Common combi fault codes…
We can all have ‘off-days’, even your boiler.
Hopefully through regular boiler maintenance and servicing, these days will be few and far between – but if your boiler is displaying a fault code, it’s trying to tell you that something’s amiss.
The good news is, the problem might not be serious, and you might not need a Gas Safe engineer to get it working again.
For this reason, we’ve put together a list of some of the most common errors your boiler will display, and what you should do about it.
What is a fault code?
When a boiler suddenly stops working, it will usually display a boiler fault code that will help to identify the cause of the problem and indicate what action should be taken.
All combi boilers display error codes differently, so depending on your model, the error code itself will vary.
If you’re lucky enough to own a Worcester Bosch Greenstar 8000 Style, you won’t need this guide because those boilers are so smart they spell out what the problem is on their fancy touch screens.
But if you have a boiler that tells you through codes, this is for you!
What can cause a boiler fault code?
Although it’s frustrating when your boiler encounters a problem, it’s important to remember that it’s a complex machine that’s working hard for you around the clock, every day of the year.
So, it’s unsurprising that whilst modern boilers are extremely reliable, they will inevitably encounter a glitch from time to time.
A glitch doesn’t necessarily mean that there’s something wrong with the boiler itself, it could indicate problems with its fuel supply or a frozen or burst pipe.
How do I find out what my fault code means?
Although there are many different fault codes that your boiler can display (in different combinations), there’s no need to panic when a code starts flashing.
The first step to take, is to check your boiler’s manual.
Your manual should contain a chart of fault codes for your boiler, with a description of what the code is signaling, and what action needs to be taken to address it.
If you can’t find your manual, you can look at some of the common fault codes we’ve compiled later in this blog; or you can use Google to search for an online manual for your boiler.
What are some common boiler problems that trigger fault codes?
Although different makes of boilers will display different fault codes, it is worth looking at what some of the common causes of these error codes are:
Common combi fault codes: Low pressure fault
Modern boilers require constant water pressure to function properly.
If your boiler is losing pressure, it will cause problems, such as the dreaded, no hot water, situation.
The good news is that low pressure is usually easy to spot, and, in some cases, you can address it yourself. Your boiler’s pressure gauge will alert you to the fact that it’s losing pressure.
If you find that your gauge is below one bar of pressure, adjusting this to 1.5 should solve your problem in the short term.
There are lots of possible reasons for your low-pressure fault.
If you notice your pressure gauge drops overnight, this could be caused by a leak in your system; a problem with the pressure relief valve; air in your radiators; or a problem with the expansion vessel.
If you experience frequent or sudden problems with your boiler pressure, you should contact a heating engineer for help.
Common combi fault codes: No gas, or insufficient gas pressure
Problems can arise, triggering a fault code, if your boiler is not receiving gas or if the gas pressure is too low.
This is a problem sometimes encountered by people using gas meters because gas meters have a regulator fitted to them, which can freeze in cold weather.
Common combi fault codes: No ignition
Another common fault occurs when there’s no pilot light on the boiler or the boiler pilot light keeps going out.
The pilot light is crucial because it ignites the gas, which heats the water in your central heating system.
So, if the pilot light has gone out, you’ll need to reset it following your manufacturer’s instructions.
Common combi fault codes: Condensate pipe is blocked
If your boiler has stopped working when it’s freezing outside, there’s a good chance that your boiler’s condensate pipe has frozen. This pipe is responsible for taking waste from your boiler out into a drain.
If it gets blocked with ice, your boiler will automatically shut down as a safety measure until it’s resolved. The good news is, it’s usually straightforward to resolve.
The first thing to do is locate your condensate pipe. It should be a white plastic pipe that comes out of the wall directly behind your boiler. Check that it runs from the boiler to an exterior drain.
You should then boil your kettle and let the water cool. It’s important that you don’t try to thaw the pipe with boiling water. Instead, let the water cool for approximately 15 minutes. Pour the warm water down the length of the pipe until the ice has thawed.
Once you’ve thawed the pipe, you’ll need to reset your boiler.
