Overview
On a Baxi MainEco boiler, 6 green flashes indicate a sensor error. This is typically reported when the boiler detects an abnormal reading from one of its temperature or flow sensors (for example flow, return or hot water thermistors), or when there is a related wiring/communication fault. The manufacturer documentation and service guides classify this as usually a temporary fault that the boiler may clear itself from after a reset, but it can also point to a failing sensor, loose connector, circulation problem or an intermittent PCB/parameter issue if it keeps recurring. Severity is usually low to medium. The immediate risk is loss of heating and/or hot water and potential strain on the system if it runs with incorrect sensor inputs. It is not normally an immediate safety emergency in the way a gas leak would be, but persistent sensor faults should not be ignored because they can mask overheating, poor circulation or cause repeated lockouts. Basic checks and a single reset are suitable for a homeowner; if the fault returns, or if you are unsure, you should get a Gas Safe registered engineer to diagnose and repair the boiler.
Possible Cause: Miscellaneous error
Troubleshooting Steps
Safety precautions
1) If you smell gas, hear escaping gas, or a carbon monoxide alarm sounds: leave the property immediately, call the national gas emergency number and a qualified engineer. Do not operate electrical switches or the boiler. 2) Turn off power to the boiler at the isolator if you need to work near it. Do not open sealed or gas-carrying panels or attempt gas adjustments. 3) If you are unsure or uncomfortable at any stage, stop and call a Gas Safe registered engineer.
Initial homeowner checks (non-invasive)
1) Record the fault: note the error (6 green flashes), the time it happened and any symptoms such as no heating, no hot water, or repeated lockouts. 2) Reset the boiler once: use the boiler reset button or selector (hold reset or R for 5–10 seconds as per your model). Wait for the boiler to attempt a restart and see if the fault clears. Only reset once or twice; repeated reset attempts are not recommended. 3) Check the system pressure on the pressure gauge. Baxi boilers normally run between 1.0 and 1.5 bar cold. If pressure is below about 1.0 bar, top up using the filling loop following your boiler manual and then reset. 4) Check that the gas supply is on at the meter and any external shutoffs, and that the mains electricity to the boiler is on. 5) Inspect the condensate discharge pipe (external frost or blockage) and the external flue terminal for obvious blockages like bird nests or debris. A frozen or blocked condensate or flue can create faults; thaw condensate pipe gently with warm (not boiling) water if frozen. 6) Check radiators and pump circulation: bleed any radiators that are cold at the top to remove trapped air and improve flow. Listen for the pump running when demand is set.
Diagnostic and more specific steps (do not open sealed gas parts)
1) After initial checks and thawing/clearing, reset the boiler and observe. If the error is cleared and stays cleared, monitor over the next 24–48 hours for recurrence. 2) If pressure repeatedly falls after topping up, there is likely a leak or faulty component and a professional is needed. 3) If the 6 green flashes return immediately or intermittently, treat it as an electrical/sensor issue: loose connectors, failing thermistor or PCB parameter fault are common. You should NOT attempt to replace sensors or remove internal covers unless you are a qualified engineer. 4) If you are comfortable with basic visual checks and with the power isolated, you may inspect external wiring to the boiler for damaged or loose cables where they enter the boiler, but avoid internal component work. Loose connectors at terminal blocks outside sealed compartments can sometimes be reseated safely if you know what you are doing and the mains is isolated. 5) Note any other codes, red flashes, or trends (for example faults after power cuts, after filling loop use, or when the pump runs). Take photos of the display and label/nameplate for the engineer.
When to call a professional
1) Call a Gas Safe registered engineer if the fault persists after a sensible reset and the basic checks above, if the fault returns repeatedly, or if you see other fault codes or red flashes. 2) Call an engineer for sensor replacement, PCB diagnosis or any internal electrical or gas component work. 3) Call a professional immediately if you smell gas, suspect a leak, or have a carbon monoxide alarm activation.
What the engineer will likely do
A qualified engineer will run the boiler action log and diagnostics, check sensor resistances/voltages, inspect wiring and connectors, check pump operation and system flow/pressure, verify flue and condensate routing, and replace faulty sensors or PCB/PSU parts if required. Keep a record of the error, resets attempted and any observations to speed up diagnosis.
Final notes
A single 6 green flash that clears after a reset is often a temporary sensor or communication glitch. Recurrent or persistent 6 green flashes point to a component or wiring fault and should be handled by a Gas Safe registered engineer to ensure safe, correct diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi MainEco.