Protherm 100EC Fault codes & diagnostics

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

Combi Natural Gas 24 kW 2000 onwards Discontinued
GC Number 47-920-33

Replaced by: Tiger

Download the Protherm 100EC manualThe official installation and service manual (PDF), the document these fault codes were read from. PDF

The fault code is already on your display

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

Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-920-33 100EC 2000–present

Appliance names and production years are taken from the UK Product Characteristics Database (the SAP/SEDBUK register). Where a GC number is also printed in this model's own manufacturer manual, both sources were checked to agree.

Just want the most common Protherm 100EC faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 5 documented codes

Tap any card for details

F0

Your boiler has stopped working because the water pressure inside the system has dropped too low or there is trapped air.

High DIY-safe

What to check first

  1. Locate the filling loop valves underneath your boiler.
  2. Open the valves slowly to let water in until the pressure gauge reads between 1 and 1.5 bar.
  3. Close the valves tightly and restart your boiler.

Parts commonly replaced

  • Water pressure sensor
  • Expansion vessel

Related codes

Full explanation for F0
F1

Your boiler has safely shut itself down because it failed to ignite, meaning you currently have no heating or hot water.

High Engineer only £100-£250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F1
F2

Your boiler has temporarily stopped working because the water temperature inside the system has dropped too low or a sensor has stopped reading it correctly.

High Engineer only £90-180

Parts commonly replaced

  • Central heating temperature sensor (NTC thermistor)
  • Wiring harness

Related codes

Full explanation for F2
F3

Your boiler has stopped working because scale or debris has built up and blocked the internal heat exchanger.

High Engineer only £150-350

Parts commonly replaced

  • Heat exchanger
  • Flushing chemicals
  • In-line scale reducer

Related codes

Full explanation for F3
F4

Your boiler is struggling to accurately measure your hot water temperature, which usually means it will temporarily stop providing hot water until the sensor is fixed.

Medium Engineer only £90-180

Parts commonly replaced

  • Domestic Hot Water NTC Thermistor
  • Wiring harness

Related codes

Full explanation for F4