i0 Low

Sime i0 Fault Code

Also written as I0 — same fault.

The Sime i0 fault code means: your boiler is struggling to read the temperature from the outdoor sensor, which tells the system how hard to work based on the weather.. It appears on 1 Sime model in our database.

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: External sensor temperature
Typical repair cost £120-200 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Sime models that show the i0 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Sime i0 fault code information on this page is sourced and cross-checked against the official Sime service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026