Medium
1021 on Worcester Bosch Greenstar 8000 Style Combi

Your boiler is struggling to accurately measure the temperature of the water being heated for your taps and shower, so it has stopped heating it to prevent overheating.

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.

What the manual says

Manufacturer manual

Cylinder charging or hot water temperature sensor faulty

Shared across the Greenstar 8000 Style range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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 →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like 1021: 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.

All 33 fault codes on the Greenstar 8000 Style Combi

Seeing a different code? Every code this model can display, in one place.

20020120220320420822422722830530681510171037106510681073107410751076292029272946294829502963296429652966296729702971

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

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 →