Common problems guide

The most common Sime iQE Simplicity 20T problems

The faults most likely to send a Sime iQE Simplicity 20T 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 6 most common faults

01

ALL 21 High Engineer

Your boiler has detected a general internal fault that requires a professional inspection to determine the exact cause.

Est. cost: £100-£350
Parts: Printed Circuit Board (PCB), Wiring harness, Gas valve

Full guide for ALL 21

02

ALL 22 High Engineer

Your boiler has detected an internal fault that it cannot resolve itself and has locked out for safety.

Est. cost: £120-350
Parts: Main PCB, Ignition Electrode, Gas Valve

Full guide for ALL 22

03

ALL 23 High Engineer

Your boiler has detected an internal electronic error that requires a professional to reset and inspect the system.

Est. cost: £100-£250
Parts: Main PCB (Printed Circuit Board), Wiring Harness

Full guide for ALL 23

04

ALL 24 High Engineer

Your boiler has encountered a critical system error that requires a professional manual reset and internal diagnostic check to function again.

Est. cost: £100-250
Parts: Main PCB, Control Knobs/Interface

Full guide for ALL 24

05

ALL 25 High Engineer

Your boiler has encountered a system-level failure and has stopped working entirely to ensure safety.

Est. cost: £100-350
Parts: Main PCB (Control Board), Air Pressure Switch, Wiring Harness

Full guide for ALL 25

06

ALL 26 High Engineer

Your boiler has encountered a deep-seated system error or sensor failure that requires a professional engineer to diagnose and reset.

Est. cost: £100-£250
Parts: Main PCB, Temperature Sensor, Wiring Harness

Full guide for ALL 26

Parts most often replaced

on the Sime iQE Simplicity 20T

Printed Circuit Board (PCB)Wiring harnessGas valveMain PCBIgnition ElectrodeGas Valve

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 8-code list for the Sime iQE Simplicity 20T