BoilerFaultCodesWorcester Bosch T4 Fault Code
The Worcester Bosch T4 fault code means: your boiler is struggling to switch between providing heating and hot water because of an internal valve check failure.. It appears on 11 Worcester Bosch models in our database.
Technical description: Three way valve test
What causes the Worcester Bosch T4 fault?
This fault occurs when the boiler's internal computer tries to move the diverter valve to switch between heating and hot water, but the movement isn't detected correctly. The most common reason is that the valve has become stuck due to a buildup of debris in the system or the small electric motor that moves the valve has failed.
This fault is frequently seen at the start of autumn when the heating is turned back on after being idle all summer, which can cause internal moving parts to seize.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Worcester Bosch models that show the T4 fault code
Click a model for the full fault code list and model-specific repair notes.
- 24 RSF Repair: £180-350
- 26 CDi Xtra Repair: £180-£350
- 30i Junior Repair: £180-280
- 35 RSF Repair: £180-350
- Danesmoor Repair: £180-£350
- Greenstar CDi Compact Repair: £150-250
- Greenstar Si Combi / i System Repair: £180-350
- Heatslave Repair: £180-350
- Hideaway Repair: £180-350
- Junior 24i Repair: £180-350
- Junior 28i Repair: £180-350
Source
Worcester Bosch T4 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026