Bosch boiler faults: fault codes, causes and parts
All 377 fault codes for Bosch boilers, plus the faults that show no code. Type your code or choose your symptom.
A fault on a Bosch boiler usually means that the appliance has locked out or is reporting a fault code. Note the code on the display and look it up below: for each code you will find its meaning, checks you can safely do yourself and the parts that are often checked or replaced. Frequently searched: EA (flame not detected (no ionisation)), --353 (the boiler is waiting), CC (outdoor temperature). No code? Check the water pressure (1.5 to 2 bar when cold), the power supply and the gas tap, and reset once. If the fault comes back, call in a certified gas engineer. If you smell gas, call your national gas emergency number or 112 immediately.
Most searched Bosch faults
These are the codes that visitors to this site search for most often.
See the 25 most searched fault codes of 2026, with the figures behind them →
Bosch boiler faults without a code on the display
Not every fault shows a code. These are the symptoms we come across most often with Bosch appliances, with what you can check yourself before calling someone out.
No hot water, but the heating works
The boiler heats the radiators but not the tap water. Check that the hot water function is switched on in the display (on some appliances it is called DHW mode or summer mode) and that the hot water valve under the boiler is open. If the water stays cold, this usually points to a fouled or leaking DHW heat exchanger, a faulty diverter valve or a DHW sensor giving incorrect readings. View the matching parts → Read the full explanation: no hot water, six causes →
Water pressure keeps dropping
Top up to 1.5–2 bar when cold and bleed the radiators. If the pressure drops again within a few days, the expansion vessel has often lost its pre-charge or there is a small leak in the system. A pressure relief valve that keeps dripping gives the same symptom. View the matching parts → Read the full explanation: water pressure keeps dropping – expansion vessel or leak? →
Boiler shuts down and locks out
The appliance shuts down and only starts again after a reset. Reset once; if the fault comes straight back, leave the reset button alone. Repeatedly resetting a locked-out boiler is not without risk: the safety device trips for a reason. View the matching parts → Read also: is the boiler cutting out because of a sensor or overheating? →
Radiators stay cold
Check that the thermostat is calling for heat, that the radiator valves are open and that the system has enough pressure. If everything stays cold while the boiler is firing, the circulation pump is often not running or the diverter valve is stuck in the hot water position. View the matching parts → Read the full explanation: radiators stay cold →
Ticking, humming or boiling noise
A banging noise at start-up usually points to air or limescale in the heat exchanger. A humming pump may have a worn bearing. A rattling fan is often rubbing or dirty. View the matching parts → Read also: replacing the fan – what to rule out first →
Water under the boiler
Check where it is coming from: the condensate drain (siphon blocked or cracked), the pressure relief valve (pressure too high or faulty expansion vessel) or a fitting. If in doubt, switch the boiler off and close the water supply. View the matching parts → Read the full explanation: water under the boiler – condensate or a leak? →
Bosch boiler fault codes 1 to 150 of 377
The code list is split over 3 pages. Looking for a specific code? Use the search at the top of this page: it searches all 377 codes in one go.
Order parts for your Bosch boiler
Bosch parts are available from Instalpunt; shipping to EU countries from €17.95 (€24.95 for more distant countries), usually 3–5 working days.
Frequently asked questions about Bosch faults
What does a fault on a Bosch boiler mean?
The appliance is reporting that it has shut itself down or cannot continue. The code on the display points to the group of causes: ignition, water pressure, a sensor, the fan or communication with the thermostat. The table above shows, for each code, what Bosch means by it.
How do I reset my Bosch boiler?
The reset button is on the control panel of your Bosch boiler; on most appliances you hold it down for two to five seconds until the display responds. Reset once. If the same fault comes straight back, resetting will not solve it and a part or a setting needs attention. Always consult the manual for your model: the position of the button and how long to press it differ per series.
Can I fix a Bosch fault myself?
You can top up the water pressure, bleed the system, replace filters and clean a siphon yourself. Anything behind the sealed section of a gas appliance is a different matter: work on the gas valve, burner, heat exchanger and flue must be carried out by a certified gas engineer.
How much does it cost to fix a fault?
That depends on the cause and whether a part is needed. With us you can see the part straight away with its price and delivery time, so you know where you stand in advance. If you call in an engineer, always ask about the call-out charge and hourly rate before they come.
Where can I find the right part for my Bosch?
Check the rating plate for the exact type and use it to search our catalogue, or search directly by the manufacturer number of the faulty part. Each part lists the appliances it fits.