Error EH 258

Safety precautions:

- If you smell gas, evacuate the property immediately, call the gas emergency number and do not operate any electrical switches or the boiler. Call a Gas Safe engineer.

- If the boiler is displaying EH 258, avoid working inside the boiler casing unless you are a Gas Safe qualified engineer. There are live electrical parts and gas components inside.

- Isolate electrical power to the boiler at the external isolator or fuse if you plan to carry out any basic external checks. Do not isolate the gas supply unless instructed by an engineer.

Initial homeowner checks (non-invasive):

1. Note the error exactly and any accompanying cause code or flashing indicator. Take a clear photo of the display for the engineer.

2. Try a simple reset: press and hold the boiler reset button (or follow the model-specific reset procedure) for about 3 seconds, then allow the boiler to attempt a restart. If you see an EA or flame-related code, do not continue to reset repeatedly; call an engineer.

3. Check the boiler’s external power: ensure the boiler switch and fused spur are on, and the house fuseboard hasn’t tripped. Replace a blown external fuse at the fused spur if you are competent to do so.

4. Visually inspect any external wiring or removable terminal covers that are designed to be accessible without removing the main casing. Look for obvious damage, corrosion, or water stains around the boiler that suggest ingress.

5. If a reset clears the code and the boiler runs normally, monitor for recurrence and book a service/engineer visit to investigate the underlying cause.

Specific diagnostic and fix steps (should be carried out by a Gas Safe engineer):

1. Confirm the fault code and read the cause code from the information menu (spanner/return button) to collect diagnostic data before powering down.

2. Isolate electrical power and gas supply, then remove the boiler casing following manufacturer instructions.

3. Visually inspect the HCM / control box and its plug and wiring harness for loose connections, bent pins, burn marks, corrosion or water ingress. Check the code plug (if applicable) is present and seated correctly and that the HCM fitted matches the boiler model/specification.

4. Re-seat connectors and check that ribbon or spade connectors are secure. If contacts are corroded or damaged, replace the connector or wiring harness per manufacturer guidance.

5. Perform electrical checks: verify supply voltages to the HCM, continuity of harnesses and integrity of the connector pins. Check for any PCB damage or component failure on the HCM/control box.

6. If the HCM is confirmed faulty or invalid for the boiler, replace it with the correct, manufacturer-approved HCM/control box and refit correctly. Ensure any replacement is compatible with the GB162 model.

7. After replacement or repair, restore power and gas, run the boiler through a full commissioning/ignition sequence, confirm flame detection and normal operation, and clear fault codes. Re-check for leaks, correct sensor readings and that no new error codes appear.

8. Record the repair details, replaced part numbers and any diagnostic cause codes for the owner’s records.

Important notes:

- This error involves gas-control electronics so a qualified Gas Safe engineer must undertake internal checks, electrical measurements and any part replacement. Do not attempt internal repairs yourself.

- If EH 258 persists after connectors are checked and the correct HCM is fitted, the control box or main PCB may need replacement; only a trained engineer should proceed.

- If you are unsure at any point, stop and contact a Gas Safe registered Worcester Bosch-approved engineer and provide the error code, cause code and photos. This will speed diagnosis and ensure the repair is safe and compliant.