Overview
On Baxi MainEco boilers the 4 green flashes most commonly indicate a temporary flame loss — the burner lit but the flame was lost and the control has flagged it. The boiler will normally attempt retries and many times will reset itself and run again. Causes for a temporary flame loss include brief interruption to the gas supply, a dirty or mis-positioned ignition electrode or flame sensor (ionisation probe), a failing gas valve, frozen or blocked condensate pipe or flue obstruction, or faults with the fan/air pressure circuit that prevent safe combustion. Severity is generally moderate: a single, occasional temporary flame loss that clears is not an immediate hazard, but repeated flame loss or recurring 4-green-flash events will lead to boiler lockout and loss of heating and hot water. If you smell gas, if the boiler locks out and will not restart, or if the fault repeats, this must be handled by a Gas Safe registered engineer. Homeowners can perform a few safe checks and a controlled reset, but they must not open the boiler or attempt gas or PCB/electrical repairs themselves. Note: some documents and models list different flash meanings (e.g., communication errors mapped to other flash patterns), so always check your exact model/service manual if available.
Possible Cause: Communication error between printed circuit board (PCB) and Control – this is a temporary error, the system should reset on its own
Troubleshooting Steps
Safety precautions:
1. If you smell gas: do not operate electrical switches, open windows and doors, evacuate the property and call the gas emergency number immediately. Do not attempt any repairs.
2. If you are unsure or uncomfortable at any point, stop and call a Gas Safe registered engineer. Do not remove covers or work on gas or electrical components inside the boiler.
3. Keep children and pets away from the boiler while inspecting it. Isolate mains power only if you know how and it is safe to do so.
Initial checks a homeowner can do:
1. Note the exact display pattern and any other lights — take a photo so you can show an engineer.
2. Check for an obvious gas supply issue: are other gas appliances working? Is the gas meter turned on? If other appliances are out, contact your gas supplier.
3. Check the boiler pressure on the gauge — aim for about 1.0–1.5 bar when cold. If pressure is very low, topping up can sometimes allow a restart (only do this if you know how to use your filling loop safely).
4. Check the external flue terminal for obvious blockages (birdnesting, debris) and check external condensate pipe (usually plastic) for signs of frost/ice or blockage.
5. Ensure the electrical supply and timer/room thermostat are providing power and calling for heat.
Specific diagnostic and safe fix steps you can try:
1. Record the fault and then attempt a controlled reset: follow the boiler manual/reset procedure for your MainEco model — typically press and hold the reset button or move the selector to R and hold for about 5 seconds. Wait and observe for a full restart sequence. Do not keep resetting repeatedly if it does not clear.
2. If reset clears and the boiler runs normally, monitor it. One-off events can be transient, but repeated occurrences need an engineer.
3. If reset does not clear and the boiler shows 4 green flashes again, check condensate pipe if accessible: if frozen, thaw with warm water (not boiling). A thaw often restores operation. If blocked, leave further work to the engineer.
4. If the boiler attempts to run but you hear no fan or the fan sounds abnormal, or the boiler shows additional fault indications, do not attempt internal repairs. This points to fan, air-pressure switch, or PCB issues.
5. If documentation or your initial note suggests a PCB/control communication fault (you mentioned a possible PCB-control communication error), try isolating electrical supply to the boiler at the fused spur for 60 seconds then restore power — sometimes a power-cycle clears a temporary electronics fault. Do not remove covers or probe internal connections.
6. If water pressure is low and you top up the system, re-check for leaks. If pressure drops again, stop and call a professional.
When to call a professional (Gas Safe engineer):
1. Any smell of gas or suspected gas leak.
2. Repeated flame loss or repeated appearance of the 4-green-flash fault, or if the boiler moves to a red lockout state and will not restart.
3. If reset does not clear the fault or if you suspect ignition electrodes, flame detection, gas valve, fan, air-pressure switch, condensate trap replacement, or a PCB/communication problem.
4. If you are not comfortable performing the simple checks listed above.
Summary: Try basic, safe checks and a single controlled reset. If the fault repeats, if there is any smell of gas, or if you see other fault lights or lockout behaviour, stop and arrange a Gas Safe registered engineer to diagnose and repair — do not attempt internal gas or electrical repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi MainEco.