ATAG 201 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 201 fault code mean?
Fault code 201 on an Atag boiler indicates that the T3 NTC thermistor — the temperature sensor responsible for monitoring domestic hot water (DHW) or boiler flow temperature depending on the model — is reporting values outside the acceptable operating range. The PCB is receiving a signal that suggests an open circuit, short circuit, or a reading that simply does not match expected parameters. On iC and iCE combi boilers, this relates specifically to the T3 hot water sensor; on iS and iR system and regular boilers, the T3 boiler sensor is the one flagged. In either case the boiler will typically lock out until the fault is resolved.
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 failed T3 NTC thermistor Common
The T3 sensor itself is the most frequent culprit. NTC thermistors degrade over time, particularly when exposed to high temperatures and limescale. A failed sensor will produce a resistance reading that falls outside the specification table, triggering code 201.
- Loose, corroded, or damaged wiring Common
The wiring harness connecting the T3 sensor to the PCB can develop loose connections, corrosion at the connector pins, or breaks — especially if the cable has been extended or disturbed during a previous service. The PCB connector at X7 pins 8 and 9 is worth checking first.
- T3 sensor not connected (iS and iR models) Sometimes
On system and regular boilers, the hot water priority DHW NTC sensor occasionally becomes unplugged during servicing or installation. If the sensor is simply disconnected rather than damaged, reconnecting it will clear the fault.
- Incorrect PCB parameter setting (non-combi boilers) Sometimes
On iS and iR boilers, parameter 2.2.8 controls whether the DHW sensor circuit is active. If this parameter is set incorrectly — for example after a PCB swap or factory reset — the boiler may raise code 201 even when the sensor hardware is perfectly healthy.
- Dirty or corroded thermistor probe Rare
A build-up of limescale, sludge, or corrosion on the sensor probe surface can distort its resistance readings without the sensor itself being electrically faulty. In some cases cleaning rather than replacing the probe resolves the fault.
How to fix it
- Check your gas supply is on DIY safe
Before anything else, confirm that the gas supply to the property is working — check other gas appliances such as a hob or fire. If there is no gas supply, contact your gas supplier rather than your boiler engineer.
- Reset the boiler (up to twice) DIY safe
Press the reset button on your Atag boiler and allow it to attempt a restart. If the fault clears and the boiler runs normally, monitor it over the next few hours. If code 201 returns, do not continue resetting — repeated resets without clearing the underlying fault can mask a developing problem.
- Check whether the boiler is still under warranty DIY safe
Atag offers a 10-year parts and labour warranty on domestic boilers, provided annual Gas Safe servicing has been carried out. If your boiler is within the warranty period, contact Atag Heating UK on 0800 680 0100 before arranging an independent repair, as the work may be covered at no cost.
- Inspect the T3 sensor wiring and connections Gas Safe engineer
A Gas Safe engineer should visually inspect the full T3 wiring harness and all connector pins for signs of corrosion, heat damage, or loose contacts. On combi models the relevant PCB connector is at X7 pins 8 and 9. Disconnecting the plug before resistance testing ensures the PCB does not interfere with the reading.
- Test T3 sensor resistance with a multimeter Gas Safe engineer
With the sensor connector removed from the PCB, a Gas Safe engineer should measure the resistance across the T3 probe terminals and compare the reading against Atag's resistance-versus-temperature table. A reading significantly outside the expected range at the current water temperature confirms sensor failure. This test also checks the wiring harness continuity at the same time.
- Visually inspect the probe and surrounding area for leaks Gas Safe engineer
The engineer should physically examine the DHW probe for cracks, scale deposits, or corrosion. Any water leak near the sensor can alter its readings and cause intermittent faults. Leaks should be repaired before the sensor is replaced.
- Clean the probe if contamination is found Gas Safe engineer
If the sensor resistance is close to specification but not quite within range, limescale or sludge on the probe surface may be the cause. The engineer may elect to clean the probe rather than replace it immediately, then retest.
- Check and adjust parameter 2.2.8 (iS and iR boilers only) Gas Safe engineer
On system and regular boilers, the engineer should navigate to parameter 2.2.8 on the PCB menu and verify that the DHW sensor setting matches the actual boiler configuration. If adjustment alone does not resolve the fault, a factory reset via parameter 2.8.0 may be required.
- Replace the T3 NTC thermistor if faulty Gas Safe engineer
If testing confirms the sensor is out of specification and cleaning does not help, the Gas Safe engineer will fit a replacement T3 NTC sensor. After fitting, the boiler should be reset and monitored through a full heating cycle to confirm the fault has cleared. If the code persists after sensor replacement, a PCB factory reset should be performed.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If code 201 continues after sensor replacement and a PCB factory reset, further diagnostics are needed — potentially including PCB inspection or replacement. Only a Gas Safe registered engineer should carry out this work. You can find one at gassaferegister.co.uk.
Parts you may need
- Atag T3 NTC thermistor (DHW / boiler sensor) · from £25
- Sensor wiring harness · from £35
- PCB connector / terminal block · from £15
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–£280, depending on the underlying cause.
Frequently asked questions
Is Atag fault code 201 dangerous?
Code 201 is not a gas leak or combustion safety fault, so it does not pose an immediate danger. However, because the boiler locks out when this code appears, you will lose heating and hot water. It should still be treated as a priority repair, particularly in cold weather or if you have young children or elderly occupants in the home.
Can I fix Atag fault code 201 myself?
The only safe DIY steps are confirming your gas supply is on and resetting the boiler once or twice to see if the fault self-clears. Everything else — testing sensor resistance, adjusting PCB parameters, replacing the probe — requires a Gas Safe registered engineer. Working on boiler sensors and wiring without the appropriate qualifications is unsafe and could also invalidate your warranty.
My boiler is an Atag iS or iR, not a combi — does code 201 mean something different?
Yes, slightly. On iS and iR system and regular boilers, code 201 refers to the T3 boiler sensor rather than a domestic hot water probe. It can also be triggered by an incorrect setting in PCB parameter 2.2.8 rather than a physically faulty sensor. An engineer should check the parameter first on these models before assuming the sensor itself needs replacing.
How much does it cost to fix Atag fault code 201 in the UK?
For most people, a straightforward T3 sensor replacement including the engineer call-out and one hour's labour comes to around £100–£280 depending on your location and whether the work falls within normal working hours. Weekend or emergency call-outs can push the cost higher. In a small number of cases where the PCB also needs to be replaced, the bill can rise to £400 or more — but this is uncommon for a code 201 fault on its own. If your boiler is within Atag's 10-year warranty period, the repair may be covered at no charge, so always check with Atag (0800 680 0100) first.