Biasi Prisma Fault codes & diagnostics

8 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 24 kW 1998-2002 Discontinued

Replaced by: Prisma 24 SE

Choose your boiler type

The Prisma range comes in 3 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

24SR 8 codes · 1 GC number 24S 8 codes · 1 GC number 28S 8 codes · 1 GC number
Each model has its own manual A family spans several manuals, one per model or generation. Pick your model above and its page has the manual its fault codes were read from.

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
8 High
Just want the most common Biasi Prisma faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 8 documented codes

Tap any card for details

Fault on gas feeding

Your boiler is not receiving the gas it needs to create a flame, causing the system to shut down for safety.

High DIY-safe £120-350 Shared across the Prisma range

What to check first

  1. Check if other gas appliances in your home, such as a hob, are working correctly.
  2. Ensure your gas meter has credit if you use a prepay meter.
  3. Check that the gas isolation valve under the boiler is in the 'on' or 'open' position.
  4. Try resetting the boiler by turning the selector knob to the 'R' position for a few seconds.

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Printed Circuit Board (PCB)
Full explanation for Fault on gas feeding
Faulty flame detection

Your boiler is struggling to detect if the burner has successfully lit, which has caused the system to shut down for safety.

High Engineer only £120-220 Shared across the Prisma range

Parts commonly replaced

  • Flame detection electrode
  • Ionisation lead
  • Electronic control board (PCB)
Full explanation for Faulty flame detection
Faulty gas valve

Your boiler has stopped working because gas is not being successfully supplied to the burner, causing the system to shut down for safety.

High Engineer only £220-£380 Shared across the Prisma range

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Wiring Harness
Full explanation for Faulty gas valve
Faulty ignition

Your boiler is unable to light the flame needed to heat your water or radiators, causing the system to shut down for safety.

High Engineer only £100-220 Shared across the Prisma range

Parts commonly replaced

  • Ignition electrode
  • Ignition lead
  • Printed Circuit Board (PCB)
Full explanation for Faulty ignition
Faulty ignition device

Your boiler is unable to light the flame needed to heat your water and home, causing the system to shut down for safety.

High Engineer only £140-£280 Shared across the Prisma range

Parts commonly replaced

  • Ignition Electrode
  • Ignition Lead
  • Printed Circuit Board (PCB)
Full explanation for Faulty ignition device
Flue gas pressure switch

Your boiler has detected a problem with how it safely vents exhaust fumes, which is preventing it from lighting to ensure your safety.

High Engineer only £120-250 Shared across the Prisma range

Parts commonly replaced

  • Flue gas pressure switch
  • Venturi tube
  • Fan assembly
  • Air pressure switch tubes
Full explanation for Flue gas pressure switch
Gas injectors plugged

Your boiler has stopped working because the small nozzles that deliver gas to the burner have become blocked, preventing the flame from starting safely.

High Engineer only £120-£250 Shared across the Prisma range

Parts commonly replaced

  • Gas Injectors
  • Burner Assembly
  • Gas Valve (if debris is present)
Full explanation for Gas injectors plugged
Lock state

Your boiler has shut down completely for safety because it failed to light or stay lit after several attempts.

High DIY-safe £90-280 Shared across the Prisma range

What to check first

  1. Check if your gas supply is active and credit is on the meter
  2. Press the 'Reset' button (located next to the lockout lamp) for five seconds
  3. Monitor if the blue burner light stays on after the reset

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Ignition PCB
Full explanation for Lock state
Status & diagnostic codes

2 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

10 Diagnostic Your boiler has detected an issue with the external outdoor temperature sensor, which helps it decide how hard to work based on how cold it is outside. 12 Diagnostic Your boiler has detected a failure to ignite, meaning it cannot light the burner to provide you with heating or hot water.