressure gauge on the front or underside of your Glow-Worm boiler. When the system is cold it should sit between 1.0 and 1.5 bar. If the needle is below 1.0 bar, top up the system using the filling loop (usually a braided or plastic-covered hose beneath the boiler) until the gauge reaches approximately 1.2–1.3 bar, then close the filling loop valves fully.
- Reset the boiler DIY safe
Press and hold the reset button for approximately one second until the fault code stops flashing. The boiler will attempt to restart. Allow a full restart cycle — this can take a couple of minutes. Do not reset more than two or three times in quick succession, as repeated failed ignition attempts can mask a worsening fault.
- Monitor whether the fault returns DIY safe
If the boiler restarts successfully, keep an eye on it over the next hour or so. If F74 reappears within a short period, the underlying issue is unlikely to resolve itself and a Gas Safe engineer should be booked. A one-off reset that holds for several days may indicate a transient wiring issue, but recurring faults always need professional diagnosis.
- Arrange inspection of the water pressure sensor and wiring harness Gas Safe engineer
A Gas Safe registered engineer will disconnect and inspect the wiring harness to the WPS, check connector condition and polarity, and measure the voltage at the sensor terminals. If the sensor is reading incorrectly or not at all, they will replace it with a compatible Glow-Worm part. This work involves accessing the inside of the boiler and should never be attempted by a homeowner.
- System flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of sludge or scale in the sensor port or circuit, a chemical power flush may be recommended to remove debris from the entire primary circuit. A corrosion inhibitor should be added afterwards to prevent recurrence. This is particularly advisable in hard-water areas or on older systems without prior inhibitor treatment.
- PCB inspection and replacement if required Gas Safe engineer
If all sensor and wiring checks come back satisfactory but F74 persists, the engineer will assess the main PCB. PCBs can be damaged by moisture ingress or age-related component failure. Replacement requires specialist parts sourcing and careful re-programming of boiler settings — strictly engineer-only work.
- Call a Gas Safe registered engineer Gas Safe engineer
If the pressure check and reset have not resolved the fault, contact a Gas Safe registered engineer experienced with Glow-Worm boilers. You can verify any engineer's registration at GasSafeRegister.co.uk. If you smell gas at any point, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
Parts you may need
- Glow-Worm water pressure sensor (WPS) · from £25
- Wiring harness / sensor loom · from £35
- Main PCB (Glow-Worm compatible) · from £180
- Central heating pump (Grundfos or equivalent) · from £65
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–£350, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F74 the same as F73?
They are closely related but not identical. Both F73 and F74 flag up heating circuit pressure sensor faults on Glow-Worm boilers. F74 specifically points to a sensor signal or wiring fault — the board is receiving an incorrect or absent signal from the pressure sensor — whereas F73 is more commonly associated with the actual pressure reading being outside the acceptable range. In practice, an engineer will diagnose both codes in a similar way, starting with the sensor and its wiring before investigating the PCB.
Can I fix a Glow-Worm F74 fault myself?
The only safe homeowner steps are checking and topping up the system pressure to around 1.2 bar via the filling loop, and resetting the boiler once or twice. If those steps don't clear the fault, the repair involves working inside the boiler — inspecting and replacing the pressure sensor, testing wiring, or assessing the PCB — all of which must be carried out by a Gas Safe registered engineer. Opening the boiler casing yourself is unsafe and will likely void any remaining warranty.
How much does it cost to fix an F74 fault in the UK?
For most households the repair falls between £150 and £350 including parts and labour. A straightforward pressure sensor swap is typically at the lower end (around £150–£200), while a pump replacement or more involved work tends to push toward £300–£350. PCB replacement is less common for this fault code but can cost £250–£400 on its own; if your engineer suspects a board fault, get a quote in writing and compare it against the cost of a new boiler, especially if your Glow-Worm is over ten years old.
Why does my Glow-Worm keep showing F74 after I reset it?
A fault that clears on reset but returns repeatedly almost always means the root cause — a failing pressure sensor, corroded wiring connection, or sludge blockage — is still present. Each reset simply restarts the boiler until the sensor fails to respond again. Repeated resets without fixing the underlying problem can put extra wear on other components, so it's best to book an engineer promptly rather than keep resetting and hoping the fault goes away on its own.