343 High

Worcester Bosch 343 Fault Code

The Worcester Bosch 343 fault code means: your boiler is detecting an unusual temperature imbalance that suggests the internal heat exchanger or circulation system isn't working as it should, so it has shut down for protection.. It appears on 4 Worcester Bosch models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: ED CH: flue gas temperature rising but supply temperature not
Typical repair cost £180-350 (including labour)

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 →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like 343: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Worcester Bosch models that show the 343 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Worcester Bosch 343 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 7 July 2026