Unit attempts to start but quickly shuts down with E03, fan may run briefly, no sustained flame.

Safety: Turn off power before removing covers. Do not operate the heater with the flue disconnected.

Steps:

• Check flue terminal: Go outside and inspect the Baxi flue terminal. Remove any obstructions such as leaves, nests, or snow. Ensure the terminal guard (if fitted) is not clogged.
• Inspect air intake: For concentric systems, ensure the air intake annulus is clear. For twin-pipe systems, check both intake and exhaust pipes.
• Check condensate trap and flue collector: On condensing Baxi water heaters, a blocked condensate trap can flood the flue collector. Remove and clean the trap, ensuring the condensate hose is not kinked or frozen.
• Air pressure switch and tubing: With power off, locate the air pressure switch and its silicone tubes. Ensure tubes are not split, kinked, or blocked with moisture. Re-seat them firmly.

Professional help: If E03 remains, a technician should test the fan operation, measure pressure switch operation with a manometer, and replace the air pressure switch or fan assembly if faulty.