J6-21 Medium
Your boiler has detected that the water coming back from your radiators is hotter than the water going out, which usually means there is a blockage or the internal sensors are confused.
Technical description: Return temperature is higher than flow temperature. Burner operates itself after a small amount of time but if the problem repeats you need to Contact a heating engineer
Typical repair cost: £100-280 (including labour)
DIY fix possible
Check that all radiator valves (TRVs) are fully openEnsure the boiler pressure is between 1.0 and 1.5 barReset the boiler to see if the temporary sensor error clears
Protect yourself from future repair bills
A boiler cover plan would cover faults like J6-21 — no call-out fees, no surprises.
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Parts commonly replaced
- Flow temperature thermistor
- Return temperature thermistor
- Circulation pump
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