Vaillant ecoFIT Pure 825 Fault codes & diagnostics

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

Combi Natural Gas 25 kW 2016-present

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3 Emergency 36 High 5 Medium 5 Low

All 49 documented codes

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con

Your boiler's electronic control boards are failing to talk to each other, which stops the system from operating as it should.

High Engineer only £200-450

Parts commonly replaced

  • Main PCB
  • Display PCB
  • Communication ribbon cable

Related codes

Connection

Your boiler's main control panel is unable to "talk" to the computer brain inside the unit, meaning it cannot receive or process your commands.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • UI connection cable

Related codes

F.00

Your boiler has lost its connection to the temperature sensor, meaning it no longer knows how hot the water is and has stopped working to keep you safe.

High Engineer only £120-220

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Wiring harness

Related codes

F.01

Your boiler has lost the connection to the sensor that monitors the temperature of the water returning to the unit, causing it to stop working for safety reasons.

High Engineer only £120-220

Parts commonly replaced

  • Return temperature sensor
  • Wiring harness

Related codes

F.10

Your boiler has stopped working because a sensor that measures water temperature is providing an incorrect signal, so the system has safely shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • NTC flow temperature sensor
  • Wiring harness

Related codes

F.11

Your boiler has detected a faulty signal from a heat sensor, causing it to shut down for safety to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC thermistor sensor
  • Wiring harness

Related codes

F.12

Your boiler is struggling to detect the temperature of your hot water due to a wiring issue or a faulty sensor, so it has stopped heating to prevent potential damage.

High Engineer only £120-220

Parts commonly replaced

  • Cylinder temperature sensor
  • Wiring harness

Related codes

F.13

Your boiler is having trouble 'reading' the temperature of your hot water supply because a wire or sensor has stopped communicating correctly.

Medium Engineer only £120-220

Parts commonly replaced

  • NTC sensor
  • Wiring harness
F.20

Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • NTC sensor
  • Pump
  • Primary heat exchanger

Related codes

F.22

Your boiler has stopped working because the amount of water circulating inside the system has dropped too low to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop, which is usually a small silver braided hose with one or two valves under your boiler.
  2. Open the valve(s) slowly to allow mains water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.
  3. Close the valve(s) tightly and restart the boiler.

Parts commonly replaced

  • Pressure sensor
  • Pressure relief valve
  • Expansion vessel

Related codes

Showing 11–20 of 49
F.23

Your boiler has sensed that the water inside is heating up too rapidly compared to how quickly it is moving, causing it to shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • Heat Exchanger
  • NTC Thermistor

Related codes

F.24

Your boiler has detected that the water inside it is heating up much faster than it safely should, causing it to shut down to prevent damage.

High Engineer only £150-350

Parts commonly replaced

  • Central heating pump
  • System filter
  • Heat exchanger
  • Flow sensor

Related codes

F.25

Your boiler has detected that the gases it is producing are too hot to be safe, so it has automatically shut down to prevent damage.

Emergency Engineer only £150-350

Parts commonly replaced

  • Flue gas temperature sensor
  • NTC thermistor
  • Primary heat exchanger

Related codes

F.27

Your boiler has detected a signal that suggests a flame is present even when the burner is turned off, causing it to shut down for your safety.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Flame detection probe

Related codes

F.28

Your boiler is trying to start but cannot get the flame to light or stay lit.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

F.29

Your boiler is attempting to start or run, but the flame inside is cutting out unexpectedly, causing it to stop heating.

High Engineer only £150-350

Parts commonly replaced

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

Related codes

F.32

Your boiler's fan, which is essential for safely removing exhaust gases, has stopped working correctly, so the boiler has shut down to prevent unsafe operation.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

F.49

Your boiler is having a communication breakdown with its internal controls or the thermostat, meaning it cannot receive signals to heat your home or water.

High Engineer only £120-250

Parts commonly replaced

  • eBUS PCB
  • Wiring harness
  • External controls
F.61

Your boiler has detected an internal electrical issue with the mechanism that controls the gas supply, meaning it has safely shut itself down to prevent operation.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Gas valve wiring harness

Related codes

F.62

Your boiler has detected that the gas control system is not closing correctly when it should, which automatically stops the boiler from operating for safety reasons.

Emergency Engineer only £200-400

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Showing 21–30 of 49
F.63

Your boiler's computer has encountered an internal memory error, meaning it can no longer communicate with its own operating system to run safely.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)

Related codes

F.64

Your boiler has detected an internal communication error between its electronic components, meaning it has shut down as a precaution to prevent further issues.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)
  • Wiring harness

Related codes

F.65

The main computer inside your boiler is overheating and has shut the system down to prevent internal damage.

High Engineer only £200-450

Parts commonly replaced

  • Main electronic PCB
  • Wiring harness

Related codes

F.67

Your boiler has detected an internal communication error with its main control unit, causing it to shut down for safety reasons.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB
  • Flame rectification electrode

Related codes

F.68

Your boiler is struggling to keep the flame alight consistently, causing it to shut down for safety reasons.

