Error H.03 – .01

Safety precautions:

1) If you smell gas, leave the building immediately and call the gas emergency service — do not operate the boiler or electrical switches.

2) Before opening or touching any internal parts isolate the electrical supply to the boiler at the external isolator or consumer unit and, if you are able and it is safe, turn off the gas supply. Only proceed if you are comfortable and competent working around appliances; otherwise call a qualified engineer.

3) Take photos of any wiring, connectors or labels before you remove or disturb them so the original routing can be restored.

Initial checks a homeowner can do (safe, non-invasive):

1) Note the error code and time and take a clear photo of the display. This helps the engineer later.

2) Try a simple reset: press and hold the boiler reset button for 3–10 seconds (follow the boiler’s manual). Wait for the boiler to attempt to restart and watch whether the H.03 – .01 returns.

3) Check mains power and the boiler’s external isolator switch. Ensure the fuse/MCB feeding the boiler hasn’t tripped.

4) Check boiler water pressure is in the normal range (usually around 1–1.5 bar). Low pressure can cause unrelated faults; top up if the pressure is low and you know how to do so safely.

5) Look for obvious signs of water ingress, condensation or corrosion around the boiler and its control/display — moisture can cause communication faults.

Specific diagnostic and (basic) fix steps (if you are competent and have isolated electricity):

1) Isolate electrical power to the boiler. Confirm power is off.

2) Remove the outer cover or user control fascia only if you are comfortable doing so and the manual permits it. Be cautious of sharp edges.

3) Locate the ribbon cable/connector that links the HMI/display module to the main PCB. This is usually a flat flex or multi-pin connector.

4) Inspect the connector and cable for bent pins, corrosion, brown/black burn marks, or signs of moisture.

5) If the connector looks intact, carefully disconnect and then firmly reseat it. Make sure it clips home and is properly aligned.

6) Refit covers, restore power and test the boiler. If the code has cleared, monitor for recurrence. If H.03 – .01 returns intermittently, log how often and under what conditions (cold start, after power loss, etc.).

7) If you find obvious damage to the cable, connectors, or PCB (burn marks, broken pins), stop and call a professional — do not attempt soldering or PCB repairs yourself.

When to call a professional and what to tell them:

1) Contact a Gas Safe registered engineer (or local equivalent) if the code does not clear after a reset, if you found damaged wiring/connectors, if the fault returns repeatedly, or if you are not confident performing the basic checks above.

2) Provide the engineer with the boiler model, serial number, the exact error (H.03 – .01), when it first appeared, any photos you took, and a description of steps you have already tried.

3) PCB or HMI replacement, wiring repairs and in-depth diagnostics are specialist tasks and should only be carried out by a qualified engineer. Repeatedly resetting the boiler without fixing the underlying fault is not recommended.

Final notes:

H.03 – .01 is often a temporary communication issue that can be resolved by reseating connectors or replacing a faulty HMI cable/module. If the problem is persistent it usually indicates a failed HMI or main PCB which requires professional diagnosis and replacement. Keep the boiler isolated and seek a Gas Safe engineer if you are unsure or if the fault persists.