High
66 H.....1..... on Ideal Isar M30 / M100

Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, causing it to shut down as a safety precaution.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

FLOW THERMISTOR ERROR

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

H1View →

Related guides you might find useful

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →

New boiler cost 2026

Realistic UK prices for a swap.

Read →