How to Fix / Troubleshooting

Safety: Switch off power before removing covers. Flue work at height or on roofs should be done by a professional.

Check flue terminal: From outside, inspect the Defro flue terminal for obstructions (ice, leaves, bird nests, plastic bags). Clear any blockage without damaging the terminal.
Inspect air intake: On concentric flue systems, ensure the air intake ring is not blocked by debris or snow.
Check condensate trap: With power off, locate the condensate trap under the heat exchanger. If full of sludge, carefully remove, clean with water, and refit, ensuring the water seal is restored.
Inspect pressure tubes: Open the boiler casing and locate the air pressure switch and its silicone tubes. Ensure tubes are not kinked, split, or filled with water. Blow gently through the tubes (with boiler off) to clear moisture, then reconnect firmly.
Reset boiler: Restore power and reset. If E03 returns, the pressure switch or fan performance must be tested by a technician using a manometer.

Boiler shows E03 and shuts down, fan may run briefly, no ignition sequence starts.

Possible Causes