ATAG 104 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 104 fault code mean?
Fault code 104 on an Atag boiler indicates that the system water pressure has dropped below 1 bar, or that the boiler's controls have failed to detect any pressure increase from the pump after it has been running at full load for around 5 seconds. In simple terms, either there is not enough water pressure in the heating circuit, or the pump is not doing its job of circulating water effectively. The boiler locks out as a safety measure to prevent damage from running with insufficient pressure or flow.
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
- Low system water pressure Common
Over time, all sealed central heating systems lose a small amount of pressure, but a sudden or repeated drop below 1 bar is the most frequent trigger for code 104. This can be the result of a slow leak at a radiator valve, pipe joint, or within the boiler itself — or simply that the system has never been topped up after recent work.
- Air trapped in the system or boiler Common
Air locks prevent the pump from building pressure in the circuit. If radiators feel cold at the top or have not been bled recently, trapped air could be preventing a proper pressure reading, causing the boiler to log a 104 fault even when the gauge looks acceptable.
- Faulty water pressure sensor Sometimes
The pressure sensor (also called a pressure transducer) converts water pressure into an electrical signal read by the PCB. If the sensor drifts or fails, the digital readout seen by the PCB can disagree with the analogue gauge on the front of the boiler, triggering a false 104 fault even when actual pressure is fine.
- Pump failure or blockage Sometimes
If the circulating pump has seized, is running too slowly, or has a blocked impeller, it will not generate the pressure increase the boiler expects to detect. This can be caused by a build-up of sludge or scale, or by electrical failure of the pump itself.
- Internal blockage or heavy sludge Rare
A significant accumulation of magnetite sludge or limescale within the heat exchanger or pipework can restrict flow to the point where the pump cannot raise pressure as expected, mimicking a pump or pressure fault.
- PCB fault Rare
If the PCB is misreading signals from the pressure sensor or pump, it may incorrectly flag a 104 fault even when all physical components are in good order. This is typically only diagnosed once all other causes have been eliminated.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of your Atag boiler. If the needle or digital display reads below 1 bar, low pressure is almost certainly the immediate cause. A healthy system should sit between 1 and 1.5 bar when cold.
- Top up system pressure via the filling loop DIY safe
Locate the filling loop — usually a braided silver hose with one or two valves beneath the boiler. Slowly open the valve(s) and watch the gauge climb. Stop when the pressure reaches around 1.2–1.5 bar, then close the valve(s) fully. Never overfill above 2 bar. If you are unsure where the filling loop is, consult your Atag user manual.
- Bleed the radiators to release trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve slightly until a hiss of air escapes and water dribbles out, then close it. Work your way back towards the boiler, bleeding each radiator in turn. After bleeding, recheck the system pressure and top up again if it has dropped below 1 bar.
- Reset the boiler DIY safe
Once the pressure is in range and radiators have been bled, press and hold the reset button on the boiler until it restarts. Allow it to complete a full heating cycle and monitor whether the 104 code returns. Only attempt a reset two or three times — repeated resets without fixing the root cause can mask a more serious issue.
- Check for visible leaks around the system DIY safe
Inspect all accessible pipework, radiator valves, the pressure relief valve discharge pipe (usually exits through an outside wall), and any visible connections under or around the boiler. Damp patches, white limescale staining, or rust marks are signs of a slow leak. Do not attempt to repair any leaking component yourself — note the location and report it to your engineer.
- Have a Gas Safe engineer verify pressure sensor accuracy Gas Safe engineer
An engineer will compare the analogue gauge reading with the digital sensor value read by the PCB. If these differ, the sensor wiring harness should be checked for continuity, and the connectors inspected for corrosion or damage. A faulty sensor will need replacing.
- Have a Gas Safe engineer test and inspect the pump Gas Safe engineer
The engineer will check that the pump is receiving the correct voltage and that it spins freely without restriction. If the pump is seized or its output is too low, it will need to be freed, serviced, or replaced. The engineer may also carry out a system flush if sludge is found to be the cause.
- Call a Gas Safe registered engineer if the fault persists or pressure keeps dropping Gas Safe engineer
If code 104 returns shortly after you have topped up the pressure, there is almost certainly an ongoing leak or an internal component fault. A qualified engineer can pressure-test the system, trace any leak, and carry out the necessary repairs safely. Do not keep repeatedly topping up the system without investigating the cause.
Parts you may need
- Water pressure sensor / transducer · from £25
- Central heating circulating pump (replacement) · from £80
- Filling loop assembly · from £20
- Pressure relief valve · from £15
- Radiator bleed key · from £3
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–£350, depending on the underlying cause.
Frequently asked questions
Why does my Atag boiler keep showing fault code 104 even after I top up the pressure?
If the pressure drops back to a level that triggers code 104 within a few hours or days, it almost always means water is escaping from the system somewhere — either a slow external leak at a joint, valve, or radiator, or an internal leak past a seal inside the boiler. Repeatedly topping up masks the problem rather than solving it. A Gas Safe engineer should carry out a pressure test to locate the source. If the pressure holds steady but the fault still appears, the pressure sensor itself may be giving an inaccurate reading.
Can I fix Atag fault code 104 myself?
Homeowners can safely top up the system pressure via the filling loop, bleed radiators to remove trapped air, and reset the boiler — and in many cases this is enough to clear the fault if low pressure or air was the only cause. However, if the fault returns, or if you cannot bring the pressure up to 1–1.5 bar, you should call a Gas Safe registered engineer. Work on the pump, pressure sensor, pipework, or PCB must always be carried out by a qualified professional.
How much does it cost to fix Atag fault code 104 in the UK?
For most people, the repair falls somewhere between £100 and £350. A straightforward callout to repressurise the system and check for leaks typically costs £80–£120 in labour. Replacing a pressure sensor adds around £25–£50 in parts. A pump replacement is the more significant repair, with parts and labour usually coming to £200–£400. In the uncommon situation where the PCB is at fault, expect to pay £300–£500 or more — but this is only reached after all other components have been ruled out.
What pressure should my Atag boiler run at?
Atag boilers, like most UK combi and system boilers, operate correctly with a cold system pressure of between 1 and 1.5 bar. The pressure will rise slightly — typically to around 1.5–2 bar — when the heating is at full temperature, which is normal. If the gauge reads below 1 bar the boiler may lock out with code 104, and if it exceeds 2.5–3 bar the pressure relief valve will open to release water. The ideal resting pressure to aim for when topping up is around 1.2 bar.