ATAG 1P3 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 1P3 fault code mean?
The ATAG 1P3 fault code indicates a repeated 'Flow Check Failed' condition. Specifically, it is triggered when the boiler detects three occurrences of a 1P1-type flow error within a 15-minute window. A 1P1 event occurs when the boiler's supply (flow) temperature rises by between 7°C and 15°C in a single second — a spike that points to water circulation dropping sharply through the heat exchanger. When this pattern repeats three times in quick succession, the boiler escalates to 1P3 and locks out to protect itself from overheating. In plain terms: water is not moving through the boiler quickly enough, so heat is building up dangerously fast. The 1P3 code is part of the broader '1P' fault family on ATAG i Series (2nd and 3rd generation) boilers, which includes 1P1 (single flow check event), 1P2, 1P4, and 1P9. If you are seeing 1P1 or 1P2 on your display instead, the underlying cause and the diagnostic approach are broadly the same as described here.
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 circulation pump is responsible for pushing heated water around the system. If the impeller has seized, worn out, or is drawing power without actually moving water, heat accumulates in the heat exchanger and triggers rapid temperature spikes. This is the single most frequent cause of a persistent 1P3 lockout.
- System sludge or debris restricting flow Common
Over time, corrosion by-products and limescale build up inside the pipework, radiators, and heat exchanger. When sludge reaches a critical level it restricts water flow to the point where the boiler overheats on every firing cycle. A stuck or contaminated pump impeller is often a consequence of heavy sludge loading rather than a standalone failure.
- Trapped air in the system Common
Air pockets in the boiler or pipework act as a blockage, reducing effective water circulation. If your radiators feel cold at the top while the bottom is warm, trapped air is likely present. Enough air in the system can cause the same rapid temperature spike that triggers a 1P event.
- Closed or partially closed isolating valves Sometimes
If a valve on the flow or return pipework is shut or only partially open — perhaps after maintenance work — the restriction will starve the boiler of circulation and reproduce the 1P3 condition consistently.
- Blocked plate heat exchanger (combi boilers) Sometimes
On combi models, the plate heat exchanger between the central heating circuit and the domestic hot water circuit can become scaled or blocked. A partial blockage reduces flow on every heating call and is a known cause of recurring flow check faults on ATAG combis.
- Faulty diverter valve (combi boilers) Sometimes
If the three-way diverter valve cartridge is stuck or not moving correctly, flow may be misdirected or restricted during heating mode. This can produce intermittent flow starvation and the repeating 1P1 events that escalate to 1P3.
- Flow or return temperature sensor fault Sometimes
If the NTC thermistor measuring the flow or return temperature is giving inaccurate readings, the boiler's control system may interpret a normal situation as a dangerous temperature spike and log false 1P events. A failed sensor can mimic all the symptoms of genuine circulation problems.
- External heat source interference Rare
Systems with a solar thermal panel or other supplementary heat source can feed unexpectedly warm water back into the boiler's return circuit. This can cause the boiler to see a misleadingly high temperature differential and trigger a flow check fault even when circulation itself is fine.
How to fix it
- Check your gas supply is live DIY safe
Before anything else, confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply to the property, contact your gas supplier rather than investigating the boiler further.
- Reset the boiler DIY safe
Locate the reset button on the ATAG control panel — it is typically marked with a flame or reset symbol. Press and hold it for around three seconds until the boiler attempts to restart. Allow a full firing cycle to complete and observe whether 1P3 returns. Limit resets to two or three attempts; repeated resetting without addressing the root cause will not resolve the fault and may mask a worsening problem.
- Check all isolating valves around the boiler are fully open DIY safe
Trace the flow and return pipes close to the boiler and confirm that any gate valves or ball valves are in the fully open position. A valve that was partially closed during a recent service is an easy and overlooked cause of this fault.
