Error H.01 – .00

Safety precautions

1) If you smell gas: immediately turn off the gas at the meter if it is safe to do so, open windows, do not operate electrical switches, and call your emergency gas supplier and a Gas Safe engineer. Evacuate if you feel unsure.

2) If there is a visible water leak or smoke, isolate electrical supply to the boiler at the fused spur and turn off the gas supply to the appliance if safe.

3) Do not remove the boiler casing or try to access the gas valve or PCB unless you are a Gas Safe qualified engineer. High-voltage, live wiring and gas components are hazardous.

Initial checks a homeowner can safely do

1) Read and note the exact displayed code and any sub-code (H.01 – .00). This is useful for the engineer.

2) Reset the boiler: locate the reset button (usually on the front control panel), press and hold for 3–5 seconds and allow the boiler to attempt a restart. If the code clears and the boiler runs normally, monitor it for recurrence. Do not repeatedly reset more than once or twice if it keeps returning.

3) Check basic supplies: confirm mains electricity to the boiler (display is on), confirm gas supply to the property by lighting another gas appliance (hob) if safe, and check system water pressure is within normal range (typically 1–1.5 bar when cold). Low pressure can cause other faults and should be corrected before further checks.

4) Visually inspect the outside of the boiler for obvious damage, water ingress, scorch marks or loose wiring visible externally. Do not open the boiler casing. If you see water on or around electrical connections or the gas valve area, turn the boiler off and call an engineer.

What an engineer will check and do (diagnostic and fix steps)

1) Isolate electricity and gas, remove the boiler front cover and perform a visual inspection of the PCB, wiring loom and gas valve/air-gas unit for loose connectors, damaged wires, corrosion or signs of overheating.

2) Reseat and secure connectors between the PCB and the gas valve/combustion control unit; check pin integrity and continuity on communication lines with a multimeter or oscilloscope when required.

3) Test the gas valve / air-gas unit operation and the PCB outputs to the valve. If communication signals are absent or abnormal the engineer will determine whether the fault is wiring, the valve/air-gas module, or the PCB itself.

4) Check for related faults that can present similarly (poor circulation, flow/return thermistor faults, flue or fan problems) and clear any stored temporary codes, then run the boiler through ignition cycles to confirm stable operation.

5) Replace or repair faulty parts as required: this may include repairing wiring, replacing the gas valve/air-gas unit (combustion module), or replacing the PCB. After any replacement the engineer will carry out a full gas safety check, combustion test where applicable, and full functional test of heating and hot water.

6) Update configuration/parameters if needed and confirm there are no other underlying issues (e.g., repeated water ingress, vibration or corrosion causing connectors to fail).

When to call a professional

1) If the reset does not clear the code, if it returns repeatedly, or if you see any sign of gas smell, water ingress or visible electrical damage, contact a Gas Safe registered engineer immediately. Communication and gas valve faults require competent testing and often part replacement.

2) Any work that involves removing the boiler cover, testing live electronics, or replacing gas-carrying components must be carried out by a qualified Gas Safe engineer.

Final notes

- Temporary H.01 – .00 faults can sometimes be cleared by a reset and basic checks, but recurrent or permanent versions of the fault indicate an underlying fault that requires a trained engineer.

- Do not repeatedly reset a persistently faulting boiler; call a Gas Safe engineer to avoid unsafe conditions and to ensure a correct, safe repair.