Baxi 4 Green Flashes Fault Code: Causes, Fixes & Repair Costs
What does the Baxi 4 Green Flashes fault code mean?
On Baxi boilers that use LED flash sequences to communicate status — including models in the EcoBlue and 600 ranges — four consecutive green flashes indicate a communication error between the boiler's PCB (printed circuit board) and its control system. Baxi classifies this as a temporary fault, meaning the boiler may attempt a self-recovery without any intervention. However, if the flashes persist or return repeatedly, it is a sign that something is preventing the PCB and control from exchanging signals correctly — whether that is a wiring issue, moisture ingress, or a deeper fault on the board itself. It is worth noting that a malfunctioning PCB can sometimes produce misleading flash codes, so the underlying problem may differ from what the sequence alone suggests.
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
- Loose or corroded wiring connections Common
Wiring that has worked loose over time, or terminals that have developed corrosion, can interrupt the data signals that travel between the PCB and the boiler's control components. Even a single unreliable connection is enough to trigger a communication fault, and the issue can be intermittent, making it harder to diagnose without specialist equipment.
- PCB component failure Common
Individual components on the board — resistors, capacitors, transistors — degrade with age or can fail prematurely due to manufacturing variation. When one of these fails, it may not cause a complete board failure but can disrupt specific communication pathways, resulting in a persistent four-flash error.
- Moisture or water damage to the PCB Sometimes
Internal leaks from a heat exchanger, pump seal, or pipework joint can allow water droplets or condensation to settle on the PCB. Even small amounts of moisture can cause corrosion of the circuit tracks or short circuits between components, leading to erratic behaviour and communication errors.
- Overheating of the PCB Sometimes
Blocked ventilation around the boiler casing, or a fault causing the boiler to run hotter than normal, can expose the PCB to temperatures it was not designed to handle. Sustained heat stress degrades component insulation and solder joints, eventually disrupting board function.
- Annual service overdue Sometimes
Without regular servicing, minor issues — dirty components, slightly loose connectors, early signs of corrosion — go undetected and compound over time. In some cases, four green flashes may be the first visible symptom of general deterioration that a service would have caught earlier.
How to fix it
- Attempt a manual boiler reset DIY safe
Turn the boiler's selector dial to the reset position (marked 'R' on most Baxi models) and hold it for approximately five to six seconds. You should hear the boiler attempting to fire. Release the dial and return it to your desired heating setting. Wait a few minutes to see whether normal operation resumes and the flashing stops. Only attempt this once or twice — repeated resets without resolution can place additional stress on the PCB.
- Allow time for an automatic self-recovery DIY safe
Baxi describes this fault as a temporary error that the boiler may clear on its own. If the boiler has just been powered back on after a power cut or was switched off at the mains, wait 10 to 15 minutes for it to complete its start-up sequence before concluding there is a fault.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in your home — a hob or gas fire, for example — are working normally. If they are not, the issue may be with your gas supply rather than the boiler. Contact your gas supplier if you suspect a supply interruption. Do not attempt to inspect the gas valve or any internal gas components yourself.
- Inspect visible external wiring for obvious damage DIY safe
With the boiler switched off at the mains, visually check whether any external cables or connectors going into the boiler show signs of obvious physical damage — fraying, scorch marks, or loose plugs at accessible external points. Do not remove any boiler casing panels or touch internal wiring; this must be left to a qualified engineer.
- Call a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
If the four green flashes return after a reset, or if the boiler is not producing heat or hot water, you need a Gas Safe registered engineer. They can safely remove the casing, test the wiring loom, check for moisture damage, and use diagnostic tools to confirm whether the PCB itself needs repair or replacement. PCB work, wiring repairs, and any internal component work must not be attempted by anyone who is not Gas Safe registered.
Parts you may need
- Baxi PCB (printed circuit board) · from £75
- Boiler wiring loom / harness · from £45
- Control panel / user interface board · from £60
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–£350, depending on the underlying cause.
Frequently asked questions
Is four green flashes on a Baxi boiler serious?
It can range from a minor temporary glitch to a more significant PCB or wiring fault. Because Baxi classifies it as a temporary error, a single reset is always worth trying first. If the flashes clear and the boiler runs normally, you may not need any further action. But if the fault returns, or if you have no heating or hot water, treat it as a genuine fault requiring an engineer visit — do not keep resetting the boiler hoping it will go away.
How much does it cost to fix a Baxi PCB?
Most people with this fault end up paying somewhere between £120 and £350, which covers engineer call-out, diagnosis, and wiring or minor board repairs. A full PCB replacement is a less common but more expensive outcome — parts for a Baxi PCB typically cost £60–£90 from a trade supplier, but with labour and diagnostic time the total bill often lands in the £400–£650 range. If your boiler is over ten years old and needs a new PCB, it is worth comparing that cost against a new boiler installation.
Can I replace the Baxi PCB myself to save money?
No — and it is strongly inadvisable. The PCB sits inside a gas appliance, and any internal work must be carried out by a Gas Safe registered engineer. Attempting it yourself could void your boiler's warranty, create safety hazards, and may be illegal under UK gas safety regulations. The part cost alone is relatively modest; it is the skilled labour involved in safe removal, fitting, and testing that accounts for most of the bill.
Will getting my Baxi boiler serviced prevent this fault?
Regular annual servicing is one of the best ways to catch the early warning signs that lead to a four-flash fault — things like slightly corroded connectors, minor leaks near the PCB, or components showing early signs of wear. A Gas Safe engineer can address these during a routine service before they develop into a full fault. Annual servicing is also a condition of most Baxi warranty packages, so keeping up with it protects both your boiler and your cover.