Bosch fault code A4
Fault code A4 on a Bosch boiler (including EHLE) means: The flue gas safety device TTB is activated and the burner stops 1.5 min after the start. 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 A4 on a Bosch boiler indicates: The flue gas safety device TTB is activated and the burner stops 1.5 min after the start.
Applies to
Bosch EHLE, 17, 23, 27, 34, 39, 78.
Fault code A4 on other Bosch series
The same code does not mean the same thing on every series. Check the rating plate to see which series you have:
- A4 on ZSE, ZSN, ZWE, ZWN, 18-6 KE, 24-4 MFK: Flue gases are escaping from the draught diverter.
- A4 on VRC, 24 Open: Escape of flue gas from the draught diverter.
Matching parts
Bosch parts are available from Instalpunt; shipping to EU countries from €12.95 (€16.95 for more distant countries), usually 3–5 working days.
For fault code A4, the part involved is usually in the category: burner.
This message does not point directly to one faulty part. These are the parts that most often fit this appliance:
- Bosch gasket for e.g. EHLE 23 87168336790 € 25.49
- Nefit maximum thermostat Clixon € 32.67
- Bosch plug for NTC € 32.67
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 Bosch boiler fault code A4 mean?
The flue gas safety device TTB is activated and the burner stops 1.5 min after the start.
How do I fix fault code A4 on a Bosch 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 Bosch fault code A4?
That depends on the exact cause. In the Bosch parts overview at Instalpunt you will find the current price and stock for each part.
Can I fix fault code A4 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 Bosch boiler faults and codes at a glance →