Chaffoteaux 304 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 304 fault code mean?
Fault code 304 on a Chaffoteaux boiler is a deliberate safety lockout that activates when the reset button has been pressed more than five times within a 15-minute period. The boiler recognises this pattern as a sign that something more serious may be wrong and refuses to accept any further reset attempts from the front panel. Importantly, code 304 is not itself a component failure — it is the boiler's way of telling you to stop repeatedly restarting it and find out why it keeps tripping in the first place. The underlying problem could be anything from a persistent ignition fault to a flame-sensing issue, and that root cause is what genuinely needs addressing.
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 reset attempts by the user Common
The most immediate reason for code 304 is simply that the boiler has been reset too many times too quickly — more than five presses within 15 minutes. This is usually the result of frustration when the boiler keeps cutting out, but the repeated resets trigger this secondary lockout on top of whatever the original fault was.
- Underlying ignition or flame-detection fault Common
If the boiler was locking out repeatedly before code 304 appeared, there is very likely an ignition problem or a faulty ionisation electrode causing it to fail to detect a stable flame. Each failed start leads to a lockout, the user resets, and the cycle repeats until the 304 threshold is reached.
- Intermittent gas supply issue Sometimes
A temporary interruption to the gas supply — such as low pressure at the meter, a partially closed service valve, or a prepayment meter running out of credit — can cause repeated lockouts as the burner cannot light or stay alight. The user resets repeatedly trying to restore heat, triggering the 304 lockout.
- Combustion or air supply problem Sometimes
A blocked flue, restricted air intake, or a faulty fan can prevent proper combustion. The boiler's safety sensors detect abnormal combustion conditions and shut the unit down, leading to repeated manual resets and eventually code 304.
- PCB or control electronics fault Rare
An intermittently failing printed circuit board can cause the boiler to lock out without any clear pattern or secondary fault code. This is less common but should be considered if no other cause is identified, particularly in older boilers.
How to fix it
- Stop pressing the reset button DIY safe
Further reset attempts will not clear this lockout and may complicate the diagnosis. The boiler's front panel controls will not respond to the reset button while code 304 is active, so pressing it again serves no purpose.
- Switch the boiler off at the isolator and wait 30 seconds DIY safe
Locate the dedicated double-pole isolator switch — usually on the wall near the boiler — and turn it off. Leave the power off for at least 30 seconds to allow the boiler's electronics to fully discharge and reset. Then switch the isolator back on. This power cycle clears the 304 lockout counter and should allow the boiler to restart normally.
- Check your gas supply is working DIY safe
Before the boiler restarts, confirm that other gas appliances in the property (such as a gas hob) are working normally. If nothing is working, there may be an interruption to your gas supply — check you have credit on a prepayment meter, or contact your gas supplier. Do not proceed with further resets if you suspect a gas supply failure.
- Note any other fault code that appears after restart DIY safe
Once power is restored, watch the boiler display carefully as it goes through its startup sequence. If a different fault code appears — for example, an ignition fault or flame-detection error — make a note of it. That secondary code is likely the original problem that caused all the repeated lockouts. Look it up on BOYLA or share it with your engineer.
- Allow one reset attempt if another fault clears DIY safe
If the boiler displays a different fault code and you have identified and corrected its cause (for example, the system pressure was low and you have topped it up via the filling loop), you may press reset once. Wait 60 seconds to see if it holds. If it locks out again, do not press reset a second time.
- Call a Gas Safe registered engineer if the boiler continues to lock out Gas Safe engineer
If the boiler trips again after restart, or if code 304 returns, a qualified Gas Safe registered engineer needs to diagnose the root cause. Issues such as a faulty ignition electrode, a failing PCB, a combustion fault, or a flue/fan problem all require professional investigation and cannot safely be tackled by a homeowner. An engineer will use diagnostic equipment to identify exactly why the boiler was locking out in the first place.
Parts you may need
- Ionisation / ignition electrode · from £35
- Ignition lead and connector · from £20
- Gas valve · from £150
- Boiler PCB (printed circuit board) · from £220
- Flue fan / combustion fan assembly · from £130
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 £100–£350, depending on the underlying cause.
Frequently asked questions
Why won't my Chaffoteaux boiler respond to the reset button?
If your boiler is showing code 304, it has entered a safety lockout specifically because the reset button was pressed too many times in a short period. The front panel reset is temporarily disabled. To unlock the boiler, you need to cut the power at the external isolator switch, wait 30 seconds, and switch it back on. This is the only way to clear a 304 lockout.
Does fault code 304 mean my boiler has a serious fault?
Not necessarily — code 304 itself is a protective measure rather than a component failure. However, the reason you were repeatedly resetting the boiler almost certainly points to an underlying fault that does need attention. Common culprits include ignition problems, a faulty ionisation electrode, or an intermittent gas supply issue. If another fault code appears after you restore power, that is the real fault to investigate.
How much will it cost to fix the fault behind a Chaffoteaux 304 error?
If the power cycle clears the lockout and the boiler runs normally with no repeat trips, there may be no repair cost at all. Where an engineer is needed, a diagnostic call-out typically costs £60–£100, and straightforward repairs such as an electrode or sensor replacement usually bring the total to around £100–£350. If the underlying cause turns out to be a PCB replacement, costs including parts and labour can rise to £450–£700, though that is the less common scenario.
How do I stop fault code 304 from coming back?
The key is to use the reset button sparingly — press it once, wait at least 60 seconds, and if the boiler trips again, stop and call an engineer rather than pressing reset repeatedly. Getting the boiler serviced annually is the best way to catch developing faults before they cause repeated lockouts. Also make sure your system pressure stays between 1 and 1.5 bar and that the condensate drain is not frozen in cold weather, as both of these are common causes of lockouts that tempt users into repeated resets.