Low
H03 on Worcester Bosch Greenstar Heatslave II

Your boiler is struggling to read the temperature outside, which may cause it to run less efficiently than usual.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description

Outdoor sensor not fitted/detected or outdoor sensor is open circuit or closed circuit

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 →

Related guides you might find useful

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

New boiler cost 2026

Realistic UK prices for a swap.

Read →