Common problems guide

The most common Sime Format C Combi problems

The faults most likely to send a Sime Format C Combi into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.

The 3 most common faults

01

Bi-colour orange led High Engineer

Your boiler has stopped working because it cannot properly read the temperature of the water inside the heating system.

Est. cost: £90-180
Parts: Central heating NTC thermistor, Wiring harness

Full guide for Bi-colour orange led

02

Green led flashing High Engineer

Your boiler has temporarily stopped working because it cannot safely clear its exhaust gases.

Est. cost: £150-300
Parts: Air pressure switch, Flue fan, Pressure switch silicone tubes

Full guide for Green led flashing

03

Red led on High Engineer

Your boiler has safely shut down because it failed to ignite or detected an issue with the exhaust gases.

Est. cost: £100-250
Parts: Ignition electrode, Flue/smoke thermostat, Gas valve

Full guide for Red led on

Parts most often replaced

on the Sime Format C Combi

Central heating NTC thermistorWiring harnessAir pressure switchFlue fanPressure switch silicone tubesIgnition electrode

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

You can smell gas or see signs of a leak
The Sime shows an Emergency or High severity code
The boiler keeps locking out after repeated resets
You've tried the DIY checks and the fault hasn't cleared
There's visible water leaking from the boiler
The flame is yellow or orange instead of blue

See the full 3-code list for the Sime Format C Combi