Worcester Bosch EH Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch EH fault code mean?
The EH fault code — most commonly displayed as EH 258 — tells you that the boiler's Heat Control Module (HCM) has been flagged as defective or is not communicating correctly with the rest of the boiler's control system. The HCM is the component responsible for governing temperature regulation and coordinating the efficient running of the burner and heating circuits. When the boiler detects a problem with this module — whether that's an internal failure, a wiring issue, or an incompatible unit — it locks out and displays EH 258 to prevent unsafe operation. On certain Greenstar variants this fault may also be described in service documentation as a burner control unit fault (sometimes cross-referenced with fault designation 9U when an incorrect HCM has been fitted). In all cases, the boiler will not fire until the underlying issue is resolved by a qualified engineer.
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 (HCM) Common
The HCM itself has developed an internal fault and is no longer operating within acceptable parameters. This is the most straightforward explanation and typically requires the module to be replaced with a compatible unit sourced for your specific boiler variant.
- Loose or corroded HCM wiring connections Common
The wiring harness or plug connections that link the HCM to the boiler's control circuit have worked loose or have corroded over time. This can cause intermittent or persistent EH 258 faults even when the HCM is otherwise functional. An engineer will inspect and reseat or replace any affected connectors.
- Incorrect or incompatible HCM fitted Sometimes
A previous repair may have resulted in the wrong HCM variant being installed. Worcester Bosch Greenstar models can require specific HCM versions, and fitting an incompatible unit will trigger EH 258 (sometimes accompanied by reference to fault code 9U in engineer diagnostics). The fix is to source and fit the correct module for the exact boiler model.
How to fix it
- Check that the boiler has not simply tripped on a one-off glitch — attempt a single reset DIY safe
Press and hold the reset button on your Worcester Bosch boiler for approximately 3 seconds, then release. Allow the boiler a minute or two to attempt a restart. If the EH 258 code returns immediately or after only a short period of operation, do not keep resetting — proceed to the next steps.
- Verify your gas supply is working DIY safe
Check that other gas appliances in your home (a gas hob, for example) are working normally. If they are not, contact your gas supplier, as the issue may be a supply interruption rather than a boiler fault. If gas supply is confirmed, the EH 258 fault is internal to the boiler.
- Do not attempt to open the boiler casing or touch any internal components Gas Safe engineer
The HCM, its wiring, and the associated control electronics are housed inside the boiler casing. Accessing these components without Gas Safe registration is both dangerous and a legal requirement in the UK. Leave the casing intact and do not attempt to disconnect or reconnect any plugs or modules yourself.
- Contact a Gas Safe registered engineer to diagnose and repair the HCM fault Gas Safe engineer
A qualified engineer will open the casing safely, identify whether the fault is due to a failed HCM, loose connections, or an incorrectly fitted module, and carry out the appropriate repair. They will cross-reference the installed HCM part number against the boiler's technical specification to confirm compatibility before deciding whether to reseat connections or replace the unit entirely.
Parts you may need
- Heat Control Module (HCM) — Worcester Bosch Greenstar compatible · from £180
- Wiring harness / HCM connector plug · from £25
- Boiler control PCB (if engineer diagnoses broader control fault) · from £220
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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I fix an EH 258 fault myself on my Worcester Bosch boiler?
No. The EH 258 fault is entirely internal to the boiler and involves the Heat Control Module and its associated wiring. Working on these components without Gas Safe registration is illegal in the UK and could be dangerous. The only homeowner action worth trying is a single boiler reset and confirming your gas supply is on. Beyond that, you need a Gas Safe registered engineer.
How much does it cost to fix a Worcester Bosch EH 258 fault in the UK?
For most households the repair falls in the range of £150–£380, covering engineer call-out and labour plus parts. If the engineer finds only a loose connection that needs reseating, you'll be at the lower end. If the HCM itself needs replacing, costs typically sit between £250 and £380 once parts and labour are included. In rare cases where the fault points to a wider PCB or control board issue, costs can exceed £400 — it is worth asking your engineer for a written quote before authorising any parts.
What is the difference between EH 258 and fault code 9U on Worcester Bosch boilers?
EH 258 is the primary fault code indicating the Heat Control Module is defective or has a connection problem. Fault designation 9U is a related sub-code that specifically flags the situation where an incorrect or incompatible HCM has been fitted to the boiler — for example, following a previous repair where the wrong module version was sourced. An engineer encountering 9U will focus on confirming the fitted HCM's part number matches Worcester Bosch's specification for your exact model before doing anything else.
My Worcester Bosch boiler keeps showing EH 258 after being repaired — what should I do?
Recurring EH 258 faults after a repair usually suggest either that the replacement HCM was not the correct variant for your boiler model, or that there is an underlying wiring or PCB issue that was not fully resolved. Ask your engineer to double-check the HCM part number compatibility and inspect the wiring harness thoroughly. If your boiler is over 10–12 years old and experiencing repeated control module faults, it may be worth getting a replacement boiler quote alongside the repair estimate — a new A-rated condensing boiler typically costs £2,000–£4,500 installed, but avoids ongoing repair bills on ageing components.