Burner does not ignite, furnace attempts ignition then locks out with A01 on display

Safety first: Turn off the main power switch to the De Dietrich furnace and close the gas shut‑off valve before removing any covers. Wait for all hot surfaces to cool.

Step-by-step checks:

• Verify gas supply: Ensure the external gas cock to the boiler is fully open. Check that other gas appliances in the home work. If not, contact your gas supplier.
• Reset the boiler: On the De Dietrich control panel, press the reset/unlock button (often marked with a flame or reset symbol) once and wait for a new ignition attempt. Do not reset repeatedly if you still hear no ignition.
• Inspect ignition electrode and cable: Remove the burner access panel. Locate the ignition electrode on the burner assembly. Check for cracked porcelain, heavy soot, or a loose/high‑voltage cable connection on the ignition transformer. Clean light deposits with a non‑metallic abrasive pad and re‑seat the cable firmly.
• Check ionisation (flame) electrode: Ensure the flame sensor is correctly positioned in the burner flame path and not bent away. Clean gently with fine emery cloth to remove oxidation.
• Listen for gas valve operation: During an ignition attempt, you should hear a click from the gas valve. If the fan runs and you see sparking but there is no click and no flame, the gas valve or its wiring may be faulty and must be tested with a multimeter by a qualified technician.
• Do not adjust gas settings: Never attempt to adjust the gas valve pressure or burner settings yourself. Incorrect adjustment can cause carbon monoxide production.

When to call a professional: If A01 reappears after basic checks, or if you suspect a faulty gas valve, ignition transformer, or control board, contact an authorised De Dietrich service technician for combustion and safety checks.