Common problems guide

The most common Glow-worm Ultracom 2 SXI problems

The faults most likely to send a Glow-worm Ultracom 2 SXI into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

9 documented codes
6 most common
0 DIY-safe checks
6 engineer needed

The 6 most common faults

01
F1 High Engineer

Your boiler has failed to light the flame needed to provide heating or hot water, and it has now locked itself out for safety.

Why it happens: This fault occurs when the temperature sensor inside your hot water storage tank detects that the water has dropped to a near-freezing level. To protect the internal components from being damaged by expanding ice, the boiler temporarily pauses operation until the water temperature rises to a safe level. This is typically caused by extremely cold incoming mains water or a storage tank located in an unheated area like a garage or loft.

Est. cost
£120-250
Parts
Ignition lead, Ignition electrode, Gas valve
Full guide for F1
02
F.2 High Engineer

Your boiler has detected that it is running too hot and has automatically shut down to prevent damage to its internal parts.

Est. cost
£120-250
Parts
NTC thermistor, Pump, Heat exchanger
Full guide for F.2
03
F.3 High Engineer

Your boiler has detected an issue with the internal fan that pushes exhaust gases out of your home, so it has safely shut down to prevent unsafe operation.

Est. cost
£180-350
Parts
Fan assembly, Printed Circuit Board (PCB), Wiring harness
Full guide for F.3
04
F.4 High Engineer

Your boiler is successfully igniting but is unable to keep the flame burning, causing it to shut down shortly after starting.

Est. cost
£150-350
Parts
Ignition electrode, Flame rectification probe, Gas valve
Full guide for F.4
05
F.10 High Engineer

Your boiler has lost the electrical connection to a sensor that measures the temperature of the water returning to it, so it has shut down as a precaution.

Est. cost
£120-220
Parts
Return NTC thermistor, Wiring harness
Full guide for F.10
06
F.13 High Engineer

Your boiler's computer memory has encountered an internal error and can no longer process instructions correctly, meaning the unit will not start.

Est. cost
£250-450
Parts
Main PCB
Full guide for F.13
Parts most often replaced

on the Glow-worm Ultracom 2 SXI

Ignition leadIgnition electrodeGas valvePCBNTC thermistorPump

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 Glow-worm 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
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See the full 9-code list for the Glow-worm Ultracom 2 SXI

Related guides

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