Welcome to the Maintracts Services blog, your trusted source for all your ATG boiler needs! With over four decades of experience since our establishment in 1974, Maintracts Services has become a leading name in the heating industry.

Our team of expert heating engineers possesses extensive knowledge and expertise in dealing with ATG boilers, offering a comprehensive range of services including installation, maintenance, and repair. In today’s blog post, we will be delving into a topic that is crucial for every ATG boiler owner or technician to understand: common ATG boiler error codes.

These error codes can be perplexing, but worry not! We are here to provide you with valuable insights into what these error codes signify and how to effectively troubleshoot them.

Whether you’re a homeowner, a landlord, or a heating professional, join us as we unravel the mysteries of ATG boiler error codes and help you maintain optimal performance and reliability for your heating system.

ATAG boiler error codes

When an error occurs with your ATAG boiler, you will encounter an error symbol like the one depicted above. Typically, this symbol is accompanied by one of the codes listed in the tables below.

Fault code System behaviour
10 Outside sensor error
20 Flow sensor error
40 Return sensor error
61 Bus communication error
78 Water pressure out of range
105 Venting program active when power turned on / interrupted (runs for 7 mins)
110 Exceeded safety temperature
111 Exceeded maximum pressure
119 X2 link position 4 and 5 missing
129 Fan doesn’t start
133 No flame after 5 ignition attempts
151 Fan error or control unit defective (speed control is not achieved)
154 Return greater than flow – flow temperature increases too fast
101 Overheat error
102 Pressure sensor fault
104 Flow check failed
108 Pressure needs filling to < 0.5 bar
1P4 Pressure needs filling to 0.5 – 0.8 bar
109 Pressure > Pmax error
110 Send probe damaged
112 Return probe error – could be damanged
114 Outdoor sensor error – could be damaged
141 Central heating flow switch is open
201 Domestic hot water (DHW) probe damaged
203 Tank probe damaged
303 Printed circuit board (PCB) error
304 Too many resets have occured
501 No flame detected
612 Fan fault – fan isn’t starting up

IC & IS Combi Boiler Error Codes

Fault code System behaviour
50 Domestic hot water (DHW) sensor error
117 Pressure greater than 3 bar or pump pressure increase is too high
118 Pressure less than 1 bar or pump pressure increase too low (no pump detection).

Other Fault Codes


If you come across a spanner symbol, it could indicate one of two possibilities. Firstly, it may signify that your boiler is in service mode. Secondly, it could indicate a blockage issue.

In the case of a blockage, the code is typically temporary and will clear itself or will eventually lock the boiler after several repeated attempts if the error persists.

ATAG Error code 133

When encountering error code 133, it indicates that your boiler’s flame is failing to ignite after five attempts. This issue is commonly attributed to a frozen condensate drain pipe. Fortunately, a frozen condensate pipe is often a simple problem to resolve.