ATAG 1P4 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 1P4 fault code mean?
The 1P4 fault code on an Atag boiler (also displayed as F1P4 or fault code 108) indicates that the system water pressure has dropped below 0.8 bar — the minimum threshold the boiler needs to operate safely. When your central heating system loses pressure, the boiler locks out as a protective measure rather than run dry. In most cases this is a straightforward fix: the system simply needs topping up via the filling loop to bring pressure back into the correct cold-fill range of 1.0–1.1 bar. Once the system heats up, it is normal for pressure to climb to around 1.8–2.4 bar — this is just water expanding with heat. However, if pressure drops repeatedly, an underlying fault such as a water leak, a weeping pressure relief valve, or a faulty pressure sensor is likely and will need professional attention.
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
- Gradual pressure loss through normal system operation Common
Over time, small amounts of water vapour escape from the system through micro-pores in seals, radiator joints, and pump glands. This is perfectly normal and means most boilers need a top-up every year or two. If the boiler has been unused for an extended period, the pressure can drift low enough to trigger the 1P4 lockout.
- Radiators recently bled Common
Bleeding a radiator releases trapped air but also expels a small volume of water, which lowers system pressure. If you have recently bled one or more radiators and not refilled the system afterwards, the resulting pressure drop can easily trigger the 1P4 fault.
- Water leak on the system Common
A dripping pipe joint, a weeping radiator valve, or a leaking connection at the boiler itself can cause pressure to fall steadily. If you find yourself needing to repressurise the boiler more than once every few months, a leak is the most likely explanation. Check radiator tails, under-floor connections, and the area directly beneath the boiler for damp patches or limescale staining.
- Pressure relief valve (PRV) weeping or discharging Sometimes
The PRV is a safety device that opens to release water if system pressure gets too high. Over time the valve seat can degrade and allow a slow, continuous drip — usually visible as a trickle from the overflow pipe outside the property. A weeping PRV will cause the system to lose pressure gradually even when there are no other leaks.
- Faulty pressure sensor Sometimes
If the pressure gauge or electronic pressure sensor has drifted out of calibration or failed, the boiler may display the 1P4 code even when the actual system pressure is adequate. This is worth suspecting when the gauge reads correctly but the fault persists after successful repressurisation.
- Flow-check failure with thermal differential Rare
In some cases the 1P4 lockout follows three consecutive flow-check failures within a short window — typically accompanied by an unusually rapid temperature rise (ΔT approaching 37 °C within 15 minutes). This pattern can point to a circulation problem, a faulty temperature sensor (T1/T2), or a blockage, and requires diagnosis by a Gas Safe engineer rather than a simple top-up.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front or underside of the boiler. A reading below 1.0 bar when the system is cold confirms low pressure is the cause of the 1P4 fault. If the gauge reads 1.0 bar or above, the fault may point to a sensor issue rather than genuinely low pressure — skip to the engineer step.
- Locate the filling loop and repressurise the system DIY safe
The filling loop is typically a short, braided flexible hose with one or two small valve handles, usually found beneath the boiler. Slowly open the valve(s) by a quarter-turn — you should hear water entering the system. Watch the pressure gauge rise and stop filling once it reaches 1.0–1.1 bar. Close the valve(s) fully. Do not overfill past 1.5 bar; if you do, bleed a small amount of water from a radiator bleed valve to bring the pressure back down.
- Reset the boiler DIY safe
Press and hold the reset button on the boiler control panel (typically marked with a flame symbol or the word 'Reset') for 3–5 seconds, then release. The boiler should attempt to restart. Allow a few minutes for it to complete its start-up sequence and check whether the 1P4 code has cleared.
- Check for visible leaks around the system DIY safe
While the boiler restarts, take a few minutes to inspect radiator valve connections, any visible pipework, and the area directly beneath the boiler for damp patches, rust staining, or limescale deposits. If you spot active drips or wet patches, do not keep repressurising — switch the boiler off and call a Gas Safe engineer, as topping up a leaking system will only mask the problem.
- Monitor the pressure over the following days DIY safe
Once running normally, keep an eye on the pressure gauge over the next 48–72 hours. A pressure that remains stable suggests a one-off top-up was all that was needed. If the reading drops again within a few days, there is an ongoing leak or component fault that needs professional investigation.
- Call a Gas Safe registered engineer if the fault persists or pressure keeps dropping Gas Safe engineer
If the 1P4 code returns after repressurising, if you cannot locate the filling loop, if pressure is falling repeatedly, or if you suspect a PRV, sensor, or internal component fault, contact a Gas Safe registered engineer. They can pressure-test the system to find hidden leaks, test the pressure sensor, inspect the PRV, and investigate any underlying circulation or sensor faults that may be contributing to the lockout.
Parts you may need
- System pressure sensor · from £25
- Pressure relief valve (PRV) · from £20
- Filling loop assembly · from £18
- Expansion vessel · from £55
- T1/T2 flow and return temperature sensors · from £30
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 £80–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Atag 1P4 fault myself without calling an engineer?
In many cases, yes. If the pressure gauge shows below 1.0 bar, repressurising the system via the filling loop and resetting the boiler is a straightforward DIY task that costs nothing. However, if pressure keeps dropping after you top it up, or if you cannot identify a filling loop, you should call a Gas Safe engineer — repeatedly topping up a leaking system is not a fix.
What pressure should my Atag boiler be at?
When the system is cold, the pressure should sit between 1.0 and 1.1 bar. As the water heats up it expands, so a reading of 1.8–2.4 bar when the heating is running is entirely normal. If the cold pressure is consistently below 0.8 bar, the boiler will lock out and display the 1P4 fault.
Why does my Atag boiler keep losing pressure and showing 1P4?
If you are topping up the system more than once every couple of months, there is almost certainly a leak somewhere. Common culprits include a weeping pressure relief valve (look for a trickle from the external overflow pipe), a dripping radiator valve, a loose pipe joint, or a failing expansion vessel. A Gas Safe engineer can carry out a proper pressure test to pinpoint the source. Ignoring a persistent pressure loss can lead to more expensive damage over time.
Could the 1P4 fault be caused by something more serious than low pressure?
Usually not — the vast majority of 1P4 faults are simply low water pressure, fixed by a quick top-up. However, in rare cases a faulty pressure sensor can trigger the code even when actual pressure is adequate, and a pattern of three successive flow-check failures leading to a 1P4 lockout can indicate a circulation problem or temperature sensor fault. If your boiler is locking out repeatedly or showing other fault codes alongside 1P4, have a Gas Safe engineer carry out a full diagnostic. If a component like the PCB ever needs replacement, costs can rise to £400 or more — though this is uncommon for a pressure-related fault.