Overview
Error code 304 on an ATAG boiler means the control unit has detected repeated reset attempts and has entered a protective state. The boiler’s electronics count how many times the unit has been restarted in a short period; if that number exceeds a preset limit the boiler will show 304 and either temporarily block operation or lock out to prevent damage or unsafe running. The underlying reason is not the resets themselves but whatever fault is causing the repeated restarts: an intermittent electrical fault, persistent sensor or gas/ignition problem, a PCB/controller fault, or repeated user resets after unsuccessful attempts to clear a different error. Severity is moderate to high. A single occurrence after a known transient (power cut, momentary fault) may clear after a proper reset and monitoring, but repeated or persistent 304 indicates an ongoing fault that needs diagnosis. Basic checks can be done by a competent homeowner (power, pressure, frozen condensate pipe, wiring to the fused spur), but because this code often masks a deeper electrical or control fault and involves gas and mains electricity, a qualified Gas Safe registered engineer should be called if the code does not clear after simple, safe checks or if you are unsure. Do not repeatedly press the reset button as that can make the issue worse and may force a full lockout.
Possible Cause: Too many resets have occured
Troubleshooting Steps
Safety first:
Do not open the boiler casing or touch internal components unless you are a qualified Gas Safe engineer. If you smell gas, evacuate the property and call the emergency gas number immediately. Isolate power to the boiler at the fused spur before any non-electrical checks. Take care with water and electricity around the appliance.
Initial checks a homeowner can safely perform:
1) Note the circumstances and timing: did the code appear after a power cut, freeze, or after pressing reset multiple times? Record the code and any other messages or symbols on the display.
2) Power supply: check the boiler is switched on at the fused spur and the household consumer unit (check RCDs/MCBs have not tripped). If mains power was unstable, allow the boiler to sit powered off for 1 minute, then turn it back on and observe.
3) Boiler reset procedure: perform a single reset using the boiler’s reset button or control panel (follow the manual). Wait and observe for several minutes. Do not press reset repeatedly. If the code clears and the boiler runs normally for an hour, monitor for recurrence.
4) System pressure: check the pressure gauge. If pressure is very low (<0.5 bar) or very high (>pmax shown in manual), follow the manufacturer’s guidance to repressurise or isolate. Low pressure can cause fault sequences leading to resets.
5) Condensate pipe: in freezing weather check the condensate drain outside for ice blockages and thaw gently if frozen. A blocked condensate can provoke repeated faults.
6) Look for visible leaks or obvious signs: water leaks, scorch marks, loose external wiring at isolator, or fault LED patterns. Take photos for an engineer if you call one.
If the error returns after the single reset or the boiler will not restart:
1) Do not continue to reset the boiler repeatedly. Repeated reset attempts will increase lockout and may hide the root cause.
2) Isolate the boiler power at the fused spur for five minutes, then restore power and attempt one reset. If 304 reappears immediately, this suggests an internal electrical/PCB or persistent fault.
3) If you have basic electrical knowledge and a multimeter and the boiler is isolated, you can check that the fused spur has live, neutral, earth present and that the incoming supply fuse is intact. If uncertain, stop and call an engineer.
4) If there were other codes before the 304 (for example ignition, sensor or pressure codes), record them. The earlier code is the likely root cause; give those details to the engineer.
When to call a professional (and why):
1) Call a Gas Safe registered heating engineer if the 304 code persists after the safe checks above, if other fault codes appear, or if you are not comfortable performing the basic visual checks. This code often indicates a recurring fault or an electronic/PCB issue that requires specialist diagnosis and tools.
2) The engineer will perform controlled diagnostics: read full error/history from the boiler, test sensors, ignition and gas supply, check the PCB and wiring, and perform safe electrical tests. They can confirm whether the fault is a temporary glitch, a failed component (fan, sensor, PCB), or an installation/electrical supply issue.
3) If the engineer advises isolation until repair, follow that instruction. Do not attempt internal repairs yourself.
What to tell the engineer when you call:
Provide the boiler model, the exact error code (304) and any other codes that appeared, the sequence of events (power cut, frost, repeated resets), photographs of the display and external pipework (condensate), and any actions you have already taken.
Summary: try one controlled reset and simple external checks (power, pressure, condensate, visible damage). If 304 persists or other faults are present, stop and arrange a Gas Safe qualified engineer to diagnose and repair the underlying cause rather than repeatedly resetting the boiler.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.