Common problems guide

The most common Heatline Vizo Plus 24 problems

The faults most likely to send a Heatline Vizo Plus 24 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.
26 documented codes
6 most common
3 DIY-safe checks
3 engineer needed

The 6 most common faults

01
18 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

What to try first

Locate the filling loop, which is a silver braided hose connected to two valves beneath the boiler.Open both taps on the filling loop slowly to let water into the system until the pressure gauge reads between 1 and 1.5 bar.Close both taps tightly and check the boiler display to see if the fault code has cleared.

Est. cost
£80-120
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for 18
02
19 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.

What to try first

Locate the external filling loop (a flexible silver hose pipe) underneath your boiler.Open the small valve or tap attached to the loop slowly until you hear water entering the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close the valve tightly and check that the error code has cleared.

Est. cost
£80-120
Parts
Filling loop, Pressure relief valve, Expansion vessel
Full guide for 19
03
26 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

What to try first

Locate the external filling loop, which is usually a flexible metal or braided hose connected to two valves beneath the boiler.Open both valves slowly to allow water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close both valves tightly and ensure the pressure gauge stays steady.

Est. cost
£80-120
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for 26
04
900 High Engineer

Your boiler has detected an electrical issue where it is not receiving the correct voltage required to run its components safely.

Est. cost
£150-300
Parts
Printed Circuit Board (PCB), Wiring harness, Transformer
Full guide for 900
05
20 High Engineer

Your boiler has detected a critical internal hardware fault with its main electronic control unit, preventing it from operating safely.

Est. cost
£250-450
Parts
Main Printed Circuit Board (PCB), Wiring harness
Full guide for 20
06
600 High Engineer

Your boiler is failing to start up properly after multiple attempts.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, Printed Circuit Board (PCB)
Full guide for 600
Parts most often replaced

on the Heatline Vizo Plus 24

Filling loopPressure relief valvePressure sensorExpansion vesselPrinted Circuit Board (PCB)Wiring harness

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 Heatline 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 26-code list for the Heatline Vizo Plus 24

Related guides

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