Error 104

Safety first:

1. If you smell gas, evacuate the property and call the gas emergency number immediately. Do not operate electrical switches or the boiler.

2. Before touching the boiler, turn it to standby/off at the programmer and isolate mains power if you will open any covers. Only qualified engineers should remove sealed covers or access gas/electrical components.

3. Wear gloves and eye protection for any plumbing tasks. Allow the boiler to cool before touching hot pipes or components.

Initial homeowner checks you can safely perform:

1. Read and note the exact error code and any other display messages. Try a single reset of the boiler using the normal reset procedure in the manual (don’t repeatedly reset). If the code returns, proceed with checks.

2. Check the system pressure gauge on the boiler. Most ATAG systems run around 1–1.5 bar when cold. If pressure is very low, the boiler may flag a flow problem. If the pressure is low and you know how to use the filling loop, repressurise the system to the correct level following the boiler manual. If you are unsure, call an engineer.

3. Check for obvious leaks around radiators, pipes and valves. A leak will drop pressure and affect flow. If you find a leak, isolate the water and call a plumber/engineer.

4. Bleed radiators to remove trapped air which can cause poor flow or pump cavitation. After bleeding, re-check and top up system pressure if needed.

5. Ensure all manual isolation valves to and from the boiler are in the open position (especially any pump isolation valves or zone valves). Confirm any motorised valves are in the correct position for heating demand.

6. Listen for the pump when the system should be calling for heat. You may be able to hear or feel the pump running. If the pump is silent when it should be running, or sounds loud/rough, this suggests a pump issue requiring an engineer.

More specific diagnostic steps (if comfortable and safe to observe):

1. After checking pressure and valves, restart the boiler once and observe: does the error clear and the boiler operate normally, or does 104 return quickly? Temporary clearing followed by reappearance points to intermittent sensor, pump or wiring issues.

2. Inspect visible wiring and connections around the flow/temperature sensor and any accessible flow switch for loose connectors or obvious damage. Do not probe live terminals unless you are qualified.

3. Check for blockages: if your system has a magnetic filter or boiler strainer, inspect it if you know how to do so safely and isolate the boiler first. A blocked filter or debris in the system can hinder flow.

4. Consider whether external conditions could cause flow problems (frozen external pipes in cold weather, recently drained or refilled system causing airlocks, or recent work on the heating system).

When to call a professional:

1. Call a Gas Safe registered heating engineer if the error persists after the basic checks, if you find low pressure you cannot safely correct, if you hear pump failure, if any wiring or internal components look damaged, or if there are leaks. Do not attempt to replace the pump, flow sensor, PCB or any gas-carrying parts yourself.

2. Provide the engineer with: the exact error code (104), what you observed, the pressure reading, whether bleeding radiators or repressurising changed anything, and any recent changes to the system.

Important notes:

1. Do not keep resetting the boiler repeatedly; persistent faults need proper diagnosis. Repeated restarts can mask an underlying issue and may lead to lockouts or component stress.

2. Electrical and gas-related repairs must be performed by qualified professionals. Sensors, pumps, control boards and sealed components require specialist tools and testing.

3. If the boiler is still under warranty, check terms before any work is carried out and use an authorised service engineer where required.