21E High
Your boiler has detected that the water returning from your radiators is too hot, which has caused it to shut down for safety.
Technical description: Central heating return sensor overheating
Typical repair cost: £120-250 (including labour)
DIY fix possible
Ensure all radiator valves (TRVs) are fully open and not stuck.Check that the internal boiler pressure is between 1 and 1.5 bar.Reset the boiler using the control knob or button to see if the fault clears.
Protect yourself from future repair bills
A boiler cover plan would cover faults like 21E — no call-out fees, no surprises.
*Affiliate links — we may earn a commission at no extra cost to you. Prices shown are starting prices.
Parts commonly replaced
- Central heating return NTC sensor
- Circulation pump
- Main heat exchanger (flushing service)
*Affiliate links — we may earn a commission at no extra cost to you.