Overview
EH 258 on a Worcester Bosch Greenstar i (System/Combi) means the boiler has detected a fault with the Heating Control Module (HCM) or the control box that the HCM plugs into. The HCM is an electronic module that helps regulate boiler operation and temperature control; if it is incompatible, not seated correctly, has a bad connection, or is electrically faulty, the boiler will log EH 258 and will often lock out or stop operating until the fault is resolved. Two common causes are (1) a defective HCM or control box board and (2) loose/poor connections between the HCM and the control box (or an incorrect HCM fitted). Severity: this is a significant control fault. While the fault itself does not usually indicate an immediate gas leak or fire risk, it prevents safe and reliable operation of the boiler and can leave you without heating and hot water. Because the HCM and control box are internal electrical components and the work involves access inside the sealed boiler casing, resolving EH 258 is generally not a DIY job. Basic non-invasive checks by a homeowner are fine, but any opening of the boiler or electrical/part replacement must be carried out by a qualified Gas Safe engineer or an accredited Worcester Bosch technician.
Possible Cause: Control box or the heat control module (HCM) is defective.
Troubleshooting Steps
Safety precautions:
- Do not open the boiler casing or attempt to repair or replace internal components unless you are a Gas Safe registered engineer. Working inside the boiler is dangerous and illegal for unqualified persons.
- If you smell gas, evacuate the property and call the gas emergency number immediately. If there is any doubt about safety, isolate gas and electricity and get professional help.
Initial homeowner checks you can safely do:
1. Record the exact fault: note the EH 258 message and any accompanying cause code or flashing indicators shown on the display. Press the spanner/return button to view the cause code if your model shows it. Take a clear photo of the display.
2. Try a single reset: follow the boiler manual reset method (press the reset button or hold the control knob per your model instructions) for 3 seconds. If the fault clears and the boiler runs, monitor it. If it returns, do not keep resetting repeatedly; that can hide an intermittent fault.
3. Check external power supply: verify the boiler isolation switch is on and the fused spur or RCD hasn’t tripped at your consumer unit. Make sure other external controls (room thermostat, programmer) have power and are calling correctly. Replace batteries in wireless controls if applicable.
4. Check for recent work or part changes: if the HCM or any control module was recently replaced or the boiler serviced, note that an incorrect or non‑compatible HCM can trigger this code. If a third-party part was fitted recently, mention this to the engineer.
5. Photograph external wiring and the area around the boiler and any visible connectors (without opening the casing) so you can show these to the engineer.
Diagnostic and fix steps for a qualified engineer (summary of professional procedure):
1. Verify fault history and read cause code(s) from the information menu. Identify if this is a persistent fault or intermittent/locking fault.
2. Confirm the HCM part number and software/firmware compatibility for that specific Greenstar i model. If an incorrect HCM is fitted, replace with the correct unit.
3. With power isolated, remove the boiler casing and visually inspect the control box and HCM connections for loose pins, broken connectors, corrosion, burn marks or poor seating. Check ribbon/plug connections and any earth/ground continuity.
4. Re-seat the HCM and connectors and retest. If reseating clears the fault and the module tests OK, run the boiler through a full demand cycle and re-check for stored codes.
5. If the HCM or control box is found defective, replace the faulty module(s) following manufacturer instructions and use genuine Worcester Bosch parts. After replacement, clear fault codes, run commissioning checks, and confirm correct operation of CH and DHW under load.
6. Check related sensors and wiring (flow/return NTCs, flame and ignition circuits) because a control fault can sometimes be triggered by secondary sensor issues or intermittent signals.
7. After repair, reset the boiler and confirm no further EH 258 or linked cause codes reappear. Run the system, check temperature regulation and ensure there are no further communication errors between modules.
When to call a professional and why:
- If the fault persists after a safe reset or if you see the EH 258 code at all, stop and call a Gas Safe registered heating engineer or Worcester Bosch technical support. The repair requires opening the boiler and handling live electrical and gas components; only a qualified engineer should perform these tasks.
- Provide the engineer with the exact error code, any cause code, details of any recent servicing or parts changes, and the photos you took. This helps speed up diagnosis and ensures the engineer brings the correct replacement parts.
Notes:
- Do not attempt to jury‑rig or bypass control modules. Repeatedly resetting the boiler without fixing the underlying fault can mask a failing component and lead to more complex failures.
- After a professional repair, the engineer should clear the code and show you the boiler operating normally; ask for a report and part numbers if parts were changed so you have a record for warranty/service history.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar I System / Combi Boiler.