ATAG 20 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 20 fault code mean?
Fault code 20 on an ATAG boiler signals that the boiler's control system has detected a problem with the flow sensor, known internally as T1. Specifically, the temperature or signal reading coming from T1 has fallen outside the expected operating range, so the boiler cannot confirm that water is circulating correctly through the primary circuit. The boiler locks out as a precaution to prevent it running dry or overheating. The root cause could be the sensor itself, the wiring connecting it to the PCB, a circulation problem preventing adequate flow, or a blockage in the pipework. A Gas Safe registered engineer will need to diagnose which of these is responsible.
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 degraded flow sensor (T1) Common
The T1 flow sensor can fail over time due to limescale build-up, corrosion, or simple component wear. When it does, the resistance values it reports to the PCB drift outside the acceptable range, immediately triggering fault code 20. This is the most frequent finding when an engineer investigates this code.
- Wiring or connector fault on the T1 circuit Common
The wiring loom and plug connectors linking the T1 sensor to the PCB are vulnerable to corrosion, chafing, and loose seating — particularly in older or heavily cycled boilers. A poor connection can produce intermittent or out-of-range signals that mimic a failed sensor, so an engineer will test the full harness before condemning the sensor itself.
- Circulation pump failure or reduced output Sometimes
If the pump is not moving water at the correct rate — whether due to bearing wear, air locking, or a seized impeller — the system flow conditions may not match what the sensor expects, causing the boiler to flag a flow check failure. The pump is often the culprit when the sensor and wiring both test within specification.
- Blockage or restriction in the primary circuit Sometimes
Sludge deposits, a partially closed isolation valve, or a blocked strainer can restrict water movement enough to cause abnormal flow readings at the T1 sensor. This is more common in older systems that have not been chemically flushed.
- PCB fault misreading the sensor signal Rare
In rare cases the PCB itself develops a fault on the input channel that reads the T1 sensor, producing false out-of-range errors even though the sensor and wiring are both healthy. An engineer can confirm this by testing the sensor in isolation.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button on the ATAG control panel as described in your user manual, then wait for the boiler to attempt a restart. If the code clears and the boiler runs normally, monitor it over the next few hours. A one-off lockout can occasionally result from a temporary flow disruption. If fault code 20 returns after one or two resets, stop resetting and move to the next steps — repeated resets without fixing the underlying problem will not help and could mask a developing fault.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler. ATAG boilers typically require a cold-fill pressure of around 1.0–1.5 bar. If the needle is below 1 bar, low system pressure can affect circulation and indirectly trigger flow-related faults. Top up using the filling loop following the instructions in your boiler's user guide, then reset and see if the code clears. If pressure drops again within a short time, there is likely a leak — call an engineer.
- Check that all heating zone valves and isolation valves are open DIY safe
Walk around your system and confirm that any manual isolation valves on the boiler's flow and return pipes are fully open (handle parallel to the pipe). Also check that motorised zone valves are not stuck closed. A partially closed valve can starve the primary circuit and upset flow sensor readings.
- Have a Gas Safe engineer test the T1 sensor resistance Gas Safe engineer
An engineer will use a multimeter to measure the resistance of the T1 sensor at known water temperatures and compare the readings against ATAG's published reference values. If the resistance is out of specification, the sensor is faulty and should be replaced. This work requires accessing internal boiler components and must be carried out by a Gas Safe registered engineer.
- Have the engineer inspect and test the T1 wiring harness Gas Safe engineer
If the sensor tests within range, the engineer will next check the wiring loom from the sensor back to the PCB, testing for continuity, shorts, and connector corrosion. Damaged sections of harness or corroded connectors will need repair or replacement.
- Have the circulation pump inspected and tested Gas Safe engineer
If both the sensor and wiring are healthy, the engineer will check that the pump is running, spinning freely, and delivering adequate flow. A seized, air-locked, or worn pump should be replaced. Some ATAG models allow the pump to be manually bled; an engineer can do this safely during the same visit.
- Consider a power flush if system sludge is suspected Gas Safe engineer
If the engineer finds evidence of heavy sludge or magnetite in the system — common in older installations without a magnetic filter — a power flush may be recommended to clear blockages that are restricting flow past the sensor. Adding an inhibitor and magnetic filter afterwards helps prevent recurrence.
- If all components test healthy, have the PCB investigated Gas Safe engineer
A Gas Safe engineer or ATAG-authorised technician can determine whether the fault lies in the PCB's sensor-input circuitry. PCB repair or replacement is specialist work and should only be undertaken after other causes have been ruled out.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If you have completed the homeowner-safe checks above and the boiler continues to display fault code 20, contact a Gas Safe registered engineer. Before booking a private repair, check whether your boiler is still within its ATAG warranty period — ATAG boilers carry a standard 10-year warranty (extendable to 14 years). If covered, contact ATAG on 0800 680 0100 or reach out to your original ATAG Selected installer.
Parts you may need
- ATAG T1 flow temperature sensor · from £25
- Wiring harness / sensor loom (ATAG compatible) · from £35
- Central heating circulation pump (Grundfos or Wilo equivalent) · from £85
- ATAG PCB (main 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix ATAG fault code 20 myself?
The only homeowner-safe actions are resetting the boiler once or twice, topping up system pressure via the filling loop if it is low, and confirming that all isolation valves are open. Beyond that, diagnosing and replacing the T1 sensor, testing wiring, or working on the pump all require access to internal boiler components and must be carried out by a Gas Safe registered engineer. Attempting this work yourself would be unsafe and would likely invalidate your ATAG warranty.
How much does it cost to repair ATAG fault code 20 in the UK?
For most households, the repair falls between £120 and £350 including parts and labour. A straightforward T1 sensor swap — the most common fix — typically comes in at the lower end of that range once you include a call-out charge. Replacing the circulation pump adds more, and if the fault turns out to be the PCB the bill can climb higher; PCB replacement on ATAG boilers can cost £300–£500 or more depending on the model and labour rates in your area. Prices in London and the South East tend to run higher than in northern regions.
My ATAG keeps showing code 20 after I reset it — is it safe to keep resetting?
No. Repeatedly resetting the boiler without addressing the underlying cause is not recommended. Each time the boiler restarts without adequate flow, there is a risk of overheating internal components. Limit resets to two or three attempts while carrying out the basic homeowner checks (pressure, isolation valves), then call a Gas Safe engineer to investigate properly.
Is ATAG fault code 20 covered under the ATAG warranty?
ATAG boilers come with a standard 10-year parts and labour warranty, which can be extended to 14 years when the boiler is registered and serviced by an ATAG Selected installer. If your boiler is within this period, a T1 sensor failure or wiring fault would typically be covered at no cost to you. Contact ATAG's UK support line on 0800 680 0100 or get in touch with the installer who originally fitted the boiler to arrange a warranty assessment before paying for a private repair.