Medium Lockout
Fault 03 / 21 on Worcester Bosch Greenstar CDi Compact

Your boiler's built-in thermostat controls has developed an electrical fault and can no longer tell how warm your room is.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Room temp sensor faulty: There is a short circuit on the room temperature sensor built into the FW 100.

What causes this fault?

This fault occurs when the electrical components inside the thermostat unit fail, preventing it from sending a clear signal to the boiler. The most common reason is an internal short circuit or a degraded sensor probe that can no longer accurately measure the air temperature in your home.

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 fault codes

A1View →C6View →EAView →

Related guides you might find useful

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →