Overview
The E430 code with a ‘Flame crossed’ message on a Baxi 600 Combi normally indicates the boiler has detected a safety problem related to water pressure/flow and/or flame detection and has locked out to protect the appliance. In your context the likely immediate cause is that an internal water pressure test has failed (system pressure too low or a pressure/sensor fault), which can prevent the boiler from allowing and maintaining a safe ignition sequence. The ‘flame crossed’ wording suggests the control has either lost a flame signal during start-up or seen an abnormal flame-sensing condition while the pressure test failed, so it will not run until the condition is corrected and the fault cleared. Severity is moderate to high: the boiler has locked out for safety so you will be without heating and/or hot water until resolved, but it is not immediately dangerous unless you smell gas. Some checks and simple fixes (checking pressure, topping up the system, resetting the boiler) are suitable for a competent homeowner. Any persistent fault, repeated pressure loss, signs of gas smell, visible leaks, or anything to do with gas, ignition components, internal sensors, pump, or the PCB should be handled by a Gas Safe registered engineer. Do not attempt internal repairs on gas or electrical components yourself.
Possible Cause: Water pressure test failed.
Troubleshooting Steps
Safety precautions:
- If you smell gas: evacuate the property immediately, do not operate electrical switches or the boiler, turn off the gas supply at the meter if it is safe to do so, ventilate the area and call the gas emergency number / Gas Safe engineer. Do not try to relight the boiler.
- Switch off electricity to the boiler and isolate power before touching any internal parts. If you are unsure, leave the boiler alone and call a professional.
- Do not work on gas valves, ignition or internal wiring unless you are Gas Safe registered.
Initial checks a homeowner can do:
1. Check the boiler display and note the exact code (E430) and any other indicators or lights.
2. Check the pressure gauge on the boiler: target pressure for a cold system is normally about 1.0–1.5 bar (Baxi guidance typically 1–1.5 bar). If pressure is clearly below 0.5 bar it can trigger low-pressure faults.
3. Look for visible leaks around the boiler, pipework, radiators, and the filling loop. Also check radiators for cold tops (air) which can indicate trapped air and low system pressure.
4. Confirm gas and electrical supplies: make sure the gas supply is on (check other gas appliances) and the boiler display/power is on.
Specific diagnostic and DIY fix steps:
1. If pressure is low, repressurise the system using the filling loop: locate the filling loop (usually a flexible braided hose with two isolating valves). With the boiler off, open the valves slowly and watch the pressure gauge. Stop when pressure reaches about 1.0–1.5 bar. Close both valves securely and remove or secure the filling loop if yours requires it. Do not overpressurise past the manufacturer’s recommended maximum.
2. Bleed radiators if some radiators are cold at the top to release trapped air. Re-check the pressure after bleeding and top up again if needed. If you have to top up frequently within days, there is likely a leak or failed component.
3. Reset the boiler after correcting pressure: use the boiler’s reset method (press and hold the reset button or turn the selector to R and hold for 5–10 seconds depending on model). Wait for the boiler to attempt to restart. If the code clears and the boiler runs normally, monitor pressure and performance for a few cycles.
4. If the fault persists after repressurising and resetting, try these checks:
- Check condensate pipe (if weather cold) for freezing or blockage. Thaw gently with warm (not boiling) water if frozen.
- Confirm there are no external blockages at the flue or vents.
- Ensure the gas supply is stable (check other gas appliances).
When to call a professional (Gas Safe engineer):
- The E430 / ‘flame crossed’ message remains after you have correctly pressurised and reset the boiler.
- The boiler loses pressure repeatedly or you find a leak in pipework/components.
- You suspect a pressure sensor, flow switch, pump, ignition, flame detection electrode, gas valve, PCB or other internal component fault.
- Any time you smell gas, or you are uncertain about any step.
Why call a professional: ignition, flame sensing, gas valve, control board, pump and sensor repairs involve live gas and electrical work and must be carried out by a Gas Safe registered engineer to ensure safety and correct diagnosis. Provide the engineer with the exact error code (E430 + Flame crossed), what you have already checked (pressure levels, whether you topped up, reset attempts), and any other symptoms (leaks, unusual noises, repeated lockouts) to speed up diagnosis.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 600 Combi Gas Boiler.