Common problems guide

The most common Sime Format B Combi problems

The faults most likely to send a Sime Format B 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.
5 documented codes
5 most common
2 DIY-safe checks
3 engineer needed

The 5 most common faults

01
Red LED on (solid) High DIY-safe

Your boiler has stopped working because it cannot light the gas flame or it cannot detect that the flame has started.

What to try first

Check that your gas emergency control valve is in the 'open' positionVerify if other gas appliances in your home, such as a hob, are receiving gasPress and hold the reset button for a few seconds to restart the ignition sequence

Est. cost
£100-280
Parts
Ignition Electrode, Flame Sensing Probe, Gas Valve
Full guide for Red LED on (solid)
02
Red LED after flow switch High DIY-safe

Your boiler has detected that water isn't moving through the system properly, causing it to overheat and shut down for safety.

What to try first

Check that the boiler pressure is between 1 and 1.5 bar on the gaugeEnsure all radiator valves are fully open to allow water flowCheck for any visible leaks around the pipeworkTry resetting the boiler using the control knob

Est. cost
£120-280
Parts
Circulating Pump, Water Flow Switch, Limit Thermostat
Full guide for Red LED after flow switch
03
Red LED on (after safety stat) High Engineer

Your boiler has shut down because it has detected an internal temperature that is too high or a problem with how exhaust gases are leaving the flue.

Est. cost
£120-350
Parts
Overheat Thermostat, Circulation Pump, Flue Thermostat
Full guide for Red LED on (after safety stat)
04
LED flashing after fan High Engineer

Your boiler has stopped working because it cannot safely clear away waste gases, likely due to a mechanical issue with internal air circulation.

Est. cost
£180-350
Parts
Fan assembly, Air pressure switch, Fan wiring harness
Full guide for LED flashing after fan
05
Orange LED flashing High Engineer

Your boiler has detected a problem with the temperature sensor that controls your central heating, meaning it cannot accurately measure how hot the water is.

Est. cost
£90-160
Parts
Central heating NTC temperature sensor, Connecting wiring harness
Full guide for Orange LED flashing
Parts most often replaced

on the Sime Format B Combi

Ignition ElectrodeFlame Sensing ProbeGas ValveCirculating PumpWater Flow SwitchLimit Thermostat

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 5-code list for the Sime Format B Combi

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →