ATAG 112 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 112 fault code mean?
Fault code 112 on an Atag boiler indicates a problem with the return temperature sensor, also known as the T2 or NTC probe. This sensor sits on the return pipework and continuously monitors the temperature of the water coming back into the boiler from the central heating circuit. It works by changing its electrical resistance as temperature rises or falls, feeding that information to the PCB so the boiler can regulate itself correctly. When code 112 appears, the PCB has detected that the T2 sensor is reporting a value outside the expected range — or no signal at all — suggesting the probe itself, its wiring, or the PCB connection may be at fault.
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
- Damaged or failed NTC return probe Common
The T2 thermistor can fail over time due to heat cycling, limescale build-up around the sensor pocket, or simply age-related degradation. When the probe's resistance drifts outside the range the PCB expects for real-world return temperatures, the boiler flags code 112 and locks out. This is the most frequently identified root cause of this fault.
- Loose, corroded, or damaged wiring to the T2 sensor Common
The wiring harness running from the return probe back to the PCB can suffer from loose connector pins, corrosion at the plug, or chafed cable insulation. Even a marginal increase in resistance caused by a poor connection can send the PCB an out-of-range signal and trigger the fault. Engineers will check the full length of the harness and reseat or replace connectors as needed.
- Moisture or corrosion at PCB connectors Sometimes
If there is a small water leak nearby — for example from an air vent — or if combustion products are recirculating into the casing, moisture can track along wiring and reach the PCB terminals. Corrosion at the board-side connector can mimic a failed sensor by distorting the resistance signal the PCB receives.
- PCB failure Rare
If the return probe and all associated wiring test within expected resistance values on a multimeter, the fault may lie with the PCB itself misreading the signal. PCB replacement is a last-resort diagnosis made only after all sensor and wiring checks have been exhausted.
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. Wait for the boiler to go through its start-up sequence. If code 112 clears and the boiler runs normally, monitor it over the next few hours. A one-off reset that holds can indicate a temporary sensor glitch. If the code returns promptly, move on — repeated resets will not fix an underlying hardware fault.
- Check the system pressure DIY safe
Look at the pressure gauge on the boiler. Atag recommends a cold-system pressure of around 1.0–1.5 bar. If the reading is below 0.8 bar, top up via the filling loop until it reaches roughly 1.2–1.5 bar, then attempt another reset. While low pressure alone is not the cause of code 112, a correctly pressurised system rules out any pressure-related knock-on effects and gives the engineer a clean baseline to work from.
- Check for any obvious leaks near the return pipework DIY safe
Have a careful visual look around the boiler casing and the visible return pipework for drips or damp patches. Do not touch any internal components — just note what you see so you can report it to the engineer. A leak close to the sensor or its wiring could explain corrosion-related faults.
- Inspect the T2 wiring harness and connectors Gas Safe engineer
A Gas Safe engineer will open the boiler casing and trace the wiring from the return probe back to the PCB. They will look for signs of chafing, heat damage, or corrosion at each connector and reseat or replace any suspect plugs. This step must not be attempted by a homeowner — internal boiler components carry live voltages and the task requires specialist knowledge.
- Test the return probe resistance with a multimeter Gas Safe engineer
The engineer will disconnect the T2 sensor and measure its resistance across the terminals, comparing the reading against Atag's published resistance-versus-temperature chart for NTC probes. A reading significantly outside the expected range at the current return temperature confirms a failed sensor. If resistance looks correct, attention shifts to the wiring and PCB connector.
- Replace the T2 return probe if faulty Gas Safe engineer
If the probe tests out of specification, the engineer will fit a replacement NTC sensor into the return pipework pocket, reconnect the wiring, and retest. The boiler should then be run through a full heat cycle to confirm code 112 does not return and that return temperatures are being read accurately.
- Check for moisture or corrosion at the PCB and rectify any leak source Gas Safe engineer
If the wiring and probe both test correctly, the engineer will examine the PCB connectors under good light for signs of oxidation or water ingress. Any leak source — such as a weeping air vent — must be fixed first. Corroded connector pins may be cleaned or the harness replaced. This work requires Gas Safe competency and familiarity with Atag board layouts.
- Replace the PCB if all other checks are clear Gas Safe engineer
When probe and wiring both measure correctly but fault code 112 persists, the PCB is the remaining suspect. PCB replacement is carried out by the engineer, who will source the correct board for the specific Atag model, fit and configure it, and run commissioning checks before leaving. This is the most expensive possible outcome so should only be reached after all earlier steps are completed.
- Call a Gas Safe registered engineer Gas Safe engineer
If you have attempted a reset and pressure check without success, the fault requires hands-on diagnosis by a Gas Safe registered engineer. For Atag boilers still within their warranty period (up to 10 years standard, extendable to 14 years with annual services), contact Atag directly on 0800 680 0100 before booking an independent engineer — a warranty repair could save you the full cost of parts and labour.
Parts you may need
- Atag T2 NTC return temperature sensor (probe) · from £20
- Sensor wiring harness / connector set · from £35
- Atag PCB (control board) — model specific · 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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Atag fault code 112 myself?
The safe homeowner actions are limited to resetting the boiler (once or twice) and checking the system pressure. Beyond that, diagnosing a faulty return probe involves opening the boiler casing, using a multimeter on internal components, and potentially replacing parts — all of which require a Gas Safe registered engineer. Do not attempt to open the boiler or touch internal wiring yourself.
How much does it cost to fix Atag error 112 in the UK?
For the most common repair — replacing the T2 return probe — you can typically expect a total bill of around £150–£320, covering call-out, labour, and the part. If the fault turns out to be corroded wiring rather than the probe itself, costs are usually at the lower end of that range. A PCB replacement, which is the rarest outcome, can push the total to £450–£600 or more depending on the engineer's rates and your location. Always check whether your boiler is within its Atag warranty before paying for any repair.
My Atag boiler is within warranty — will code 112 be covered?
Atag offers a standard 10-year parts-and-labour warranty, extendable to 14 years, provided the boiler has been serviced annually by a Gas Safe registered engineer. If your boiler meets these conditions, contact Atag's UK customer line on 0800 680 0100 before booking an independent engineer. A warranty claim could cover the full cost of a return probe or even a PCB replacement.
What is the T2 sensor on an Atag boiler and why does it matter?
The T2 sensor is an NTC thermistor fitted to the return pipework — the pipe that carries cooled water back into the boiler from the central heating circuit. It measures the temperature of that returning water and feeds the reading to the PCB, which uses it to control the boiler's output and protect against overheating or running inefficiently. If the T2 sensor fails or its wiring becomes faulty, the PCB can no longer trust the return temperature data and shuts the boiler down to prevent damage, displaying code 112.