BoilerFaultCodes.com
H9-01 Low Warning

Your boiler is struggling to understand the outdoor temperature, which may cause your heating to run less efficiently or at the wrong temperature.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Outside sensor fault – try resetting the boiler

What causes this fault?

This fault occurs when the boiler loses its connection to the probe located on the outside of your house designed to monitor weather conditions. The most common reason is that moisture or corrosion has damaged the sensor's internal wiring, or the electrical signal between the sensor and the boiler's main control board has been interrupted.

This issue often comes to light during the first cold snap of autumn when the boiler attempts to use outdoor temperature data to adjust your heating for the winter.

Typical repair cost £90-160 (including labour)

DIY fix possible

Locate the reset button (usually a circular arrow or 'R') on the front control panel.Press and hold the reset button for 2 to 3 seconds.Wait for the boiler to restart; if the code reappears, a professional is required to check the wiring.

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Protect yourself from future repair bills

A boiler cover plan would cover faults like H9-01 — no call-out fees, no surprises.

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