Atag Sov. Fault codes & diagnostics

30 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

System Natural GasLPG 15-35 kW 2000-2009 Discontinued

Replaced by: iS Series

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Severity at a glance

Each cell = one fault code. Hover to identify.
23 High 1 Medium 6 Low

All 30 documented codes

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10

Your boiler has lost communication with the sensor that measures the temperature outside, which may cause your heating to behave unpredictably.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness

Related codes

20

Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely regulate heat.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC temperature sensor
  • Wiring harness

Related codes

40

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning from your radiators, preventing it from heating your home safely.

High Engineer only £120-£190

Parts commonly replaced

  • Return temperature sensor (NTC)
  • Wiring harness
  • Main control board

Related codes

50

Your boiler has detected a problem with the sensor that manages your hot water, which means it cannot accurately tell if the water is at the right temperature.

High Engineer only £90-180

Parts commonly replaced

  • T3 Hot Water Sensor
  • NTC Sensor Wiring Harness

Related codes

61

Your boiler has lost the ability to communicate with its internal control systems or the external thermostat, meaning it cannot receive instructions to heat your home.

High Engineer only £120-350

Parts commonly replaced

  • Communication Module
  • Main PCBA (Control Board)
  • Wiring Harness
78

Your boiler has detected that the water pressure is either too low or too high, or the sensor that monitors this pressure has lost its connection.

High DIY-safe £90-180

What to check first

  1. Check the pressure gauge on the front of the boiler or via the digital display.
  2. If the pressure is below 1.0 bar, use the filling loop valves to carefully top it up to 1.5 bar.
  3. If the pressure is above 3.0 bar, bleed a radiator to release excess pressure.
  4. Reset the boiler once the pressure is back within the normal range (1.0 to 2.0 bar).

Parts commonly replaced

  • Water pressure sensor
  • Filling loop
  • Expansion vessel

Related codes

105

Your boiler is busy removing air bubbles from the pipes after the water pressure was topped up, and it will return to normal operation in about seven minutes.

Low DIY-safe

What to check first

  1. Wait approximately 7 to 10 minutes for the automatic countdown to complete
  2. Ensure the blue filling loop taps are fully closed
  3. Check that the water pressure gauge is between 1.0 and 1.5 bar

Related codes

110

Your boiler has shut down because it has detected an internal temperature that is too high, often caused by a lack of water circulation.

High DIY-safe £100-£350

What to check first

  1. Check if your radiator valves are open to allow water to flow.
  2. Check your boiler pressure gauge and top it up to 1.5 bar if it is low.
  3. Restart the boiler using the reset button to see if the fault clears.

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Main Heat Exchanger (Flushing)

Related codes

117

Your boiler has shut down because there is too much water pressure in the system, which can put strain on the internal seals.

High DIY-safe £0-120

What to check first

  1. Check that the filling loop valves are fully closed
  2. Bleed water from a radiator using a radiator key to reduce pressure
  3. Monitor the pressure gauge until it returns to between 1.0 and 1.5 bar
  4. Reset the boiler once the pressure is within the normal range

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve

Related codes

118

Your boiler has detected that the water pressure is too low to work properly, so it has automatically reduced its power to protect the system.

Medium DIY-safe £0-120

What to check first

  1. Locate the external filling loop, which is usually a flexible silver hose with two small valves.
  2. Slowly open both valves to allow water into the system while watching the pressure gauge.
  3. Close both valves tightly once the pressure reaches between 1.2 and 1.5 bar.

Parts commonly replaced

  • Filling loop hose
  • Pressure sensor
  • Expansion vessel

Related codes

Showing 11–20 of 30
119

Your boiler has detected that a critical internal safety link or a water pressure switch connection is broken, preventing it from starting up.

High Engineer only £90-180

Parts commonly replaced

  • Link wire
  • Water pressure switch
  • Wiring loom harness

Related codes

129

Your boiler's fan is failing to start, which means the system cannot safely vent exhaust gases and has shut down as a precaution.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Control PCB
  • Wiring harness
133

Your boiler has tried to start several times but cannot light a flame, meaning you will not have any heating or hot water until it is fixed.

High DIY-safe £100-£350

What to check first

  1. Check if your gas supply is active by testing other gas appliances like a hob
  2. Ensure your gas meter has credit if you use a pre-payment meter
  3. Check that the condensate pipe (the plastic pipe leading outside) isn't frozen if the weather is cold
  4. Press the Reset button once to see if the boiler restarts

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • Ignition Lead

Related codes

151

Your boiler has stopped working because the fan that clears away waste gases is not spinning at the correct speed or has lost communication with the main control board.

High Engineer only £250-£550

Parts commonly replaced

  • Fan assembly
  • Control Management Unit (PCB)
  • Wiring harness

Related codes

154

Your boiler has stopped working because it cannot move water through the system properly, often due to a blockage or a pump issue.

High DIY-safe £150-£350

What to check first

  1. Check that your radiators are turned on and the valves are open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar on the pressure gauge
  3. Try resetting the boiler by pressing the 'Reset' button

Parts commonly replaced

  • Circulation pump
  • Flow/Return temperature sensors
  • Main heat exchanger (flushing)

Related codes

180

This is a normal status message showing the boiler is returning to standard operation after a maintenance test, and it is not an actual fault.

