Baxi 3 Green Flashes Fault Code: Causes, Fixes & Repair Costs
What does the Baxi 3 Green Flashes fault code mean?
Three green flashes on a Baxi boiler indicate that the boiler has detected a temporary loss of flame during operation and has shut down as a precautionary measure. The boiler will typically attempt to relight itself automatically — this is not an immediate lockout. However, if the boiler tries to re-establish the flame five times without success, the fault will escalate to 3 red flashes, which signals a full lockout requiring a manual reset. In short: three green flashes is the boiler's way of saying the flame went out unexpectedly, and it is trying to sort itself out. If it cannot, you may need to investigate further or call an engineer.
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
- Frozen condensate pipe Common
During cold weather, the plastic condensate pipe that carries waste water from the boiler to an external drain can freeze solid. This blocks drainage and prevents the boiler from firing safely. It is the single most frequent cause of flame-loss faults in winter. Look for a white or grey plastic pipe, typically 21–32 mm in diameter, exiting through an outside wall.
- Interrupted gas supply Common
If there is no gas reaching the burner, there can be no flame. Check whether other gas appliances in your home — such as a hob or gas fire — are working normally. If they are not, there may be a wider supply interruption. Contact your gas supplier or check for reported outages in your area before assuming the boiler is at fault.
- Faulty or dirty flame sensing electrode Common
The flame sensing electrode monitors whether a flame is actually present once the burner has ignited. If this electrode is coated in residue or has deteriorated, it may fail to detect the flame even when combustion is occurring — causing the boiler to shut down unnecessarily. This is a common cause of intermittent or repeated flame-loss faults.
- Low system pressure Common
Baxi boilers need adequate water pressure to operate correctly. If the pressure gauge reads below 1.0 bar, the boiler may struggle to fire or maintain a flame. The correct cold-system pressure is 1.0–1.5 bar. You can check this on the pressure gauge, usually visible on the front of the boiler.
- Faulty ignition electrode Sometimes
The ignition electrode produces the spark that lights the burner. If it is cracked, worn, or incorrectly positioned, it may fail to generate a reliable spark. Without a proper initial spark, no flame is established and the boiler shuts down. A Gas Safe engineer will need to inspect and replace this component if faulty.
- Gas valve fault Sometimes
The gas valve controls the flow of gas into the burner. Over time, valves can seize, wear out, or develop electrical faults, either cutting off the gas supply entirely or failing to modulate correctly. A stuck or faulty gas valve will prevent reliable ignition and is a job for a Gas Safe registered engineer.
- Faulty thermistor or sensor Sometimes
NTC thermistors measure water temperature at key points in the system. If one develops an open-circuit fault or its readings drift significantly, the boiler's control board may misread conditions and shut down the burner as a precaution. Sensor faults can sometimes produce misleading fault indicators.
- PCB (printed circuit board) fault Rare
The PCB is the electronic control centre of the boiler. A fault here can cause erratic behaviour, fault codes that will not clear, or unexplained shutdowns. It is worth noting that a PCB fault can sometimes cause the boiler to display an incorrect fault pattern, so a Gas Safe engineer will need to carry out proper diagnosis before assuming this is the cause.
How to fix it
- Check whether other gas appliances in your home are working DIY safe
Turn on a gas hob ring or check another gas appliance. If nothing is working, the issue is almost certainly with your gas supply rather than the boiler itself. Contact your gas supplier or check for local outages. Do not attempt to reset the boiler repeatedly if you suspect a supply issue.
- Check the boiler 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 1.0 bar, locate the filling loop (a braided flexible hose with one or two valves, usually beneath the boiler) and slowly open the valve(s) until the pressure reaches around 1.2–1.3 bar, then close them. Never overfill beyond 1.5 bar.
- Check for a frozen condensate pipe (cold weather only) DIY safe
If the temperature outside has dropped below freezing, inspect the condensate pipe — the plastic pipe that exits the boiler and runs to an outside drain. If it feels solid or there is visible ice, thaw it carefully by pouring warm (not boiling) water over the frozen section. Work from the boiler end outwards. Once thawed, reset the boiler as described in the next step.
- Reset the boiler DIY safe
Turn the boiler's selector dial or press the reset button (marked R on many Baxi models) and hold it for approximately 5 seconds, then release. The boiler should attempt to re-ignite. Only attempt this up to 2–3 times in total. If the boiler continues to flash or escalates to 3 red flashes after resetting, stop and move on to calling an engineer — repeated resets without addressing the root cause can mask a more serious underlying problem.
- Allow time for auto-recovery if the fault appeared after a power cut DIY safe
If the 3 green flashes appeared immediately after a power outage or after the boiler was switched off, allow 10–15 minutes for the boiler to complete its internal start-up sequence. In some cases, the boiler will recover on its own once it has gone through this process.
- Call a Gas Safe registered engineer to inspect and repair Gas Safe engineer
If the boiler continues to display 3 green flashes, escalates to 3 red flashes, or the fault keeps returning, you need a qualified Gas Safe engineer. They will check the ignition and flame sensing electrodes, inspect the gas valve, test thermistors and other sensors, and examine the PCB if required. Do not attempt to work on any of these components yourself — gas work must legally be carried out by a Gas Safe registered engineer in the UK.
Parts you may need
- Flame sensing electrode · from £25
- Ignition electrode · from £20
- Ignition lead set · from £18
- Gas valve · from £120
- NTC thermistor (flow or return sensor) · from £15
- PCB (printed circuit board) · from £180
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–£320, depending on the underlying cause.
Frequently asked questions
Will the Baxi boiler fix itself when showing 3 green flashes?
Sometimes, yes. Three green flashes indicates a temporary flame loss, and Baxi boilers are designed to attempt an automatic relight. If the underlying cause was a brief gas supply blip or a momentary sensor issue, the boiler may recover on its own within a few minutes. However, if the fault persists or you see it happening regularly, the boiler needs professional attention. If it escalates to 3 red flashes, the boiler has locked out and will need a manual reset followed by an engineer visit if it cannot hold a flame.
What is the difference between 3 green flashes and 3 red flashes on a Baxi boiler?
Three green flashes means the boiler has detected a flame loss and is in a recovery phase — it will try to relight itself automatically. Three red flashes means the boiler has made five unsuccessful attempts to relight and has locked out completely for safety. At that point, the boiler will not fire again until it is manually reset. If resetting does not resolve the red flashes, a Gas Safe engineer is required to find and fix the root cause.
Can I reset my Baxi boiler myself when it is showing 3 green flashes?
Yes, a single reset is a perfectly safe and sensible first step. Turn the dial to the R (Reset) position, hold for 5 seconds, and release. If the boiler fires up and runs normally, keep an eye on it over the next day or two. If the fault returns quickly or the boiler fails to light after 2–3 resets, stop resetting and call a Gas Safe engineer. Repeatedly resetting without addressing the cause will not fix the problem and could delay diagnosis of something more serious.
How much does it typically cost to fix a Baxi 3 green flashes fault?
For most homeowners, the repair cost falls somewhere between £100 and £320 once parts and labour are included. Simple fixes — such as repressurising the system or thawing a frozen condensate pipe — can cost nothing or just a standard callout fee of £60–£100. Replacing an ignition or flame sensing electrode typically costs £100–£200 including labour. A gas valve replacement usually sits in the £150–£300 range. A PCB replacement is less common but can cost £250–£450 depending on the specific model. If a heat exchanger is at fault (rare for this code), expect costs of £400 or more — but that would normally trigger a different fault pattern.