Common combi fault codes: Boiler is overheating
Your boiler overheating is a potentially dangerous problem, often caused by safety features failing – such as the pump, or a blockage in the system. Because of this, modern boilers are designed to lockout and display a fault code if the system detects signs of overheating to prevent serious damage.
Common combi fault codes: Component failure
Although modern boilers are resilient appliances, like any machine that uses advanced technology and complicated processes, component parts can degrade and break down over time.
If a component part of your boiler corrodes or becomes faulty, this could trigger a fault code and other warning signs, such as strange noises or changes in how your heating and hot water is functioning.
Depending on what component part has failed, it might be possible to simply repair or replace the part, rather than replacing the whole boiler.
If you’re experiencing regular breakdowns, contact The Heating People to find out whether it’s time to consider a boiler replacement.
Common combi fault codes for manufacturers
Different brands and models of boiler have different fault codes, which mean different things. But just in case you haven’t got your manual to hand, we’ve compiled some fault codes for but combi boilers from Worcester Bosch, Vaillant, and ATAG:
Worcester Bosch CDi Compact / Greenstar 25/30 Si Combi / Greenstar 27/30 i System
| Fault Code | Problem |
| T1 | Ignition test – checking the igniter spark |
| T2 | Fan test – checking the basic fan |
| T3 | Pump test – checking the basic pump |
| T4 | Three way valve test |
| T6 | Ionisation oscillator test |
| 9A 362 | error – incorrect HCM fitted |
| 9U 233 | Heat Control Module (HCM) error – problem with code plug |
| B7 257 | Internal error – possible control board problem |
| C6 215 | Fan problem – fan running too fast |
| C7 214 | Fan problem – fan not running |
| D1 240 | Return sensor error – sensor may be wet or damaged |
| E2 222 | Flow sensor short circuit error |
| E5 218 | Flow temperature too high |
| E9 219 | Safety sensor fault – temperature too high, sensor short circuit or open circuit |
| EA 227 | No flame detected or flame signal loss during operation |
| F0 237 | Internal error |
| F7 228 | Flame error – false flame or flame detected before burner started |
| FA 306 | False flame fault – flame detect |
| FD 231 | Mains power fault – electrical power interruption |
| FA 364 | Gas valve EV2 leak test failed – gas valve leak |
| FB 365 | Gas valve EV1 leak test failed |
| A1 281 | Pump stuck or running with air in the system |
| C1 264 | Airflow stopped during operation |
| C4 273 | Airflow present during last 24 hour |
| D1 240 | Return sensor short circuit |
| D4 271 | Temperature difference between flow and safety sensor exceeds limit |
| E9 224 | Max thermostat activated – flue gas thermostat overheat |
| EA 227 | No ionisation detected after ignition |
| EF 349 | Central heating boil detected – boiler operating at minimum burner load with temperature difference greater than 18°C between Flow & Return. |
| NO CODE 212 | Safety or flow temperature rising too fast |
ATAG
Alt attribute: ATAG logo
| Fault code | Problem |
| 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 (run 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 damaged |
| 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 occurred |
| 501 | No flame detected |
| 612 | Fan fault – fan isn’t starting up |
ATAG IC and IS combi boiler error codes
| Fault code | Problem |
| 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) |
Vaillant
| Fault code | Problem |
| F.00 | Interruption: Flow sensor: NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, NTC defective |
| F.01 | Interruption: Return sensor: NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, NTC defective |
| F.02 | Interruption: DHW outlet sensor: Only in conjunction with F.91NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronics |
| F.03 | Interruption: Cylinder sensor: Only in conjunction with F.91NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronics |
| F.10 | Short circuit: Flow sensor: NTC defective, short circuit in cable harness, cable/casing |
| F.11 | Short circuit: Return sensor: NTC defective, short circuit in cable harness, cable/casing |
| F.12 | Short circuit: DHW outlet sensor: Only in conjunction with F.91NTC defective, short circuit in cable harness, cable/casing |
