ATAG 304 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 304 fault code mean?
Fault code 304 on ATAG boilers (including the iC, iS, and iR ranges) means the boiler has been reset too many times within a 15-minute window and has locked itself out as a safety measure. Crucially, 304 is a secondary lockout code rather than the root fault — it is the boiler's way of telling you that an underlying problem kept causing it to trip, and that repeatedly pressing reset is not solving it. Common companion faults that trigger repeated resets include ignition failures (such as fault 501, 'no flame detected'), low system pressure, and gas supply interruptions. Identifying and fixing that underlying issue is the only lasting solution.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Repeated ignition failures triggering auto-reset cycles Common
The most common reason for code 304 is an ignition problem — typically fault 501 ('no flame detected') — that has caused the boiler to lock out and be reset several times in quick succession. Dirty or worn ignition electrodes, a weak spark, or a faulty flame sensor can all prevent the burner from lighting reliably, leading to repeated lockouts.
- Low system water pressure Common
If the boiler's water pressure has dropped below the safe operating threshold (normally around 1–1.5 bar on ATAG boilers), it may lock out repeatedly. Each manual reset works briefly before the pressure issue causes another lockout, and several attempts within 15 minutes will trigger code 304.
- Interrupted or insufficient gas supply Sometimes
A partly closed gas isolation valve, a problem with the meter, or low mains gas pressure can prevent the burner from sustaining a flame. The boiler locks out, gets reset, fails again — and code 304 follows.
- Faulty gas valve Sometimes
A gas valve that is sticking, not opening fully, or not responding correctly to the control board's signals can cause intermittent ignition failures. This tends to produce a recurring lockout pattern that quickly accumulates enough resets to trigger code 304.
- PCB or wiring fault Rare
Code 303 is the direct PCB error on ATAG boilers, and it sometimes appears alongside or before code 304. A failing control board or loose/corroded wiring connection can cause spurious lockouts or mis-readings from sensors, prompting repeated resets. Stress the board further with more resets and the fault can worsen.
How to fix it
- Stop resetting the boiler DIY safe
Further resets will not clear code 304 and can place additional stress on the PCB. Switch the boiler off at the programmer or room thermostat and leave it alone while you carry out the checks below.
- Check your gas supply DIY safe
Confirm that the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Try another gas appliance such as a gas hob — if that also fails to light, there may be a supply issue affecting the whole property. Contact your gas network operator if you suspect a mains supply problem.
- Check and restore system pressure if needed DIY safe
Look at the pressure gauge on the boiler. The normal operating range for ATAG boilers is 1–1.5 bar when cold. If it reads below 1 bar, top up via the filling loop (two valves beneath the boiler) until the gauge reads around 1.2 bar, then close both valves securely. If the pressure has been dropping repeatedly, there may be a leak that needs a Gas Safe engineer to investigate.
- Attempt a single reset DIY safe
Once you have checked the gas supply and corrected any low pressure, press the reset button on the boiler control panel once and observe what happens over the next few minutes. If the boiler fires and runs normally, monitor it over the next day or two. If it locks out again — do not keep resetting.
- Call a Gas Safe registered engineer for full diagnosis Gas Safe engineer
If the boiler locks out again after a single reset, or if code 304 reappears, a Gas Safe engineer must attend. They will retrieve any stored fault history, inspect the ignition electrode and flame sensor for wear or contamination, test the gas valve, check the PCB for signs of damage or burned tracks, and verify all wiring connections. Only ATAG-approved service parts should be used for any replacements.
Parts you may need
- Ignition electrode · from £35
- Flame sensor (ionisation electrode) · from £30
- Gas valve · from £180
- PCB (main control board) · from £320
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting an ATAG boiler showing code 304?
No — and the boiler itself is telling you the same thing by locking out after too many resets. Code 304 is a safety mechanism that forces you to investigate the root cause rather than mask it with repeated resets. Continuing to reset without a fix can stress the PCB and potentially turn a straightforward repair into a more expensive one.
What is the underlying fault that usually causes code 304?
Most commonly it is an ignition problem — ATAG fault 501 ('no flame detected') is a frequent companion code. Low system pressure, an interrupted gas supply, or a faulty gas valve are also common culprits. A Gas Safe engineer will read the boiler's fault log to identify which fault was repeating before code 304 was triggered.
Could this be a PCB problem?
It is possible, especially if ATAG fault code 303 (a direct PCB error) has also appeared on your boiler. A faulty PCB can cause spurious lockouts that accumulate into a code 304 situation. PCB repair by a specialist typically costs £150–£250, while a full PCB replacement usually runs £400–£600 including labour — significantly more than most other repairs, so engineers will normally rule out simpler faults first.
My ATAG boiler keeps losing pressure — is that why I'm seeing code 304?
Quite possibly. If the boiler's pressure drops below the safe threshold it will lock out, and if you have been topping it up and resetting it several times, code 304 can be the result. Topping up the pressure is a temporary fix; the reason for the pressure loss — which could be a weeping radiator valve, a small leak on a pipe, or a faulty expansion vessel — needs to be found and repaired by a Gas Safe engineer.