Overview
The F0 258 (sometimes shown as EH 258 on Greenstar i/4000 interfaces) is a fault that points to a problem with the boiler’s control electronics — specifically the control box (main PCB) or the Heat Control Module (HCM). The HCM is a sub-module that helps the boiler manage temperature sensing, timing and communications with sensors and user controls; the control box is the main logic board. The error means the boiler has detected an invalid response, a communication fault or a hardware failure in one of those components. Why it occurs: common causes are a defective HCM or control box, a wrongly fitted or incompatible HCM, a loose or corroded connector/wiring fault between the HCM and control box, or intermittent electrical supply issues affecting the electronics. Severity is medium-high: the boiler may go into lockout or run unreliably and because this concerns the electronic control of a gas appliance it should not be treated lightly. Simple checks can be done by a homeowner, but diagnosing and replacing control electronics is a job for a qualified, Gas Safe registered engineer (or the manufacturer’s approved service) to ensure safe, correct repair and correct part coding/compatibility.
Possible Cause: Control box or the heat control module (HCM) is defective.
Troubleshooting Steps
Safety first: do not attempt live electrical work or gas repairs unless you are a qualified Gas Safe engineer. If you are not competent, switch the boiler off at the electrical isolator (fused spur) and contact a Gas Safe engineer or Worcester Bosch technical support. Repeated resets are not a fix and should be avoided if the fault returns immediately. If the boiler is under warranty, contact the installer or Worcester before any component is changed.
Initial homeowner checks you can safely do:
1) Record the display exactly — note the fault (F0) and cause number (258) and any secondary messages. Also note whether there is a flashing blue light or triangle. These details help an engineer.
2) Try a controlled reset: press and hold the reset or check/OK button for the time specified in your manual (around 3–8 seconds) until the display reboots. Wait to see if the code returns. If it clears and the boiler runs normally, monitor it — if it reappears, call an engineer.
3) Check mains power to the boiler: ensure the fused spur is on, the household RCD hasn’t tripped and the boiler has a steady supply. Do not open the boiler while power is on.
4) Check basic system conditions: confirm central heating demand (thermostat or timer calling for heat), and check boiler pressure is in the normal range (typically ~1–1.5 bar). Low pressure rarely causes this specific fault but is good to confirm while diagnosing.
5) Consider recent work or part changes: if the HCM or control box was recently replaced or serviced, incorrect HCM coding or a non‑compatible module is a common cause. Make a note and tell the engineer.
Visual, non-invasive checks (power isolated):
6) Isolate electrical supply at the boiler spur before opening the case. If you are comfortable and competent to remove the outer casing, you may perform a visual inspection only – do not touch live parts. Look for loose ribbon cables or multi-pin connectors between the HCM and control box, signs of burnt components, corrosion or water ingress. Gently reseat connectors only if you can do so with the power isolated and you are confident — do not force connectors or attempt soldering.
7) Check for obvious signs of water leaks or ingress around the PCB area and condensate route. Water damage to the control box will require professional replacement.
Diagnostic steps to pass to an engineer or Worcester support:
8) Use the boiler’s information/menu (spanner/return button) to read cause codes and service menus and report these to the engineer or technical support. Note whether it’s a locking error or blocking error and whether a cause code is flashing.
9) Ask the engineer to verify HCM and control box part numbers and firmware/code plug compatibility, check for intermittent connections, and run electrical diagnostics on the PCB, sensors and ignition circuits. If replacement of the HCM or control box is required it must be fitted and coded by a qualified engineer.
When to call a professional: If the error does not clear after a reset, if it returns immediately, if you find any signs of water damage, burnt components or loose multi-pin connectors you cannot safely reseat, or if the HCM has recently been changed — stop and call a Gas Safe engineer or Worcester Bosch technical support. Only a trained engineer should replace the HCM or control box, verify compatibility, and carry out any repairs that affect gas safety and electrical integrity. Provide the engineer with the exact fault and cause numbers, any recent service history or part changes, and whether the boiler is under warranty.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar I System / Combi Boiler.