Home / Fault codes / Boiler / ATAGBoiler fault codes ATAG boiler faults: fault codes, causes and parts All 234 fault codes for ATAG boilers, plus the faults that show no code. Type your code or choose your symptom.
In brief A fault on an ATAG 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: C1 18 (pressure too low (), C1 33 (no flame after 5 ignition attempts), 140* (dynamic pressure increase failed 2x (no pump…). 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 ATAG 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 →
ATAG boiler faults without a code on the display Not every fault shows a code. These are the symptoms we come across most often with ATAG 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? →
ATAG boiler fault codes 1 to 150 of 234 The code list is split over 2 pages. Looking for a specific code? Use the search at the top of this page: it searches all 234 codes in one go.
Code Cause 10 Outdoor sensor fault (e.g. open, short-circuited, out of range) 101 Maximum temperature exceeded 101 Overheating 102 Pressure sensor fault 102 Pressure sensor defective 104 No pump kick 104 Insufficient flow rate 105 No fault; air purge programme active (approx. 7 minutes) 108 Pressure < Pmin (< 0.7 bar), system must be topped up 108 Pressure too low (<0.7 bar) or pump pressure increase too low (no pump detection) 109 Pressure too high (>3 bar) or pump pressure increase too high 109 Pressure too high (>Pmax) 110 Flow sensor fault (e.g. open, short-circuited, out of range) 110 Safety temperature exceeded 110 Flow sensor fault (e.g. open, short-circuited, out of range) 111 Maximum temperature exceeded 112 Return sensor fault (e.g. open, short-circuited, out of range) 112 Return sensor fault (e.g. open, short-circuited, out of range) 114 Outdoor sensor fault (e.g. open, short-circuited, out of range) 114 Outdoor sensor fault (e.g. open, short-circuited, out of range) 117 Pressure too high (>3 bar) or pump pressure increase too high 118 Pressure too low (<0.7 bar) or pump pressure increase too low (no pump detection) 119 Link between positions 4 and 5 of X2 missing/open circuit 129 Fan fault (fan does not start) 133 No flame after 5 ignition attempts 140* Dynamic pressure increase failed 2x (no pump detection) 151 Fan fault (speed control not achieved or out of range) or defective control unit. 154 Flow temperature rising too quickly, dT too large, return > flow 180 No fault: briefly displayed when exiting chimney sweep function 181 No fault: briefly displayed when exiting 'accelerator pedal function' 197 Link at 'external safety contact' position missing/open circuit 1P4 Pressure < Pmin (0.7 - 1.0 bar), system must be topped up 1P4 Pressure between 0.7 and 1 bar, top up to above 1.3 bar 1P9 Dynamic pressure increase failed 1x 20 Coding plug not recognised. 20 Internal data fault 20 Flow temperature sensor defective 20 Domestic hot water temperature sensor incorrectly fitted 20 Gas valve assembly defective 20 Temperature monitor TB1 faulty
External monitor has tripped.
