Baxi 1 Green Flash Fault Code: Causes, Fixes & Repair Costs
What does the Baxi 1 Green Flash fault code mean?
A single green flash on a Baxi boiler is a temperature-related signal. In many cases it simply means the boiler has reached its target temperature and no action is needed — the light will clear on its own once the boiler cools slightly and cycles normally. However, if the boiler has stopped providing heat or hot water alongside this flash, it indicates the boiler's safety system has detected a temperature problem: the flow temperature has climbed too high, the water temperature is rising faster than expected, or there is an unusually large difference between the flow and return pipe temperatures. In these situations the boiler shuts itself down as a precaution. This behaviour is seen across the Baxi 400, 600, 800, and similar Heat and Combi ranges.
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
- Low system pressure Common
When the system pressure drops below around 1 bar, there is less water circulating around the heating circuit. The water that remains near the heat exchanger absorbs more heat than it should, causing the boiler to detect elevated temperatures and shut down. Checking the pressure gauge is the first and easiest thing to do.
- Sludge or debris restricting circulation Common
Over time, corrosion products and debris build up inside radiators and pipework. These deposits can partially block the heat exchanger or the system as a whole, slowing water flow. The heated water cannot travel away from the burner quickly enough, the temperature difference between flow and return rises sharply, and the boiler trips on overheat protection.
- Air in the heating system Common
Pockets of air trapped inside radiators or pipework interrupt the smooth movement of water. This creates localised hot spots and uneven circulation, which can push flow temperatures high enough to trigger the fault. Air often enters after pressure drops, repairs, or on older systems with ageing automatic air vents.
- Faulty or seized circulation pump Sometimes
The pump is responsible for moving water continuously around the system. If it has seized, is running slowly due to wear or sludge accumulation, or has an electrical fault, heat builds up rapidly around the heat exchanger. The boiler sees this as a dangerous temperature condition and locks out.
- Limescale build-up in the heat exchanger Sometimes
In hard-water areas, limescale can coat the inside of the heat exchanger, acting as an insulating layer. Water passing through cannot absorb heat efficiently, so temperatures spike locally. The NTC thermistor detects abnormally hot water leaving the boiler and triggers a safety shutdown.
- Faulty NTC thermistor (temperature sensor) Rare
The NTC thermistor monitors the temperature of water leaving the heat exchanger. If it has drifted out of calibration or failed entirely, it may report incorrect temperatures to the PCB. The boiler may respond by firing harder than necessary or by falsely detecting an overheat condition, leading to repeated lockouts even when water temperatures are normal.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, locate the filling loop (a braided flexible hose beneath the boiler with one or two valves) and slowly open the valve(s) to allow mains water in until the gauge reaches approximately 1.2 bar. Close the valve(s) firmly. Never overfill beyond 2 bar.
- Reset the boiler DIY safe
On most Baxi models, turn the control dial to the reset position and hold it for around six seconds until you hear the boiler attempt to relight. Release the dial and set it to your desired heating temperature. Allow several minutes for the boiler to complete its start-up sequence. If the single green flash returns and the boiler locks out again, do not reset more than two or three times — repeated resets without fixing the underlying issue can cause further damage.
- Bleed the radiators to remove trapped air DIY safe
Switch the boiler off and let the system cool. Starting on the top floor, use a radiator bleed key to open the bleed valve on each radiator slightly. Hold a cloth beneath the valve to catch drips. Once a steady trickle of water escapes (rather than a hiss of air), close the valve. Work your way down to the ground floor. Once all radiators are bled, recheck the system pressure and top it back up to 1.2 bar via the filling loop if needed, then reset the boiler.
- Wait and observe if the fault clears on its own DIY safe
If the green flash appeared after a power cut, or the boiler has only just been switched on, give the system 10–15 minutes to complete its start-up sequence before assuming there is a fault. A single green flash without a lockout often clears automatically once the boiler reaches its set temperature and begins to modulate.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler continues to lock out after you have checked the pressure, bled the radiators, and attempted a reset, a professional diagnosis is needed. The engineer can test the circulation pump output, measure NTC thermistor resistance with a multimeter, inspect the heat exchanger for limescale or sludge restrictions, and carry out a power flush if the system is heavily contaminated. Do not attempt to access the gas valve, PCB, pump internals, or heat exchanger yourself.
Parts you may need
- NTC thermistor (flow or return temperature sensor) · from £25
- Circulation pump (Grundfos or equivalent Baxi replacement) · from £85
- Automatic air vent · from £15
- Magnetic system filter (e.g. Magnaclean) · from £65
- Central heating inhibitor (1 litre) · from £12
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–£300, depending on the underlying cause.
Frequently asked questions
Is a single green flash on a Baxi boiler always a fault?
Not necessarily. A single green flash most commonly just means the boiler has reached its set target temperature — this is entirely normal behaviour and will clear on its own as the system settles. It only becomes a concern if the boiler has simultaneously locked out and is no longer providing heat or hot water, which suggests the safety system has detected a genuine temperature problem such as poor circulation or low pressure.
How many times can I reset my Baxi boiler for this fault?
You can safely attempt a reset two or three times. If the boiler locks out again with the same single green flash shortly after each reset, continuing to reset it without addressing the root cause is unlikely to help and could mask a worsening problem. At that point, book a Gas Safe registered engineer to diagnose whether the issue is sludge, a failing pump, limescale, or a faulty thermistor.
Could sludge in my heating system cause the 1 green flash fault?
Yes — sludge is one of the more common reasons this fault appears on boilers that are otherwise in reasonable condition. Accumulated corrosion debris slows water circulation, which means heated water lingers near the heat exchanger for longer than it should. The temperature difference between the flow and return pipes exceeds the boiler's safety threshold and it shuts down. A Gas Safe engineer can assess whether a power flush is worthwhile, and fitting a magnetic filter afterwards helps prevent the problem recurring.
What does a thermistor replacement cost and could it be the cause?
A faulty NTC thermistor is a less common but possible cause, particularly on older Baxi boilers. If the sensor drifts out of range it can misreport water temperatures, leading to unnecessary lockouts. A Gas Safe engineer will test resistance values with a multimeter to confirm a fault before replacing anything. Expect to pay around £150–£250 including the part and labour. For context, if the heat exchanger itself is found to be damaged beyond cleaning, replacement costs significantly more — often £500 or upward — which is worth considering against the age of the boiler.