Overview
Error code 61 on an ATAG boiler means the boiler has detected a bus communication error. The “bus” is the low-voltage communications pathway between the boiler’s main control PCB and other modules or peripherals (display, sensors, pumps, room/stat connections or any expansion modules). When the control unit cannot communicate reliably over that bus it will log code 61 and may lock out or fall back to a safe mode to protect the appliance. This fault can be caused by simple issues such as a loose connector, damaged cable, a disconnected room thermostat or sensor, corrosion or water ingress, or by a failing PCB or interface module. Severity ranges from a minor intermittent communication problem (boiler may still operate intermittently) to a hard fault that prevents the boiler from operating. Because the bus is part of the boiler’s control and safety systems, diagnosing and repairing persistent bus faults often requires a qualified Gas Safe engineer — homeowners can perform basic checks and resets safely, but do not attempt gas, PCB replacement or complex electrical work unless you are competent and qualified.
Possible Cause: Bus communication error
Troubleshooting Steps
Safety first:
1) If the boiler is showing error 61, do not try to bypass the fault. Isolate the boiler mains electrical supply before opening the casing. If you are unsure how to isolate safely, stop and call a Gas Safe engineer. Do not work on gas or high-voltage components unless qualified.
2) Take photographs or notes of the display, any other fault codes, and the wiring before you touch anything. That information will help an engineer if you need to call one.
Initial homeowner checks you can safely do:
1) Reset the boiler using the normal reset procedure in the user manual (usually switch off at the fused spur or boiler front panel, wait 30–60 seconds, then switch back on). Record if the fault cleared and whether it returns immediately or intermittently.
2) Check for obvious signs of damage or moisture around the boiler, especially where wiring enters/exits the casing and around the PCB area. Look for disconnected plugs, crimp terminals pulled out, rodent chew marks or water stains.
3) Check any external controls: room thermostat, wireless receivers, programmers, or smart controls. If you have recently changed or removed a thermostat or wireless module, reconnect or restore it to rule out a missing device on the bus.
4) If the boiler was recently serviced, moved or worked on, ask the person who serviced it whether any connectors were removed or left loose.
Basic diagnostic steps (for competent DIYers only; isolate mains power first):
1) With power isolated and using the photos/notes you took, open the boiler casing and visually inspect connectors on the main PCB and any plug-in modules. Gently reseat any multi-pin connectors and ribbon cables — do not force them. Ensure locking clips are engaged.
2) Inspect wiring for broken conductors, frayed insulation, or corrosion. Pay particular attention to multi-core cables that run to external controls or sensors.
3) If you have a multimeter and are competent with low-voltage wiring: with power OFF, check continuity of the bus pair(s) between the PCB connector and any external module/thermostat connectors. If continuity is open or reads inconsistent, that indicates a broken wire or poor connection. Do not attempt live voltage checks unless you are trained.
4) After visual checks and reseating connectors, restore power and observe the boiler. If the fault is intermittent and clears, try to reproduce the problem by operating external controls (call for heat via thermostat) and watch for the error returning.
When to call a professional and what to tell them:
1) If the code returns after a reset, you find damaged wiring, you see burnt components, water ingress, or you are not comfortable performing the electrical checks, stop and call a Gas Safe registered heating engineer. Bus faults frequently require specialist testing equipment and manufacturer firmware/part replacement.
2) When contacting an engineer, have the boiler model and serial number ready, note the exact error code (61) and any other codes or symbols, describe what you checked (reset, reseated connectors, any visible damage), and provide photos if possible. That speeds diagnosis and helps the engineer bring the right parts.
Important cautions:
1) Do not attempt to replace the PCB, gas valve, fan or any gas-carrying parts yourself. PCB replacement and wiring repairs that affect safety must be done by a qualified engineer.
2) Do not bypass safety interlocks or override errors — these protect against dangerous conditions.
Likely professional actions:
1) A Gas Safe engineer will run detailed diagnostics, test the bus signalling and voltages, check continuity under working conditions, and may replace a faulty cable, connector or control module. If the main PCB is at fault they will follow manufacturer procedures for replacement and configuration.
If the issue is persistent, avoid repeated resets and arrange a professional service visit. Bus communication faults affect control and safety, and while some simple connector reseating may fix the problem, a full repair should be done by a trained engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.