Temperature monitor locked out. 20 Outdoor temperature sensor not recognised. 201 Domestic hot water sensor defective (Combi) 201 Domestic hot water sensor fault (e.g. open, short-circuited, out of range) 203 Cylinder sensor - open circuit (Solo) 303 PCB fault 303 Control unit fault 304 Too many resets (5x in 15 minutes) 304 Too many resets 40 Temperature exceeded at the burner flow temperature sensor. 40 Open circuit or short circuit of the burner flow temperature sensor. 40 Internal fault 40 Internal data fault 40 Boiler block temperature limiter has been activated 40 Flame is detected although the appliance is switched off 40 Flame is not detected 50 Domestic hot water sensor fault (e.g. open, short-circuited, out of range) 50 Gradient limitation: temperature rising too quickly 50 After gas shut-off: flame is detected. 50 Reset button was pressed accidentally. 501 No flame detected 501 Ionisation fault 61 No communication via Zbus 612 Fan fault (fan does not start) 78 Water pressure sensor out of range or not connected or, if water pressure OK: pump defective. B0 Flow water temperature rising too quickly B0 Flow water temperature rising too quickly
No or insufficient flow in the system? B1 Temperature difference between flow and return too large
Insufficient flow or pump faulty? B1 Temperature difference between flow and return too large B3 Maximum flue gas temperature exceeded (if flue gas sensor fitted) B3 Maximum flue gas temperature exceeded (if flue gas sensor fitted)
Blockage in the flue gas outlet? B4 Maximum flow water temperature exceeded
No flow or too much resistance in the system? B4 Maximum flow water temperature exceeded B5 Maximum flow water temperature exceeded B5 Maximum return water temperature exceeded
No flow or too much resistance in the system? B6 Return temperature higher than flow temperature
Circulation pump fitted the wrong way round? B6 Maximum flow water temperature exceeded B7 General lockout input open B7 General lockout input open
Water pressure too low or low water pressure protection faulty? BL 01 External safety contact open BL 01 External safety contact open. External safety contact (pos. 24 and 25) is active BL 11 Maximum Delta-T of flow and return sensor on central heating (CH) repeatedly exceeded
Domestic hot water operation is still possible as normal during the lockout. During the lockout the pump continues to run at minimum capacity BL 11 Maximum temperature difference of flow and return sensor on CH repeatedly exceeded. Domestic hot water operation is normally possible during the lockout (LO). During the LO the pump continues to run at minimum capacity BL 11 Maximum temperature difference of flow and return sensor on CH repeatedly exceeded. Domestic hot water operation is normally possible during the lockout (LO). During the LO the pump continues to run at minimum capacity. BL 11 Maximum temperature difference of flow and return sensor on CH repeatedly exceeded. Domestic hot water operation is normally possible during the lockout (LO). During the LO the pump continues to run at minimum capacity. BL 11 Maximum temperature difference of flow and return sensor on CH repeatedly exceeded. BL 12 Maximum Delta-T of flow and return sensor on domestic hot water (DHW) repeatedly exceeded
Central heating system operation is still possible as normal during the lockout. During the lockout the pump continues to run at minimum capacity. BL 12 Maximum temperature difference of flow and return sensor on DHW repeatedly exceeded. CH operation is normally possible during the lockout (LO). During the LO the pump continues to run at minimum capacity BL 12 Maximum temperature difference of flow and return sensor on DHW repeatedly exceeded. CH operation is normally possible during the lockout (LO). During the LO the pump continues to run at minimum capacity. BL 60 Incorrect parameter setting of the minimum or maximum output BL 67 A Delta-T has been detected between flow and return sensor while the burner is not operating
Once the Delta-T has disappeared the lockout will be cleared BL 67 A temperature difference has been detected between flow and return sensor while the burner is not operating. Once the temperature difference has disappeared, the lockout (LO) will be cleared BL 67 A temperature difference has been detected between flow and return sensor while the burner is not operating. Once the temperature difference has disappeared, the lockout (LO) will be cleared. BL 80 Maximum flue gas temperature exceeded (if flue gas sensor fitted)
The lockout (BL) is only cleared once the flue gas temperature falls below the maximum flue gas temperature BL 80 Maximum flue gas temperature exceeded (if flue gas sensor fitted)