High Engineer only £180-350

Parts commonly replaced

  • Flame rectification electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

F.70

Your boiler’s internal computer has forgotten its identity settings, preventing it from starting up correctly.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB

Related codes

F.71

Your boiler is struggling to read its own internal water temperature, so it has shut down to prevent overheating or running inefficiently.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

F.72

Your boiler is having trouble comparing temperature readings from different parts of the system, so it has stopped heating to prevent damage.

High Engineer only £150-250

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Electronic PCB

Related codes

F.73

Your boiler's sensor is having trouble reading the internal water pressure, which means it has safely shut down to prevent damage until the issue is resolved.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

F.74

Your boiler's water pressure sensor is reporting an incorrect reading, which is preventing the system from running safely.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

Showing 31–40 of 49
F.75

Your boiler is struggling to confirm that water is moving through the system properly when it tries to start up.

High Engineer only £150-300

Parts commonly replaced

  • Pressure Sensor
  • Central Heating Pump

Related codes

F.76

Your boiler has detected that it is running too hot and has automatically shut down to prevent damage to its internal components.

High Engineer only £150-350

Parts commonly replaced

  • Primary heat exchanger
  • NTC thermistor
  • Heating pump

Related codes

F.77

Your boiler has detected an issue with the drainage system or the ventilation safety mechanism, which is preventing it from operating safely.

High Engineer only £150-300

Parts commonly replaced

  • Condensate pump
  • Flue gas non-return flap
  • PCB

Related codes

F.78

Your boiler is struggling to detect the temperature of the hot water because a sensor connection is loose or faulty, which means you may have trouble getting consistent hot water.

Medium Engineer only £120-220

Parts commonly replaced

  • Domestic Hot Water (DHW) sensor
  • Wiring harness

Related codes

F.83

Your boiler has detected that there is not enough water circulating inside the system to carry the heat away safely.

High Engineer only £150-300

Parts commonly replaced

  • NTC sensor
  • Pressure sensor
  • Heating pump

Related codes

F.84

Your boiler is confused because the temperature readings inside are not making sense, causing it to shut down to prevent damage.

High Engineer only £150-250

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Wiring harness

Related codes

F.85

Your boiler has detected that the internal temperature sensors are mismatched or incorrectly fitted, preventing it from safely regulating heat.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Wiring harness

Related codes

F.86

Your boiler has detected that the external safety switch connected to an underfloor heating system is signaling a fault or that the connection is open.

Medium Engineer only £120-250

Parts commonly replaced

  • Underfloor heating limit switch
  • Wiring harness
  • PCB

Related codes

F.87

Your boiler is failing to light the fuel mixture needed to start, meaning your heating and hot water will not turn on.

High Engineer only £150-250

Parts commonly replaced

  • Ignition electrode
  • Ignition lead
  • PCB

Related codes

F.88

Your boiler has detected an issue with the assembly that controls the gas supply, meaning it cannot ignite safely to provide heating or hot water.

Emergency Engineer only £200-350

Parts commonly replaced

  • Gas valve
  • Gas valve wiring harness

Related codes

Showing 41–49 of 49
F.89

Your boiler is struggling because the internal pump that moves hot water through your home's radiators and taps has stopped working correctly.

High Engineer only £250-450

Parts commonly replaced

  • Central heating pump
  • Wiring harness

Related codes

F.91

Your boiler is having trouble communicating with the temperature sensor in your hot water cylinder, which means it may not be able to heat your taps or shower correctly.

Medium Engineer only £120-200

Parts commonly replaced

  • Cylinder temperature sensor (NTC)
  • Wiring harness
F.xx

Your boiler has identified a generic error that generally requires a qualified technician to safely diagnose and resolve.

High Engineer only

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Sensor assembly

Related codes

P.00

Your boiler is currently running a routine 'air-clearing' test sequence, which is a normal process used to remove trapped air from the system.

Low Engineer only
P.01

Your boiler is currently running a professional diagnostic test to check it is burning fuel and heating water at its highest power level correctly.

Low Engineer only
P.02

Your boiler is currently performing a brief automatic system test and will return to normal operation on its own shortly.

Low Engineer only
P.03

Your boiler is currently waiting for a signal from your thermostat to tell it how to run, but it isn't receiving the correct communication.

Medium Engineer only £120-250

Parts commonly replaced

  • eBUS connection cabling
  • External thermostat / programmer
  • Printed Circuit Board (PCB)

Related codes

P.06

Your boiler is currently in a manual maintenance mode used for filling or draining the system, rather than a failure state.

Low DIY-safe

What to check first

  1. Check if the service/fill button was accidentally pressed
  2. Turn the boiler off at the fused spur for 60 seconds
  3. Turn the boiler back on to see if it resets to normal operation

Related codes

S.04

Your boiler is operating normally and is currently heating your home.

Low DIY-safe

What to check first

  1. No action is required as this is a standard operational status message, not a fault.