Common problems guide

The most common Vaillant Ecomax Condensing 600/800 series problems

The faults most likely to send a Vaillant Ecomax Condensing 600/800 series into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
21 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
F.22 High DIY-safe

Your boiler has detected that the water pressure inside the system has dropped too low to operate safely.

What to try first

Locate the filling loop (a silver braided hose) under your boiler or near your cylinder.Open the small valves at either end of the loop to allow mains water into the heating system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close both valves tightly and ensure the filling loop is disconnected if it is a temporary hose.

Est. cost
£80-120
Parts
Pressure relief valve, Expansion vessel, Pressure sensor
Full guide for F.22
02
F.24 High DIY-safe

Your boiler is struggling because it does not have enough water circulating inside it, causing the system to heat up much faster than it safely can.

What to try first

Check your boiler's pressure gauge and ensure it is between 1 and 1.5 barIf the pressure is low, locate the filling loop (usually a silver braided hose under the boiler)Slowly open the valves on the filling loop to top up the system pressure to 1.5 bar, then close both valves tightly

Est. cost
£90-180
Parts
System pressure sensor, Circulation pump, Central heating diverter valve
Full guide for F.24
03
F.0 High Engineer

Your boiler's temperature sensor is failing to provide a reading, which has caused the system to shut down for safety.

Est. cost
£120-200
Parts
Flow NTC thermistor, Wiring harness
Full guide for F.0
04
F.1 High Engineer

Your boiler is struggling to accurately measure the temperature of the water returning to it, which prevents it from heating your home safely and has caused it to shut down for protection.

Est. cost
£120-200
Parts
Return NTC sensor, Wiring harness
Full guide for F.1
05
F.10 High Engineer

Your boiler has detected a faulty temperature sensor reading, causing it to shut down to prevent overheating.

Est. cost
£120-220
Parts
Flow NTC thermistor, Wiring harness
Full guide for F.10
06
F.11 High Engineer

Your boiler's internal sensor that monitors the temperature of water returning to the system is reading incorrectly, causing the boiler to shut down for safety.

Est. cost
£120-220
Parts
Return NTC thermistor sensor, Wiring harness
Full guide for F.11
Parts most often replaced

on the Vaillant Ecomax Condensing 600/800 series

Pressure relief valveExpansion vesselPressure sensorSystem pressure sensorCirculation pumpCentral heating diverter valve

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Vaillant shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 21-code list for the Vaillant Ecomax Condensing 600/800 series

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →