Error 112

Safety precautions:

1) If you smell gas or suspect a gas leak, leave the property immediately and call the gas emergency number—do not investigate the boiler. 2) Before any work on the boiler, switch off the electrical supply at the fused spur and isolate from mains. Allow the boiler to cool. 3) Do not tamper with sealed gas components or the burner assembly; those tasks must be done by a Gas Safe registered engineer. 4) Use appropriate PPE (safety glasses, gloves) and a multimeter rated for service work if you will be measuring electrical values.

Initial homeowner checks (safe, low-skill):

1) Note any other fault symbols or codes on the display and the exact behaviour (CH not working, only DHW ok, spanner/service symbol). 2) Try a single reset of the boiler (switch off at the fused spur, wait 30 seconds, switch back on) and see if the code returns. Some transient errors clear, but recurring 112 indicates persistent sensor issue. 3) Check visible wiring and pipework in the boiler's accessible covers for obvious damage, corrosion, or water ingress around sensor cable entries; look for loose connectors. 4) Check system water pressure on the gauge and basic system faults (bleed radiators if cold, ensure the filling loop is closed) — low system pressure can sometimes lead to unusual behaviour though it is not the primary cause of code 112.

Specific diagnostic steps (requires basic electrical/test tools and caution):

1) Locate the return probe: consult the boiler manual or schematic. The return probe is usually mounted in the return manifold or hydraulic block and has a 2- or 3‑wire cable to the control PCB. If unsure, stop and call an engineer. 2) With the boiler isolated from mains power, carefully disconnect the probe connector at the PCB or at its plug connection (if accessible). Inspect the connector pins for corrosion, water, or damage. Reconnect firmly if clean. 3) Use a multimeter set to measure resistance (ohms). Measure across the probe wires. If the meter shows open circuit (infinite resistance) or a very low short, the probe is likely faulty or the cable is broken. 4) Dynamic test: gently warm the probe (e.g., with warm water) and observe the resistance change — a functioning thermistor/NTC probe will change resistance as temperature changes. If the resistance does not move at all, the probe is defective. 5) Check wiring continuity back to the PCB: measure continuity from the probe connector to the PCB terminal to confirm there is no break in the cable. Also check for short to earth/ground. 6) If probe resistance and cable continuity are okay, but the boiler still reports 112, inspect the PCB connector for loose pins or damage. Faulty wiring or a defective input on the PCB can cause the same code.

Possible fixes (when safe and competent to do so):

1) Clean corroded connectors and re-seat the probe connector. 2) If the probe cable is damaged and you can access a short length, a qualified person may replace the section or the whole cable; ensure correct routing and secure fastening. 3) If the probe itself is confirmed faulty, replace it with the correct ATAG spare part or equivalent specified in the manual. Ensure replacement sensor has the correct thermistor characteristics. 4) After repair or replacement, restore power and clear the fault (use the boiler reset procedure). Observe several cycles of operation to confirm normal behaviour and that the code does not reappear.

When to call a professional and final notes:

1) Call a Gas Safe / qualified heating engineer if you are not confident doing the electrical checks, if the probe needs replacing, or if the fault persists after the basic checks. 2) If you find wiring or PCB damage, or if multiple sensors are failing, do not continue to operate the boiler — arrange an engineer. 3) Always use the manufacturer’s recommended spare parts and follow the boiler service manual. 4) Repeated lockouts or additional fault codes alongside 112 are best handled by a trained engineer who can test the control PCB, bus communications and replace sensors safely.

Note: this guidance is diagnostic and safety-oriented. Do not attempt gas or burner work yourself; replacement of internal components and any work that requires opening the combustion chamber must be carried out by a qualified engineer.