ATAG 1P1 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 1P1 fault code mean?
The 1P1 fault code appears on Atag i Series boilers when the control system detects that the flow water temperature is climbing too rapidly — typically a rise of between 7°C and 15°C within a single second. This strongly suggests that water is not circulating quickly enough through the heat exchanger, causing the small volume of water present to overheat almost instantly. Atag labels this internally as a 'flow check failed' condition. If the same event happens three times within a 15-minute window, the boiler will escalate to a full lockout rather than attempting another restart, so it is important to identify and resolve the underlying cause promptly rather than repeatedly resetting.
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
- Restricted water circulation or closed valves Common
If a lockshield valve, zone valve, or service valve has been partially or fully closed — deliberately or accidentally — the volume of water flowing through the heat exchanger drops sharply. The small amount of water that remains heats up almost immediately, triggering the 1P1 response. This is the single most common cause and worth ruling out before anything else.
- Failing or seized circulation pump Common
A pump that has lost performance — whether through worn bearings, internal sludge binding, or electrical failure — cannot push water around the system at the required rate. Even if the pump appears to be running, it may not be generating enough pressure to maintain adequate flow across the heat exchanger. The boiler sees the resulting temperature spike and shuts down.
- System sludge or debris restricting flow Common
Over time, black iron oxide sludge and scale build up inside the pipework, radiators, and especially the narrow passages of the heat exchanger itself. This progressively reduces flow rate until the boiler can no longer dissipate heat fast enough. Systems without a magnetic filter are particularly vulnerable.
- Faulty flow or return temperature sensor (T1 or T2) Sometimes
The Atag i Series uses separate flow (T1) and return (T2) sensors to monitor the heat exchanger. If either sensor develops a resistance drift or an intermittent connection, it can report a temperature change that has not actually occurred in the water, causing a spurious 1P1 fault even when circulation is perfectly healthy.
- External heat input — solar thermal or secondary circuit Sometimes
On properties where the Atag boiler shares a circuit with a solar thermal system or another heat source, heat can be pushed back into the boiler's return pipework. This raises the apparent return temperature and can trigger a 1P1 even though the boiler itself is doing nothing wrong. If the fault appears mainly on warm or sunny days, this is a likely factor.
- PCB fault Rare
In rare cases, the printed circuit board misinterprets valid sensor signals or generates false temperature readings internally. This is a diagnosis-of-exclusion — an engineer should confirm that sensors, wiring, pump, and pipework are all healthy before suspecting the PCB.
How to fix it
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in the property — cooker, gas fire — are working normally. If there is no gas supply to the property, contact your gas network operator rather than the boiler engineer.
- Check all visible valves around the boiler and on radiators are open DIY safe
Look for any lockshield or service valves on the boiler pipework that may be closed (handle at 90° to the pipe). Also ensure any zone valves serving underfloor heating or hot water cylinders are not stuck in the closed position. Do not attempt to force any valve that appears seized.
- Check the system pressure and top up if low DIY safe
Look at the pressure gauge on the boiler front. For most Atag i Series models the working pressure should sit between 1.0 and 1.5 bar when cold. If it reads below 0.8 bar, use the filling loop (two braided hoses beneath the boiler with a small inline valve) to slowly admit mains water until the gauge reaches approximately 1.2 bar, then close the valve firmly. Low pressure alone can restrict flow enough to trigger a 1P1.
- Reset the boiler — but no more than two or three times DIY safe
Press and hold the reset button (refer to your Atag user manual for its exact location on your model) until the display responds, then allow the boiler to run through its ignition sequence. If the 1P1 clears and the boiler runs normally, monitor it over the next few hours. If the code returns, stop resetting — repeated resets without fixing the root cause will simply lead to a lockout.
- Engineer: verify flow and return temperatures via the A-menu diagnostics Gas Safe engineer
A Gas Safe engineer should access the Atag i Series diagnostic menu (A0 for flow temperature, A1 for return temperature) and cross-reference the displayed values against independent thermometer readings on the flow and return pipes. A significant discrepancy points to a sensor fault rather than a genuine circulation problem.
- Engineer: test T1 and T2 sensor resistance Gas Safe engineer
With the boiler isolated, an engineer should measure the resistance of both the flow sensor (T1) and return sensor (T2) using a multimeter and compare the readings against the Atag resistance-temperature chart. An out-of-range or unstable reading confirms a sensor fault and the affected sensor should be replaced.
