Worcester Bosch boilers use error codes to flag a fault before it gets worse. The EH 258 code points to a problem with the Heat Control Module, or HCM
This guide explains what EH 258 means, what causes it, and how to get it fixed. It applies to the Greenstar i Combi and other Worcester Bosch models that use the HCM.
If your boiler is showing a different code, our Worcester Bosch boiler fault code guide covers what each one means.
What the EH 258 error code means
The HCM controls the boiler’s temperature and keeps it running efficiently. When your boiler shows EH 258, the HCM isn’t talking to the rest of the system properly.
That breaks the heating control loop, so the boiler shuts down and throws the code rather than running with a fault.
What causes EH 258
There are two usual causes:
- The control box or HCM is faulty.
- A connection inside the control box has come loose.
Either one disrupts the heating control system and triggers EH 258.
How to fix the EH 258 error code
EH 258 is an internal fault. The parts that need checking sit inside the boiler casing, so this is a job for a Gas Safe engineer. Here’s what they’ll do.
Check HCM compatibility
The wrong HCM for your model can set off the code. The engineer confirms the fitted module matches your boiler’s specification against the manufacturer’s documentation.
Check for loose connections
They inspect the control box and refit or repair any loose connection, following Worcester Bosch guidelines. That usually clears the fault and the code.
Replace a faulty HCM
If the module itself has failed, it’s swapped for the correct part. The boiler is then tested to confirm EH 258 has cleared.
Why EH 258 isn’t a DIY fix
Opening the boiler casing is dangerous and, for anyone who isn’t Gas Safe registered, illegal. If EH 258 shows up, book a qualified engineer rather than trying to reset your way out of it.
Stop EH 258 coming back
Most HCM and connection faults get picked up early during a service. An annual boiler service by a Gas Safe engineer keeps your Worcester Bosch running and catches small issues before they turn into a breakdown.
Book a Worcester Bosch engineer
Got an EH 258 code you can’t clear? Our Worcester Bosch-accredited engineers will diagnose and fix it properly — and sort any other heating issue while they’re there.
Call us or book online for boiler repair in London, and we’ll get an engineer out to you.
