BoilerFaultCodesYour boiler is unable to provide hot water because it cannot accurately measure how hot the water is.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Hot water temperature sensor is defective – the plug to the hot water temperature sensor may not be connected
What causes this fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors your tap water temperature. The most common reason is that the sensor itself has developed an internal electrical failure or the wiring connecting it to the main system has come loose or become corroded. Because the boiler cannot safely verify how hot the water is, it stops heating it entirely to prevent the risk of scalding.
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 →Protect yourself from future repair bills
A boiler cover plan would cover faults like 1021 B — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
Affiliate links — we may earn a commission at no extra cost to you.