- Engineer: inspect wiring connectors for corrosion or looseness Gas Safe engineer
Connectors to both temperature sensors and to the circulation pump can suffer from vibration-induced loosening or mild corrosion over time, particularly in boiler cupboards with high humidity. An engineer should unplug and reseat each connector, checking the terminals for discolouration or moisture ingress.
- Engineer: test the circulation pump electrically and mechanically Gas Safe engineer
The engineer should confirm that the correct voltage is reaching the pump terminals when the boiler calls for heat. If voltage is present but the pump is not spinning freely, or is running well below its rated speed, the pump should be replaced. On Atag i Series boilers the pump is a serviceable component and replacement is straightforward for a qualified engineer.
- Engineer: inspect the heat exchanger and consider a power flush Gas Safe engineer
If sensors and pump check out, the engineer should assess the heat exchanger and system pipework for sludge restriction. A power flush using an appropriate descaler and inhibitor, followed by installation of a magnetic filter if one is not already fitted, is the recommended remedy. Atag recommends the use of a system inhibitor at the correct concentration after any flush.
- Engineer: investigate any external heat source connections Gas Safe engineer
If the property has solar thermal panels or any other secondary heat input connected to the same circuit, the engineer should verify that the correct anti-thermosiphon valve or check valve is fitted and functioning, preventing reverse heat flow into the Atag boiler's return.
- Engineer: replace the PCB as a last resort Gas Safe engineer
Only after all sensors, wiring, pump, and pipework have been checked and found satisfactory should the PCB be considered. An engineer may wish to contact Atag technical support (0800 680 0100) before ordering a board, as they can sometimes assist with remote diagnosis on i Series models.
Parts you may need
- Circulation pump (Atag i Series compatible) · from £95
- Flow temperature sensor T1 · from £35
- Return temperature sensor T2 · from £35
- Magnetic system filter (e.g. Magnaclean Pro2) · from £55
- PCB (Atag i Series) · from £220
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–£400, depending on the underlying cause.
Frequently asked questions
Is the Atag 1P1 fault dangerous — do I need to turn the boiler off immediately?
The 1P1 is a safety response designed to prevent overheating, so in that sense the boiler is already protecting itself by shutting down. You do not need to isolate the boiler at the mains unless you can smell gas or see any signs of water leaking. However, you should not continue resetting the boiler repeatedly — if it trips three times in 15 minutes it will lock out completely, and persistent overheating without investigation is not safe for the heat exchanger over the long term.
Can I fix a 1P1 fault myself, or does it need a Gas Safe engineer?
There are a few things a homeowner can safely check: make sure the system pressure is between 1.0 and 1.5 bar, confirm that all visible valves are open, and try a reset once or twice. Beyond that, diagnosing a sensor fault, testing the pump, or flushing the system requires a Gas Safe registered engineer. Working on the internal components of a gas boiler without Gas Safe registration is illegal and potentially dangerous.
How much does it typically cost to fix an Atag 1P1 fault in the UK?
For the most common cause — a failing circulation pump — expect to pay roughly £150–£350 including parts and labour. A temperature sensor replacement is usually £120–£180 all-in. A power flush, if sludge is the culprit, typically costs £300–£500 depending on system size. In rare cases where the PCB is at fault, the cost rises to around £300–£500 for that component alone plus labour — but this is uncommon and should only be considered once everything else has been ruled out.
My Atag boiler is still under warranty — will the 1P1 repair be covered?
Atag i Series boilers carry a 10-year manufacturer's warranty as standard, but this is conditional on the boiler being serviced every 12 months by a Gas Safe registered engineer and on installation having been carried out by an Atag-approved installer. If your service history is up to date, contact Atag directly on 0800 680 0100 before arranging any paid repair, as they may cover the cost of parts and in some cases labour.
Why does my Atag show 1P1 mainly on sunny days?
If the fault appears predominantly during warm or sunny weather, it is worth considering whether a solar thermal system is connected to the same circuit. Solar collectors can push heat back into the boiler's return pipework when the boiler is not firing, causing the return temperature to climb unexpectedly. A Gas Safe engineer should verify that the correct check valve or anti-thermosiphon device is correctly fitted between the solar circuit and the boiler.