ATAG 1P2 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 1P2 fault code mean?
The 1P2 code is a safety lockout triggered when the boiler detects that the temperature difference between its flow (T1) and return (T2) pipework has exceeded 55°C. This abnormally large differential tells the boiler that water is barely circulating through the heat exchanger, putting it at serious risk of overheating. ATAG groups three related flow-check codes together in the i Series: 1P1 fires when the flow temperature jumps by 7–15°C in a single second; 1P2 escalates when the flow-to-return differential breaches that 55°C threshold; and 1P3 triggers when three 1P1 events occur within a 15-minute window. If your display shows 1P2, the boiler has already moved beyond the first-level warning and shut itself down to avoid heat exchanger damage.
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 seized circulation pump Common
The pump is the most frequent culprit. If it has seized mechanically, lost power, or is running at insufficient speed, water flow through the heat exchanger drops sharply and the temperature differential climbs quickly to the point where 1P2 triggers. Engineers can test this by temporarily removing the PWM control cable to run the pump at full 230 V speed.
- Low system pressure Common
When pressure drops below 1 bar the boiler struggles to push water around the circuit effectively. This alone can produce enough of a flow restriction to push the flow-to-return differential over the 55°C limit and fire the 1P2 code.
- Airlocks in the heating system Common
Pockets of trapped air in the pipework or radiators prevent water from moving freely, reducing circulation and causing heat to build up in the heat exchanger. Bleeding radiators is often enough to clear a mild airlock.
- Closed or partially closed valves Sometimes
A lockshield or isolation valve left even slightly closed can drastically restrict flow. This is easy to overlook after pipework work has been carried out, or if a valve has been accidentally knocked.
- Thermostatic radiator valves restricting bypass flow Sometimes
If every radiator in the system has a TRV fitted and they all close simultaneously — for example, when the house reaches temperature — the pump has nowhere to push water and flow effectively stops. Systems without a dedicated bypass radiator or automatic bypass valve are particularly vulnerable.
- Sludge or scale blockage in the heat exchanger or pipework Sometimes
Magnetite sludge and limescale deposits narrow the internal bore of the heat exchanger and pipework over time, restricting flow progressively. This usually causes intermittent 1P2 faults before becoming a permanent lockout.
- External heat source interference Rare
Solar thermal panels or other secondary heat sources feeding into the heating circuit can raise the return temperature unexpectedly, creating a misleading temperature differential that trips the 1P2 fault even when boiler-side flow is adequate.
- PCB fault Rare
If the pump, valves, pressure and pipework have all been checked and cleared, a logic error or failure on the main PCB may be causing it to misread the thermistor signals or fail to drive the pump correctly. This is a last-resort diagnosis.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, top the system up using the filling loop — usually a flexible braided hose with one or two valves beneath the boiler. Open the valve(s) slowly until the gauge reaches 1.2–1.3 bar, then close firmly. Never exceed 2 bar.
- Bleed the radiators to release trapped air DIY safe
Starting on the ground floor and working upwards, use a radiator bleed key to open each bleed valve a quarter-turn until water dribbles out steadily with no hissing. Retighten the valve, then re-check the system pressure afterwards and top up again if it has dropped below 1 bar.
- Check that all valves throughout the system are fully open DIY safe
Walk around the property and visually inspect lockshield valves on every radiator, any isolation valves on the boiler pipework, and the gate or ball valves on the flow and return connections. All should be fully open. Partially closed valves look open but are not — turn them fully anticlockwise and then a quarter-turn back.
- Reset the boiler DIY safe
Once you have addressed pressure, airlocks, and valves, press and hold the reset button (refer to your boiler's user guide for the exact location — typically a flame symbol with a line through it) for 3–5 seconds. Allow the boiler a full 2 minutes to attempt a restart. If it locks out again immediately, do not reset more than twice more without investigating further.
- If the fault persists, call a Gas Safe registered engineer Gas Safe engineer
A Gas Safe engineer will retrieve the boiler's fault history (the last 10 stored codes), test the circulation pump at full speed, check for 230 V supply at the relevant PCB connectors, and inspect for sludge or scale. If the pump has failed it will be replaced. If sludge is found, a power flush followed by installation of a magnetic filter may be recommended. In rare cases the PCB may require replacement.
Parts you may need
- Central heating circulation pump · from £130
- Pump PWM control harness · from £25
- Flow or return thermistor (NTC sensor) · from £20
- Automatic bypass valve · from £35
- Magnetic system filter (e.g. Magnaclean) · from £55
- Main PCB (control board) · from £280
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–£380, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler when the 1P2 code appears?
Resetting the boiler two or three times is fine as a first step, but repeatedly forcing restarts without addressing the underlying cause is not safe. The 1P2 code is a deliberate safety lockout designed to prevent the heat exchanger from overheating and cracking. If the fault returns after one or two resets, stop and call a Gas Safe engineer.
What is the difference between the 1P1, 1P2, and 1P3 codes on an Atag boiler?
They are all flow-check errors but at different severity levels. 1P1 fires when the flow temperature rises by 7–15°C in just one second — an early warning of falling circulation. 1P2 is more serious: it triggers when the gap between the flow and return temperatures exceeds 55°C, meaning circulation has dropped significantly. 1P3 is a cumulative fault that trips when three 1P1 events happen within 15 minutes. All three point to the same family of causes — poor water circulation — but 1P2 and 1P3 will typically lock the boiler out rather than just warn.
How much does it cost to fix an Atag 1P2 fault?
For the most common repair — circulation pump replacement — expect to pay around £150–£380 including parts and labour. If the system is heavily sludged and needs a power flush, that typically adds £300–£500 on top. A PCB replacement, which is rarely needed, can cost £400–£550 for parts alone, at which point many engineers will recommend comparing this against the cost of a new boiler, especially if the appliance is out of warranty.
Could TRVs on all my radiators be causing the 1P2 code?
Yes. If every radiator has a thermostatic radiator valve (TRV) and no radiator is left without one, the entire circuit can close off when rooms reach their set temperature. With nowhere for the pump to push water, flow effectively stops and heat builds up in the heat exchanger, triggering 1P2. The solution is to either fit an automatic bypass valve, leave at least one radiator with its TRV set to maximum (or removed), or ask an engineer to increase the pump speed setting.