Worcester Bosch EH 258 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch EH 258 fault code mean?
EH 258 indicates an internal fault within the boiler's heat and burner control electronics. On Greenstar i Combi models, Worcester Bosch refer to this as a Heat Control Module (HCM) fault — the HCM is the electronic brain responsible for managing burner operation and temperature regulation. On CDi and some other Worcester Bosch models, the same code points to a fault in the burner control unit itself. In both cases the boiler locks out to prevent unsafe operation. The fault may stem from the module failing internally, or from a wiring or connector issue preventing the module from communicating correctly. A manual sub-description of 'internal error' is consistent with both interpretations: the control electronics have detected a condition they cannot recover from automatically.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Defective Heat Control Module or burner control unit Common
The HCM or burner control unit has failed internally — a component has broken down or the firmware has entered an unrecoverable error state. This is the most common root cause when EH 258 appears on older boilers or after a power surge.
- Loose, corroded or disconnected wiring to the control module Common
The plug and wiring harness connecting the HCM or burner control unit to the rest of the boiler can work loose over time, or contacts can corrode. Even a marginal connection can prevent the module from functioning correctly and trigger a lockout.
- Incompatible or incorrectly matched HCM Sometimes
If a control module was previously replaced with a unit that does not match the specific boiler model's requirements — perhaps after an earlier repair — the mismatch can cause persistent control errors including EH 258.
- Power surge or transient electrical fault Sometimes
A sudden voltage spike can corrupt the control electronics or cause a temporary lockout. In some cases the boiler will recover after a reset; if EH 258 returns immediately or repeatedly, the module itself has sustained damage.
How to fix it
- Reset the boiler once or twice using the reset button DIY safe
Press and hold the reset button for approximately three seconds. Wait two minutes for the boiler to attempt a restart. If EH 258 clears and the boiler runs normally, monitor it closely — a single occurrence after a power cut can be a one-off. If the code returns within a short period, further resets will not help and a Gas Safe engineer is needed.
- Check that the gas supply is active DIY safe
Confirm other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier rather than an engineer, as the fault may be with the supply rather than the boiler.
- Gas Safe engineer to inspect wiring and connector plugs on the control module Gas Safe engineer
An engineer will remove the boiler casing and carefully examine the plug connectors and wiring loom running to the HCM or burner control unit. Loose pins will be re-seated and any corroded contacts cleaned or replaced. This is often the quickest and least expensive resolution.
- Gas Safe engineer to verify HCM compatibility with the boiler model Gas Safe engineer
The engineer will cross-reference the installed module's part number against Worcester Bosch's technical documentation for the specific boiler model and serial number. If a previous repair introduced an incompatible unit, the correct replacement will be sourced.
- Gas Safe engineer to replace the Heat Control Module or burner control unit if found to be defective Gas Safe engineer
Where the module itself has failed, it must be replaced with the correct Worcester Bosch part. Programming or parameter setup may be required after fitting. The engineer should carry out a full functional test and flue gas analysis before leaving.
- Contact a Gas Safe registered engineer if you have not already done so DIY safe
EH 258 is not a fault a homeowner can safely or legally resolve. If the steps above have not already involved an engineer, book one now. You can verify any engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Heat Control Module (HCM) — Worcester Bosch Greenstar i Combi · from £220
- Burner control unit — Worcester Bosch Greenstar CDi · from £195
- Wiring harness / connector plugs (HCM loom) · from £35
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£400, depending on the underlying cause.
Frequently asked questions
Can I fix Worcester Bosch EH 258 myself?
No. EH 258 is a fault within the internal control electronics of the boiler, and working on these components requires a Gas Safe registered engineer. The only homeowner actions worth trying are a boiler reset (up to two or three times) and confirming your gas supply is active. If the code persists, call an engineer — attempting DIY repairs on control units is both unsafe and illegal in the UK.
How much does it cost to repair EH 258 in the UK?
If the fault turns out to be a loose or corroded wiring connection, the repair typically costs £120–£180 including the engineer's call-out and labour. If the Heat Control Module or burner control unit needs replacing, expect to pay £250–£400 for parts and labour combined. In rare cases where a full PCB replacement is needed (on older or less common models), costs can reach £500 or more — but this is outside the typical range most people encounter.
My boiler is over 10 years old and showing EH 258 — should I repair or replace it?
It is worth getting a diagnosis first, since a connection fix is relatively inexpensive. However, if the HCM or control unit needs replacing on an older boiler, factor in the difficulty of sourcing parts and the likelihood of further faults on ageing components. As a rough guide, if the repair quote exceeds one third of the cost of a new boiler, or if your boiler has needed repeated repairs in recent years, replacement often makes more financial sense. A new combi boiler installed by a Gas Safe engineer typically costs between £1,400 and £2,000 in the UK.
Will an annual boiler service help prevent EH 258 from occurring?
Yes. During an annual service a Gas Safe engineer checks electrical connections, inspects the control components for signs of wear or corrosion, and addresses minor issues before they cause a lockout. Regular servicing also keeps your manufacturer's warranty valid and is the single most effective way to extend the working life of your boiler.