ATAG 108 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 108 fault code mean?
Fault code 108 on an ATAG boiler — sometimes displayed as 1P4 or F1P4 — signals that the water pressure inside the central heating system has dropped below the minimum acceptable threshold of approximately 0.8 bar. When pressure falls this low, the boiler's internal safety logic prevents it from firing in order to protect the heat exchanger and pump from running dry. You may notice the pressure gauge sitting well below the normal operating range of 1.0–1.5 bar. Restoring the pressure via the filling loop is often all that is needed, but if pressure keeps falling back down, there is likely an underlying fault requiring a Gas Safe engineer.
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
- Pressure lost after bleeding radiators Common
Releasing air from radiators also releases a small volume of water, which drops system pressure. This is the single most common reason for a 108 fault to appear — especially in autumn when homeowners bleed radiators ahead of the heating season. Topping up via the filling loop almost always resolves it immediately.
- Gradual natural pressure loss over time Common
All sealed central heating systems lose a small amount of pressure over months or years through microscopic seepage and dissolved gases being released. If the boiler has been unused for an extended period or hasn't been topped up since installation, the pressure can creep below 0.8 bar without any obvious leak being present.
- Leak on the heating circuit Sometimes
A small but persistent leak on a radiator valve, a pipe joint, or a component inside the boiler itself will cause pressure to keep dropping after every top-up. Check around all visible radiators, pipework, and under the boiler for damp patches or dried scale marks. If pressure drops repeatedly within days, a leak is the most likely culprit.
- Faulty pressure relief valve (PRV) Sometimes
The PRV is designed to release water if pressure gets too high, but a worn or partially open PRV can weep continuously, slowly draining the system. Look for a damp or discoloured pipe running to an outside wall or a drain — this is the PRV discharge pipe. A weeping PRV needs replacing by an engineer.
- Failing pressure sensor or transducer Rare
The sensor that reads system pressure and reports it to the boiler's control board can develop a fault and send an inaccurately low reading, triggering the 108 code even when actual pressure is adequate. This is less common but is more likely if the gauge reads fine yet the fault persists after repressurising.
- Waterlogged or failed expansion vessel Rare
The expansion vessel contains a rubber diaphragm that accommodates changes in water volume as the system heats and cools. If the diaphragm fails or the vessel loses its pre-charge, pressure can swing erratically and drop below 0.8 bar at rest. Diagnosis and re-pressurisation of the vessel requires an engineer.
How to fix it
- Check the pressure gauge reading DIY safe
Locate the pressure gauge — on most ATAG iC models it is on the front fascia or visible through the bottom panel. If it reads below 1.0 bar, the system needs topping up. If it already reads 1.0 bar or above, the fault may point to a sensor issue rather than genuinely low pressure, and you should skip to calling an engineer.
- Locate the filling loop DIY safe
On ATAG iC and iC Economiser Plus models the filling loop is a short copper or braided-hose connection, typically a U-shaped tube with a valve at one or both ends, found on the underside of the boiler. Ensure you can see both valves and that the loop is connected at both ends before proceeding.
- Slowly open the filling loop valve(s) to top up the pressure DIY safe
Turn the filling loop valve(s) a quarter-turn to allow mains water into the system. Watch the pressure gauge rise — you are aiming for 1.0–1.1 bar for a cold system. Close the valve(s) the moment you reach this level. Do not exceed 1.4 bar. If the gauge overshoots, stop immediately; over-pressurisation will trigger the PRV to discharge water.
- Reset the boiler DIY safe
Once pressure is between 1.0 and 1.4 bar, press and hold the reset button on the boiler's control panel (typically 3–5 seconds) until the display clears the fault. Allow the boiler a minute to run through its ignition sequence. Limit resets to two or three attempts — repeated resets without resolution will not fix an underlying problem and can mask faults.
- Monitor pressure over the following days DIY safe
Keep an eye on the pressure gauge over the next 48–72 hours. A healthy system should hold its pressure with no significant drop. If the gauge falls below 1.0 bar again within a few days of topping up, you almost certainly have a leak or a faulty component that needs professional attention. Note the rate of drop to report to the engineer.
- Inspect visible pipework and radiators for signs of leaks DIY safe
Walk around the property and look at all visible radiator valves, pipe joints, and the area directly beneath the boiler for damp patches, white limescale deposits, or rust staining — all signs of a slow weep. Also check outside for any water dripping from a discharge pipe on an external wall, which would indicate the PRV is weeping. Do not attempt to tighten gas fittings; only check water-side connections visually.
- Call a Gas Safe registered engineer if pressure keeps dropping or you find a leak Gas Safe engineer
If the fault code 108 returns within days of topping up, if you have spotted a leak anywhere on the system, or if the boiler still shows the fault after repressurising and resetting, contact a Gas Safe registered engineer. They will pressure-test the system, trace any leaks, inspect the PRV, check the expansion vessel pre-charge, and test the pressure sensor — all tasks that are beyond safe DIY scope.
Parts you may need
- System pressure sensor / transducer · from £25
- Pressure relief valve (PRV) · from £20
- Expansion vessel diaphragm / replacement vessel · from £45
- Filling loop assembly · from £18
- Radiator valve (pair) · from £15
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 £100–£300, depending on the underlying cause.
Frequently asked questions
Can I top up my ATAG boiler pressure myself, or do I need an engineer?
Yes, topping up the pressure yourself is perfectly safe as long as you use the filling loop correctly and stop at 1.0–1.1 bar. This costs nothing and takes only a few minutes. An engineer is only needed if the pressure drops again repeatedly, suggesting a leak or a component fault rather than simple pressure loss.
Why does my ATAG boiler keep losing pressure and showing fault 108?
Recurring pressure loss almost always means water is escaping the sealed system somewhere. The most common culprits are a weeping pressure relief valve (check for a damp discharge pipe outside), a leaking radiator valve, a corroded pipe joint, or a failed expansion vessel diaphragm. Each top-up only masks the issue — a Gas Safe engineer should trace and fix the source. If the leak is inside the boiler, the repair cost is typically £100–£300; a failed expansion vessel is usually £150–£250 including labour.
What pressure should my ATAG boiler run at?
For a cold system (heating off and cooled down) ATAG recommends a pressure of 1.0–1.1 bar. During normal operation when the heating is running and the water is hot, it is normal for pressure to rise to around 1.5–2.0 bar. The boiler's low-pressure lockout triggers at around 0.8 bar, which is when fault code 108 (1P4) appears. If pressure regularly exceeds 2.5 bar when hot, the expansion vessel may need attention.
Is fault code 108 the same as 1P4 or F1P4 on my ATAG display?
Yes — these are all the same fault. ATAG displays the low water pressure alert as 108, 1P4, or F1P4 depending on the boiler model and firmware version. The meaning and the fix are identical regardless of which version appears on your display. If you see any of these codes, start by checking and topping up the system pressure as described above.