- Bleed your radiators to release trapped air DIY safe
Turn the heating on for 15 minutes, then switch it off and let the system cool slightly. Starting on the ground floor and working upward, use a radiator bleed key to open the bleed valve on each radiator until only water — no hissing air — escapes, then re-tighten. After bleeding, check the system pressure on the boiler gauge; if it has dropped below 1 bar, top it up via the filling loop until the gauge reads between 1 and 1.5 bar.
- Top up system pressure if it is below 1 bar DIY safe
Locate the filling loop — a braided flexible hose with one or two valves, usually beneath the boiler. Slowly open the valve(s) and watch the pressure gauge rise. Stop when the needle reaches approximately 1.2 bar and close the valve firmly. Do not overpressurise above 1.5 bar. If pressure repeatedly drops between checks, there may be a leak that an engineer needs to investigate.
- Try the boiler air purge cycle DIY safe
ATAG i Series boilers can carry out an internal air purge. Switch off the boiler at the fused spur or isolation switch, wait at least five seconds, then restore power. The boiler will run a short purge sequence on start-up that can clear minor air locks from the internal pipework.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If 1P3 returns after resetting, bleeding, and checking pressures, a qualified engineer is needed. They will check the fault log (the last ten stored faults on the ATAG system give valuable diagnostic context), test the circulation pump for correct operation and voltage, inspect the pump impeller for sludge seizure, assess the plate heat exchanger for blockage, test the diverter valve on combi models, and measure the flow and return thermistors for accuracy. Depending on findings, they may carry out a power flush, replace the pump, clean or replace the heat exchanger, or renew the diverter valve or sensors. Do not attempt any of these tasks yourself.
Parts you may need
- Circulation pump (replacement) · from £85
- Diverter valve motor/cartridge (combi) · from £55
- NTC flow/return thermistor · from £20
- Magnetic system filter (e.g. Magnaclean) · from £65
- Auto air vent · from £12
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–£450, depending on the underlying cause.
Frequently asked questions
Can I fix an ATAG 1P3 fault myself without calling an engineer?
You can safely try the initial steps: reset the boiler up to three times, bleed the radiators, check that all valves are open, and top up the system pressure if it is below 1 bar. These actions resolve the fault in a minority of cases — usually where trapped air or a brief pressure drop was the trigger. However, if the 1P3 code returns after those steps, the underlying cause almost certainly involves the circulation pump, heat exchanger, sensors, or system sludge, all of which require a Gas Safe registered engineer to diagnose and repair safely.
What is the difference between ATAG fault codes 1P1, 1P2, and 1P3?
They are all part of the same flow-check family. A 1P1 is a single event where the boiler detects its supply temperature rising by 7–15°C in one second, suggesting water flow has suddenly dropped. A 1P2 is a similar escalation at a different threshold within that family. A 1P3 specifically means the 1P1-type event has happened three times within a 15-minute period, so the boiler locks out rather than continuing to attempt firing. Think of 1P3 as the boiler saying it has already tried and failed three times in quick succession and is now refusing to carry on until the circulation problem is addressed.
My ATAG boiler is still under warranty — does that cover a 1P3 fault?
ATAG boilers come with a 10-year standard warranty, and during the warranty period ATAG dispatches their own engineers to carry out repairs. Contact ATAG directly on 0800 680 0100 to raise a warranty claim before arranging an independent engineer visit, as using a third-party engineer without prior authorisation could affect your warranty entitlement. Have your boiler's serial number and installation date to hand when you call.
How much does it cost to fix a 1P3 fault on an ATAG boiler?
For most households, the repair bill falls in the £150–£450 range. A straightforward pump replacement (including parts and labour) typically costs £200–£350. If the system needs a power flush to clear sludge, expect to add £300–£500 on top of any parts replaced. In a smaller number of cases where the plate heat exchanger has failed, costs can reach £500–£800. Getting an annual boiler service (usually £80–£120) is the best way to catch pump wear and sludge build-up before they escalate to a full breakdown.