| F.13 | Short circuit: Cylinder sensor: Combination product: Warm start sensor/cylinder sensor short circuitCombination product with actoSTOR: Short circuit cylinder sensor (NTC) only in combination with F.91NTC defective, short circuit in cable harness, cable/casing |
| F.20 | Safety switch-off: Temperature limiter: Incorrect earth connection between cable harness and product, flow or return NTC defective (loose connection), black discharge via ignition cable, ignition plug or ignition electrode |
| F.22 | Safety switch-off: Low water pressure: No or insufficient water in the product, water pressure sensor defective, |
| F.23 | Safety switch-off: Temp.spread too large: Pump blocked, insufficient pump output, air in product, flow and return NTC connected the wrong way round |
| F.24 | Safety switch-off: Temp. incr. too fast: Pump blocked, insufficient pump output, air in product, system pressure too low, non-return valve blocked/incorrectly installed |
| F.25 | Safety switch-off: Flue temp. too high: Break in plug connection for optional flue gas safety temperature limiter (STB), break in cable harness |
| F.26 | Fault: Fuel valve not working: Gas valve stepper motor not connected, multiple plug on the PCB not plugged in correctly, interruption in cable harness, gas valve stepper motor defective, electronics defective |
| F.27 | Safety switch-off: Flame simulation: Moisture on the electronics, electronics (flame monitor) defective, gas solenoid valve leaking |
| F.28 | Start-up failure: Ignit. unsuccessful: Gas meter defective or gas pressure monitor has triggered, air in gas, gas flow pressure too low, thermal isolator device (TAE) has triggered, condensate duct blocked, incorrect gas restrictor, incorrect spare part gas valve, value in D.052 does not correspond to the printed value on the current gas valve, fault on the gas valve, multiple plug on PCB incor- rectly plugged in, break in cable harness, ignition system (ignition trans- former, ignition cable, ignition plug, ignition electrode) defective, ionisa- tion current interrupted (cable, electrode), incorrect earthing of product, electronics defective |
| F.29 | Start-up failure: Ignit. unsuccessful: Gas supply temporarily stopped, flue gas recirculation, condensate duct blocked, defective earthing of product, ignition transformer has spark failure |
| F.32 | Fault: Fan: Plug on fan not correctly plugged in, multiple plug on PCB not correctly plugged in, break in cable harness, fan blocked, Hall sensor defective, electronics defective |
| F.49 | Fault: eBUS: Short circuit on eBUS, eBUS overload or two power supplies with differ- ent polarities on the eBUS |
| F.52 | Contact fault mass flow sensor/Venturi: The mass flow sensor/Venturi is not connected electrically – The plug is not plugged in correctly – The plug is not plugged in – The plug is defective – The slot is defective (loose connection) – Mass flow sensor/Venturi defective |
| F.53 | Combustion regulation fault: The combustion regulation has detected a fault – Gas flow pressure too low – Liquid gas coding resistance used when operating with natural gas – If the fault occurs again after being cleared: – Gas valve defective – Mass flow sensor/Venturi defective, wet or blocked (if the fault occurs again after being cleared): Do not wet the sensor, do not use any lubricants on the O-ring on the Venturi. |
| F.54 | Fault in the gas supply (in combination with F.28/F.29): There is insufficient gas supply to operate the unit – Gas isolator cock(s) closed – Gas flow pressure is too low – Gas valve defective |
| F.56 | Safety switch-off: CO limit exceeded: Safety switch-off: CO limit exceededA component in the combustion regulation is defective– Contact fault at the gas valve (plug not plugged in correctly or not plugged in, plug defective, slot is defective (loose connection))– If the fault occurs again after being cleared: The gas valve is defective |
| F.57 | Fault: Measuring program: Active comfort protection mode has detected a regulation fault– Ignition electrode highly corroded |
| F.61 | Fault: Fuel valve actuation: The gas valve cannot be actuated– Cable harness supply line to the gas valve is defective (short to earth, short circuit)– Gas valve defective– PCB defective |
| F.62 | Fault: Fuel valve switch-off delay: Delayed shutdown of gas valve detected– Secondary light (ignition and monitoring electrode indicates delayed extinguishing of the flame signal)– Gas valve defective– PCB defective |
| F.63 | Fault: EEPROM: Electronics defectiv |