The lockout is only cleared once the flue gas temperature falls below the maximum flue gas temperature BL 80 Maximum flue gas temperature exceeded (if flue gas sensor fitted). The lockout (LO) is only cleared once the flue gas temperature falls below the maximum flue gas temperature. BL 80 Maximum flue gas temperature exceeded (if flue gas sensor fitted). The lockout (LO) is only cleared once the flue gas temperature falls below the maximum flue gas temperature. BL 80 Maximum flue gas temperature exceeded (if flue gas sensor fitted)
The lockout is only cleared once the flue gas temperature falls below the maximum flue gas temperature BL 81 Flue gas sensor open circuit after having previously been connected to the controller
Burner is locked out until the flue gas sensor is reconnected BL 81 Flue gas sensor open circuit after having previously been connected to the controller
Burner is locked out until the flue gas sensor is reconnected BL 81 Flue gas sensor open circuit after having been connected to the controller. Burner is locked out until the flue gas sensor is reconnected. BL 82 Flue gas sensor short-circuited, heat demand blocked and pump capacity at minimum BL 84 Flue gas sensor short-circuited, heat demand blocked and pump capacity at minimum BL 85 The controller has detected no water flow. Automatic air purge cycle is started. If water flow is detected during this cycle, the air purge cycle is ended and the burner is enabled. BL 85 The controller has detected no water flow
Automatic venting cycle is started. If water flow is detected during this cycle, the venting cycle is ended and the burner is enabled. BL 85 The controller has detected no water flow BL 85 The controller has detected no water flow. Automatic air purge cycle is started. BL 86 Mains frequency deviates by more than 2.5 Hz
the mains frequency deviates by more than 2.5 Hz from the standard 50 Hz (<47.5 Hz, >52.5 Hz) BL 86 Mains frequency deviates by more than 2.5 Hz from the standard 50 Hz (<47.5 Hz and > 52.5 Hz) Bx01sc01 Fan is not running at the correct speed Bx03sc01 Flow sensor T1 open. Bx03sc02 Flow temperature T1 too high. Bx05sc01 Return sensor T2 open. Bx05sc02 Return temperature T2 too high. Bx08sc01 Safety contact open. Bx12sc01 Water pressure sensor open. Bx12sc02 Water pressure sensor short-circuited. Bx12sc03 Water pressure lower than 0.7 bar. Top up. Bx12sc04 Water pressure too high. Reduce pressure. Bx12sc05 No pressure increase at pump start. Bx13sc01 Temperature difference between T1 and T2 too high. Bx15sc01 Communication error between boilers. C 10 Outdoor sensor fault (e.g. open, short-circuited, out of range) C 20 flow sensor fault (e.g. open, short-circuited, out of range) C 20 Flow sensor fault C 20 Flow sensor fault (e.g. open, short-circuited, out of range) C 20 Flow sensor fault e.g. open, short-circuited, out of range. C 40 return sensor fault (e.g. open, short-circuited, out of range) C 40 Return sensor fault C 50 Domestic hot water sensor fault (e.g. open, short-circuited, out of range) C 50 Wiring fault. C 61 No communication via Z-bus (reset only possible by interrupting the power supply) C 61 OpenTherm room thermostat fault. C 62 Unknown OpenTherm room thermostat. C 78 Water pressure sensor fault C 78 Water pressure sensor out of range or not connected or, if water pressure OK: pump defective C 78 Water pressure sensor out of range or not connected or, if water pressure OK: pump defective. C1 05 Air purge programme active C1 05 Air purge programme active C1 10 safety temperature exceeded C1 11 maximum temperature exceeded C1 17 Water pressure too high (>3 bar) C1 17 pressure too high (>3 bar) or pump pressure increase too high C1 18 Pressure too low (<0.7 bar) or pump pressure increase too low (no pump detection) C1 18 Water pressure too low (<1.0 bar or <0.7 bar) C1 29 Fan fault, fan does not start C1 29 fan fault (fan does not start) C1 33 no flame after 5 ignition attempts C1 51 Fan fault (speed control not achieved or out of range) C1 52 Outdoor sensor setting incorrect. Parameter 555 b0 and/or b1 is incorrectly set to 'on' C1 54 Flow rising too quickly, delta T too large, return > flow
1 2 Next → Order parts for your ATAG boiler ATAG parts are available from Instalpunt ; shipping to EU countries from €12.95 (€16.95 for more distant countries), usually 3–5 working days.
Parts per Atag-series: A Series · i Series (iCon) · Q Series · E Series · P-series (iCon) · Blauwe Engel · XL-series .
View ATAG parts →
Frequently asked questions about ATAG faults What does a fault on an ATAG 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 ATAG means by it.
How do I reset my ATAG boiler? The reset button is on the control panel of your ATAG 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 an ATAG 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 ATAG? 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.
Part and compatibility data come from the technical documentation of our supplier and are refreshed daily (last updated: 2 November 2026). Technical checks and the product range are handled by Bart van der Ark 
