Overview
Error code 141 on an ATAG boiler means the central heating flow switch is reporting an "open" condition — in other words the boiler’s control thinks there is no flow in the central heating circuit when a demand for heating is present. The flow switch is a safety/interlock device that detects water movement from the boiler pump or through the CH flow pipe and prevents the burner from operating if there is no flow. Typical causes are: the pump not running, a closed motorised zone valve, airlocks or blockages in the system, low system pressure, wiring or connector faults to the flow switch, or a failed flow switch itself. Severity is generally moderate: the boiler will usually refuse to light for central heating while the fault exists, so you will lose CH until it is fixed, but it is not an immediate danger provided you do not try to bypass safety devices. Some initial checks and basic actions can be done by a competent homeowner (pressure check, bleed radiators, reset the boiler, check programmer/thermostat), but diagnosis and any work on electrical parts, the gas supply, pump replacement or replacing the flow switch should be carried out by a qualified Gas Safe engineer. Do not leave safety interlocks bypassed or perform gas work yourself.
Possible Cause: Central heating flow switch is open
Troubleshooting Steps
Safety precautions:
- If you are not competent with electrical or gas appliances, stop and call a Gas Safe registered engineer. Do not attempt gas component repairs.
- Before any electrical checks isolate the boiler at the fused spur or consumer unit. Use insulated tools and follow basic electrical safety.
- Do not bypass the flow switch permanently — it is a safety device. Temporary, supervised diagnostic tests are for trained engineers only.
- Protect the area from hot water spills if you bleed radiators or open valves.
Initial homeowner checks you can do (no tools or simple tools only):
1. Check the boiler display: note the code 141 and try a single reset using the boiler reset/central heating reset procedure in the manual. If the code recurs immediately, further checks are needed.
2. Check CH demand controls: ensure the room thermostat and programmer are calling for heat and set above the room temperature. Set the boiler to CH mode and observe.
3. Check system pressure on the boiler gauge. Typical pressure when cold should be around 1–1.5 bar (consult your manual). If pressure is very low (<0.7 bar) repressurise the system following the manufacturer instructions, then try again.
4. Listen at the boiler when CH is called: can you hear the pump running for a short time? Can you hear zone valves moving? Walk round and see if radiators begin to warm (top first). If nothing runs, that suggests pump/power/valve issue.
5. Bleed radiators starting at the highest radiator to remove any trapped air that might prevent flow and create an airlock. After bleeding, re-check system pressure and top up if needed.
Diagnostic steps (for competent DIYers with basic tools and electricians only if competent):
1. Confirm pump operation: with CH demand active, place your hand on the pump housing (turn power off first if you will touch electrical parts) or just listen for running noise. If the pump does not run but you hear a click when the boiler tries to start, the pump may be unpowered or seized.
2. Check zone/motorised valve position: if you have motorised valves, open a CH feed valve or manually operate the actuator (consult manual) to confirm flow. If valves are closed or the microswitch isn’t indicating open, the flow switch may see no flow.
3. Check wiring to the flow switch: isolate power, remove the boiler cover and visually inspect the flow switch and wiring connectors for loose wires, corrosion or obvious damage. Re-seat connectors. Only do this if you are confident and the boiler is isolated.
4. Electrical continuity test (only if you are skilled and comfortable with meters): with power isolated, test the flow switch contacts for continuity while simulating flow if possible (some flow switches have a small lever). If the switch shows open when it should be closed, the switch is faulty.
5. Check for blocked/immobile pump: if pump rotates slowly or is seized it will not create flow. Some pumps can be freed by carefully turning the shaft (manufacturer guidance required). If pump is electrically receiving power but not turning, replacement is likely.
6. Check for blockages or closed isolating valves in the flow pipe, magnetic filters clogged or an obstruction at the pump/heat exchanger area that would prevent flow.
What to do depending on findings:
- If low pressure: repressurise the system to the correct level, clear air by bleeding radiators, then reset the boiler. If the fault clears, monitor for recurrence; persistent pressure loss needs engineer attention.
- If airlock suspected: bleeding radiators and cycling the pump/valves may clear it. If you cannot clear the airlock, call an engineer.
- If pump does not run and has no electrical supply: check the fused spur and boiler fuses. If those are OK, do not attempt to replace the pump unless you are qualified — call an engineer.
- If pump runs and valves open but flow switch still reports open: likely the flow switch is defective or its wiring to the PCB is faulty. This usually requires replacement of the flow switch and/or repair by a qualified engineer.
- If wiring or connector damage is found: isolate power and have a Gas Safe engineer repair/replace wiring/components.
When to call a professional:
- If any electrical components (pump, flow switch, PCB wiring) need testing or replacement.
- If you smell gas, have gas supply concerns, or suspect ignition/gas valve issues — isolate gas if safe and call Gas Safe immediately.
- If you cannot identify the cause after the basic checks (pressure, bleeding radiators, reset) or the fault reoccurs.
Final notes:
- Do not bypass safety interlocks to make the boiler run. Permanent bypasses are dangerous and void warranties.
- When you call a Gas Safe engineer, tell them the boiler model and that it is showing ATAG error 141 (central heating flow switch open) and describe the checks you have already done. This will help them diagnose faster.
- If the flow switch needs replacement, only a qualified engineer should fit and test it and then run the boiler through safe operation checks.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.