Overview
E4 on a Baxi GA range indicates an ignition failure — the boiler has attempted to light the burner but has not detected a safe flame, so it has gone into lockout to protect the house. Common causes include no gas supply or interrupted gas flow, a faulty ignition electrode or flame sensor, a stuck or faulty gas valve, frozen or blocked condensate/drain or flue problems preventing correct operation, or an internal control/sensor fault. Some intermittent causes (like a temporary loss of gas or a frozen condensate pipe) can be cleared by a reset, while hardware faults will reoccur or prevent relighting. Severity is medium to high: the boiler will not provide heating or hot water while the fault exists, and repeated ignition attempts and resets should be avoided. If you smell gas, evacuate immediately and call your gas emergency number. For basic checks and safe resets you can attempt a few diagnostic steps yourself, but any work involving gas components, the burner, ignition electrodes, gas valves, or internal PCB/sensors must be done by a Gas Safe registered engineer. If a simple reset or thaw clears the fault but it returns, get a professional to diagnose and repair it.
Possible Cause: Ignition failure
Troubleshooting Steps
Safety first
1) If you smell gas, leave the property immediately, do not operate electrical switches or phones inside, ventilate if safe and call your gas emergency number and the gas supplier.
2) Turn the boiler off at the control panel if it is safe to do so and avoid repeatedly trying to reset it more than 1–2 times.
3) If you are unsure or uncomfortable with any step below, stop and call a Gas Safe registered engineer.
Initial checks a homeowner can do
1) Check other gas appliances (hob, oven) to confirm the gas supply is on. If none of them work, contact your gas supplier or check prepayment meter/top-up credit if relevant.
2) Check the boiler pressure gauge — target pressure is about 1.0–1.5 bar when cold. If pressure is very low (<0.5 bar) top up the system (see filling loop steps below) or call an engineer if you don’t know how to.
3) Check the condensate discharge pipe (usually plastic, runs to outside or drain). In cold weather it can freeze — feel it or look for ice. If frozen, thaw it with warm (not boiling) water or a warm cloth and insulate it afterwards.
4) Check the outside flue terminal for obvious blockages (birds’ nests, debris) and clear only if safe to reach from ground level. Do not climb unsafely to access the flue.
5) Check the boiler display for additional codes or lights and make a note of E4 and any other codes or how often it attempts to restart.
6) Replace thermostat batteries or check controls in case the boiler is not being called correctly.
Simple reset procedure
1) Follow the manufacturer’s reset instructions for your GA model: usually hold the reset button or turn the selector to R and hold for 5–10 seconds. Wait for the boiler to attempt ignition.
2) Do not keep resetting repeatedly — if the fault returns straight away, the underlying issue remains and further attempts can mask a dangerous fault.
How to top up boiler pressure (only if you are confident doing this)
1) Locate the filling loop (a braided hose beneath the boiler with two isolating valves). Confirm you have the correct instructions for your model.
2) Ensure both boiler and mains valves are closed, then open the filling loop valves slowly while watching the pressure gauge.
3) Stop when pressure reaches about 1.0–1.5 bar, then close the filling loop valves tightly and remove or stow the loop if required by your model.
4) If pressure falls again quickly, call an engineer — do not top up repeatedly without diagnosis.
If initial checks do not clear E4 or you are unsure
1) Do not open or attempt repairs to gas valves, burners, ignition electrodes, or the PCB — these must be handled by a Gas Safe engineer.
2) Call a Gas Safe registered heating engineer and give them the exact fault code (E4), any other codes, what checks you have done (gas supply, pressure, condensate thawed, flue clear), and how many reset attempts you tried.
3) Ask the engineer to check gas pressure to the boiler, ignition electrode/flame sensor condition and wiring, the gas valve operation, condensate/drain and flue integrity, and the boiler control/PCB diagnostics.
4) If the boiler relights after your checks and reset, monitor it closely for repeated lockouts and arrange a service if it recurs.
When to call urgently
1) Smell of gas — leave and call emergency services/gas supplier immediately.
2) Repeated lockouts or the boiler will not light after sensible checks and a single reset.
3) If you find a frozen condensate pipe you cannot clear, persistent pressure loss, or unusual noises or smells from the boiler.
Summary: E4 is an ignition failure that may be cleared by simple checks (gas supply, condensate thaw, pressure and a safe reset) but often requires a Gas Safe engineer to diagnose and repair ignition components, gas valve or control faults. Do not carry out internal gas or burner repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi GA Range.