Low DIY-safe

What to check first

  1. No action is required
  2. Wait a few moments for the code to disappear from the display
  3. Continue using your heating and hot water as normal

Related codes

181

Your boiler is simply finishing its internal setup process and is returning to normal operation, so there is no actual fault or problem.

Low DIY-safe

What to check first

  1. Wait a few seconds for the code to disappear automatically.
  2. No further action is required as the boiler is functioning correctly.
C 20

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, which has caused it to stop running for safety.

High Engineer only £100-£190

Parts commonly replaced

  • Flow NTC sensor
  • Wiring harness
  • Main control board (PCB)

Related codes

C 40

Your boiler has detected an issue with the sensor that monitors water returning from your radiators, preventing it from heating your home safely.

High Engineer only £120-220

Parts commonly replaced

  • Return Water Temperature Sensor (NTC)
  • Sensor Wiring Harness

Related codes

C 61

Your boiler's internal computer brain has temporarily lost its ability to talk to the control sensors and requires a manual power restart.

High DIY-safe £0-180

What to check first

  1. Locate the main electrical power switch for your boiler (usually a fused spur switch nearby).
  2. Turn the power switch to the 'Off' position and wait for 30 seconds.
  3. Turn the power switch back to 'On' to allow the system to reboot and re-establish communication.

Parts commonly replaced

  • Internal wiring loom
  • Main PCB (Printed Circuit Board)

Related codes

Showing 21–30 of 30
C 78

Your boiler has detected that water is not circulating properly, which could mean the pump has stopped working or the water pressure is too low.

High DIY-safe £250-450

What to check first

  1. Check the pressure gauge on your boiler or under the sink.
  2. If the pressure is below 1.0 bar, use the filling loop to top it up to 1.5 bar.
  3. Reset the boiler to see if the error clears.

Parts commonly replaced

  • Circulating pump
  • Water pressure sensor
  • Expansion vessel

Related codes

C1 05

Your boiler is currently running an automatic cycle to clear trapped air from the system, which typically occurs after it has been powered on or refilled.

Low DIY-safe

What to check first

  1. Wait approximately 7 to 10 minutes for the automatic cycle to finish
  2. Ensure all radiator valves are open to help move air through the system
  3. Check the system pressure gauge and top up to 1.5 bar if it has dropped

Related codes

C1 10

Your boiler has switched itself off because it has become too hot internally, often due to a lack of water flow or a blockage.

High DIY-safe £120-350

What to check first

  1. Check that your system pressure is between 1.0 and 1.5 bar on the pressure gauge
  2. Ensure all radiator valves are fully open to allow water to circulate
  3. Press the 'Reset' button on the boiler control panel after the unit has cooled down

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger cleaning

Related codes

C1 11

Your boiler has sensed that the water inside has become far too hot and has shut down as a safety precaution.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate
  2. Ensure your system water pressure is between 1.0 and 1.5 bar on the gauge
  3. Reset the boiler by pressing the reset button to see if the fault clears

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Main Heat Exchanger (Flushing)

Related codes

c1 18

Your boiler has stopped working because the water pressure is critically low and needs to be topped up.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath or near the boiler
  2. Slowly open the small valve handle(s) until you hear water flowing
  3. Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then turn the valves off tightly

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

C1 29

Your boiler has stopped working because the internal fan, which clears exhaust fumes, is unable to start up correctly.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Main PCB
  • Wiring harness

Related codes

C1 33

Your boiler has tried to light several times but has failed to produce a flame, meaning your heating and hot water will not work.

High DIY-safe £120-350

What to check first

  1. Check if other gas appliances, like a cooker, are working to ensure gas supply
  2. Check if your gas meter has credit
  3. Press the Reset button once to see if the boiler restarts

Parts commonly replaced

  • Ignition electrode
  • Ionisation probe
  • Gas valve
  • Spark generator

Related codes

C1 51

Your boiler has stopped working because the fan that removes waste gases is not spinning at the correct speed.

High Engineer only £280-£450

Parts commonly replaced

  • Fan assembly
  • Main PCB (Control Board)
  • Wiring harness

Related codes

C1 54

Your boiler has detected that water is heating up too quickly or flowing in the wrong direction, often because something is blocking the circulation of water through the system.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Bleed all radiators to remove trapped air
  4. Restart the boiler using the reset button

Parts commonly replaced

  • Circulation Pump
  • Flow/Return Temperature Sensors
  • Main Heat Exchanger (cleaning or replacement)

Related codes

F1

Your boiler is displaying an advisory message indicating that specific chemical protection or anti-freeze has been added to the system water, rather than reporting an actual breakdown.

Low DIY-safe £0-80

What to check first

  1. Press the 'Reset' button on the display panel to clear the notification.
  2. Check your boiler service records or any stickers nearby to confirm chemical levels were recently checked.
  3. Monitor the boiler for any other actual fault codes if the heating stops working.