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

Replaced by: Tiger

Download the Protherm 100EC manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. 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 →

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 to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.
  2. Slowly open the tap or taps on the filling loop until you hear water flowing.
  3. Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps fully.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for F0
F1

Your boiler has failed to light or has lost its flame, so it has shut down for safety purposes.

High DIY-safe £100-280

What to check first

  1. Check if your gas meter has credit and the gas supply to the house is turned on
  2. Check if other gas appliances in your home, like a hob, are working correctly
  3. Press the reset button on the front of the boiler to try and restart it

Parts commonly replaced

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

Related codes

Full explanation for F1
F2

Your boiler has stopped working because it cannot correctly measure the temperature of the water inside the pipes.

High Engineer only £90-160

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for F2
F3

Your boiler is unable to circulate water properly through its internal heating system, causing it to shut down for safety.

High Engineer only £150-350

Parts commonly replaced

  • Primary heat exchanger
  • System filter
  • Central heating pump

Related codes

Full explanation for F3
F4

Your boiler is struggling to accurately measure the temperature of your hot water, which usually means your taps will only run cold.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring harness connections

Related codes

Full explanation for F4
Status & diagnostic codes

9 operating states & engineer diagnostics

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

n1 Diagnostic Your boiler is currently operating in its base 10kW heating mode, which is a standard status indicator rather than a fault, meaning the appliance is functioning correctly. n2 Diagnostic Your boiler is displaying a status code indicating that it is currently operating at its lower heat output setting. n3 Diagnostic Your boiler has detected an issue with the sensor that monitors the temperature of the water circulating through your heating system. n4 Diagnostic Your boiler has identified a communication failure between the control panel and the main heating system, preventing it from operating correctly. n5 Diagnostic Your boiler's internal computer has lost communication with the main control board, which means it cannot safely manage your heating or hot water. n6 Diagnostic Your boiler has detected that the water flow inside the system is restricted or blocked, preventing it from heating your home properly. n7 Diagnostic Your boiler's internal computer is struggling to communicate with its control sensors, preventing it from safely starting your heating or hot water. n8 Diagnostic The boiler's temperature sensors have detected an issue with the water flow or heat exchange, causing the system to stop working to prevent internal damage. n9 Diagnostic Your boiler is currently unable to operate because it has detected an issue with the internal computer software or its communication with the display panel.