| F. 64 | Fault: Electronics/sensor: Flow or return NTC short circuited, electronics defective |
| F.65 | Fault: Electronics temp.: Electronics overheating due to external influences, electronics defective |
| F.67 | Fault: Electronics/flame: Implausible flame signal, electronics defective |
| F.68 | Fault: Flame signal unstable: Air in gas, gas flow pressure too low, incorrect air ratio, condensate duct blocked, ionisation flow interruption (cable, electrode), flue gas recirculation, condensate duct |
| F.70 | Fault: Invalid Device Specific Number: If spare parts fitted: Display and PCB replaced at same time and DSN not reset, incorrect or missing output range coding resistance |
| F.71 | Fault: Flow sensor: Flow temperature sensor signalling constant value:– Flow temperature sensor incorrectly positioned at supply pipe– Flow temperature sensor defective |
| F.72 | Fault: Flow/return sensor: Flow/return NTC temperature difference too great → flow and/or returntemperature sensor defective |
| F.73 | Fault: Water press. sensor: Interruption/short circuit of water pressure sensor, interruption/short circuit to GND in supply line to water pressure sensor or water pressure sensor defective |
| F.74 | Fault: Water press. sensor: The line to the water pressure sensor has a short circuit to 5 V/24 V or internal fault in the water pressure sensor |
| F.75 | Fault: Pump/ water shortage: Water pressure sensor and/or pump defective, air in the heating installa- tion, insufficient water in the product; connect external expansion vessel to the return |
| F.77 | Fault: Flue non-ret. valve/condens. pump: No response from flue non-return flap or condensate pump defective |
| F.78 | Interrupt.: DHW outlet sensor on ext. contr: UK link box is connected but the hot water NTC is not bridged |
| F.80 | Fault: actoSTOR inlet sensor: Only in conjunction with F.91NTC defective, NTC cable defective, defective plug connection on NTC, defective plug connection on the actoSTOR electronicsSensor plug has short to earth to the casing, short circuit in cable har- ness, sensor defective |
| F.81 | Fault: cylinder charging pump: Only in conjunction with F.91Cylinder is not fully charged after specified time.– Check cylinder charging sensor and cylinder sensor– Air in the actoSTOR pump– Inspect cable harness for pump– Check the impeller sensor and/or limiter in the product– Diverter valve defective– Secondary heat exchanger blocked– Pump faulty |
| F.83 | Fault: NTC temp. gradient: When the burner starts, the temperature change registered at the flow and/or return temperature sensor is non-existent or too small.– Insufficient water in product– Flow or return temperature sensor not in correct position at pipe |
| F.84 | Fault: NTC temp. diff. implausible: Flow and return temperature sensors returning implausible values.– Flow and return temperature sensors have been inverted– Flow and return temperature sensors have not been correctly in-stalled |
| F.85 | Fault: NTCs fitted incorrectly: The flow and/or return temperature sensors have been installed on the same pipe/incorrect pipe |
| F.90 | Fault: Communication: Check the cable harness from the product to the actoSTOR module (PEBus).If the product is to be operated without an actoSTOR module, setD.092 = 0. |
| F.91 | Sensor/actuator fault on actoSTOR module |
| F.92 | Coding resistance fault: The coding resistance on the PCB does not match the entered gas group: Check the resistance, repeat the gas family check and enter the correct gas group |
| F.93 | Poor combustion quality: Combustion regulation has detected poor combustion quality– Incorrect gas restrictor has been installed for the gas family (anothergas family is present)– Recirculation– Mass flow sensor/Venturi defective (wet, blocked): Do not wet thesensor, do not use any lubricants on the O-ring on the Venturi. |
| LEDactoSTOR module | actoSTOR electronics status: LED on: Communication OKLED flashing: Communication not OK LED off: No power supply |
| Err | Communication fault: Communication fault between display and PCB in the electronics box |
What should I do after I find out what my fault code means?
It is extremely important that you only perform actions on your boiler if your boiler manual instructs you to do so.
Any other work must be carried out by a Gas Safe Registered engineer.
If you are unsure about anything, you should call one of our experts at The Heating People.
Useful Links:
Vaillant servicing and repairs literature
Ideal boilers product literature
ATAG user instuctions for the i range
