Viessmann fault code F A4 and F A5
Fault code F A4 and F A5 on a Viessmann boiler (including Vitocrossal 300) means: Internal fault in the feedback signal of the gas safety valves, output relay does not switch. Reset the appliance once. If the code comes back, a part is usually faulty. You can check the water pressure and settings yourself; leave work on gas-carrying parts to a certified gas engineer.
After a safety alert (gas, flue gas, combustion), reset no more than once and call in a certified gas engineer. If you smell gas, call your national gas emergency number or 112 immediately.
Cause
Fault code F A4 and F A5 on a Viessmann boiler indicates: Internal fault in the feedback signal of the gas safety valves, output relay does not switch.
Fixing fault code F A4 and F A5: step by step
- Switch the appliance off at the mains for 30 seconds (unplug it), then switch it back on.
- Is it about communication with the thermostat? Then check the thermostat connection and wiring.
- Reset the appliance once. If the code comes straight back, do not keep resetting it.
- If the code comes back, the PCB or burner control unit is probably faulty. Have the appliance read out first to be sure.
Applies to
Viessmann Vitocrossal 300, -.
Click a model for the parts list and exploded view of that appliance.
Matching parts
Viessmann parts are available from Instalpunt; shipping to EU countries from €12.95 (€16.95 for more distant countries), usually 3–5 working days.
This message does not point directly to one faulty part. These are the parts that most often fit this appliance:
- Viessmann siphon Vitocrossal 300 € 36.69
- Viessmann inspection hatch gasket vsb 72/89 gf 10x10 l=700 mm € 46.21
- Viessmann flue gas gasket 2nd heat exchanger Vitocrossal 300 720kW € 99.21
Fix it yourself or call an engineer?
Always consult the manual for your appliance and work safely. If in doubt or if the fault comes back, call in a qualified professional. A question about a part? Write to us at info@instalpunt.nl.
Frequently asked questions
What does Viessmann boiler fault code F A4 and F A5 mean?
Internal fault in the feedback signal of the gas safety valves, output relay does not switch.
How do I fix fault code F A4 and F A5 on a Viessmann boiler?
First check the cause described above. Then restart the appliance using the reset button. If the message comes back, a part usually needs replacing or the fault needs to be repaired by a qualified professional.
Which parts do I need for Viessmann fault code F A4 and F A5?
That depends on the exact cause. In the Viessmann parts overview at Instalpunt you will find the current price and stock for each part.
Can I fix fault code F A4 and F A5 myself?
You can reset the appliance and check things like the water pressure or settings yourself. Work on gas-carrying parts must be left to a certified gas engineer.
All Viessmann boiler faults and codes at a glance →