Potterton Osprey CF 125 Fault codes & diagnostics

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

Regular Natural Gas 36.6 kW 1994-2005 Discontinued
GC Number 41-590-78

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.
2 Emergency 3 High 2 Low
Just want the most common Potterton Osprey CF 125 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 7 documented codes

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Boiler Safety Overheat neon

Your boiler has switched itself off because it has become too hot to operate safely.

High DIY-safe £100-280

What to check first

  1. Turn the boiler off and allow it to cool down for at least 30 minutes.
  2. Ensure all radiator valves are open and there is no air trapped in the system.
  3. Unscrew the small black cap on the front control panel and press the reset button firmly.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • System Thermistor
Full explanation for Boiler Safety Overheat neon
Flame Failure neon

Your boiler has lost its flame while it was supposed to be running, causing the system to shut down for safety.

High DIY-safe £120-350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the property
  2. Check if your gas meter has credit (if applicable)
  3. Press the reset button once to see if the boiler restarts

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead
  • PCB
Full explanation for Flame Failure neon
Overheat condition

Your boiler has become too hot and has automatically turned its pilot light off as a safety measure to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • PCB
Full explanation for Overheat condition
Power On Neon

This light simply shows that your boiler is receiving electrical power and is switched on.

Low DIY-safe

What to check first

  1. Ensure the main power switch for the boiler is in the 'ON' position
  2. Check that the fuse in the spur hasn't blown if the light fails to illuminate
  3. Monitor the boiler for normal operation as this light indicates standard power status
Full explanation for Power On Neon
TTB constant trip

Your boiler is automatically shutting down because it has detected dangerous exhaust fumes backing up into your home instead of escaping safely through the chimney.

Emergency Engineer only £120-450

Parts commonly replaced

  • TTB Down-draught Thermostat
  • Flue liner
  • Chimney cowl
Full explanation for TTB constant trip
TTB Trip

Your boiler has detected that hot exhaust fumes are not escaping properly through the flue and has switched itself off to prevent poisonous gases from entering your home.

Emergency Engineer only £150-350

Parts commonly replaced

  • TTB sensor
  • Flue terminal
  • Internal flue components
Full explanation for TTB Trip