There is nothing quite as frustrating as waking up to a freezing house only to find a cryptic code like “F4” or “F2” flashing on your boiler display. If you do not know what these sequences mean, you might fear a massive repair bill or a condemned unit. I have spent two decades working on UK heating systems, and while Viessmann boilers are some of the best engineered units on the market, they are not immune to the occasional lockout. This guide tells you exactly what those codes mean, which ones you can fix yourself, and which ones require a professional.
Troubleshooting the Viessmann F2 fault
The F2 code is one of the most common issues I see on the Vitodens range. It tells you that the burner has tripped the high-limit thermostat because it is getting too hot. Most of the time, this is not a burner fault at all. It is usually a circulation problem.
Before you panic, check your system pressure. If it is below 1.0 bar, the pump might be struggling to move water, which causes the heat exchanger to overheat. If the pressure is fine and a reset does not work, you likely have a seized pump or a blocked heat exchanger. Because these are “wet” components behind the main casing, you will need to find a professional to swap them out.
If you find that your heating is working but your radiators are cold at the bottom, this is a sign of sludge buildup that can lead to an F2 lockout.
Solving the Viessmann F4 flame fault
An F4 code means your boiler has failed to ignite or cannot detect a flame. This is a safety mechanism that locks the boiler out to prevent unburned gas from building up.
In my experience, this is often caused by a faulty ignition lead or a dirty ionisation probe. However, in the middle of a UK winter, it is frequently caused by a frozen condensate pipe. If the white plastic pipe leading outside your house freezes, the boiler cannot drain waste fluid and will shut down. You can often fix this yourself by pouring warm water over the external pipe to melt the ice.
Can I fix a Viessmann boiler myself?
Safety is my priority when I advise homeowners. You should only ever attempt basic maintenance that does not require opening the boiler casing.
Safe DIY fixes
- System Repressurisation: If the needle is in the red zone, you can use the filling loop to top up the water.
- The Boiler Reset: Many codes are temporary “glitches” that a single reset will clear.
- Thawing Pipes: Clearing a frozen condensate pipe is perfectly safe for a homeowner to do.
Professional repairs
Any issue involving gas valves, fans, or internal electronics must be handled by someone on the Gas Safe Register. I have seen many DIYers try to save a few pounds by opening the casing, only to cause much more expensive damage or a gas leak.
Comprehensive Viessmann fault code index
I have listed the most frequent codes I encounter below to help you identify the problem quickly.
| Code | Description of Fault | Likely Solution |
| F2 | Burner overheating/lockout | Check pressure. Possible pump failure. |
| F4 | No flame detected | Check gas supply or frozen condensate pipe. |
| 0b | Heating pump issue | The pump is blocked or has a wiring fault. |
| F5 | Air pressure switch fault | Likely a fan issue. Requires a professional. |
| F1 | Flue gas temperature too high | Check for flue blockages. |
| b1 | Communication error | Internal electronic fault. |
Estimated repair costs for Viessmann boilers
Knowing the likely cost of a repair helps you decide if a fix is worth it or if you should consider a new unit.
- New Circulating Pump: £250 to £350 including labour.
- Replacement Fan: £200 to £300 depending on the model.
- Sensor Replacement: £80 to £120 for parts and fitting.
Final thoughts on boiler maintenance
A Viessmann boiler is a high-performance machine that requires regular servicing to stay efficient. If your boiler is over 12 years old and showing frequent faults, it might be more cost-effective to replace it with a modern A-rated unit.
For more advice on keeping your home running smoothly, visit our inside your home hub or check our guide on finding a local heating engineer if your DIY fix does not work.
Complete List of Viessmann boiler fault & error codes
Now the list you’ve been waiting for. I’ve scoured the internet, read countless Viessmann boiler manuals and pulled together a comprehensive list of Viessmann boiler fault codes for you, including a description of the fault & potential solution, you can also use the search bar to find your fault code easier:
| Error Code | Description of Fault | Possible Solution |
|---|---|---|
| 2 | The safety has been activated and you’ve been locked out. | This is typically due to another fault, but try reset the boiler |
| 10 | The sensor for the outdoor temperature has short circuited. | To get this issue fixed, you will need to call a boiler engineer. |
| 19 | There’s a fault with the communication of the outdoor temperature sensor | To get this issue fixed, you will need to call a boiler engineer. |
| 20 | This is an issue with the temperature sensor, but it can be in relation to it short circuiting, or the system flow or supply temperature. | To get this issue fixed, you will need to call a boiler engineer. |
| 21 | The sensor for the return water temperature has short circuited. | To get this issue fixed, you will need to call a boiler engineer. |
| 25 | The temperature of the boiler whilst in operation has been consistently high. | Reset the boiler. |
| 26 | When the boiler is operation there has been a continuous relay test. | Reset the boiler. |
| 28 | There is an issue with the supply temperature sensor | To get this issue fixed, you will need to call a boiler engineer. |
| 30 | The sensor for the water temperature has short circuited | To get this issue fixed, you will need to call a boiler engineer. |
| 35 | The boiler is not activating | You can fix this by turning the emissions switch back to its original setting & resetting the boiler |
| 38 | There is an issue with the sensor for the water temperature | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| 52 | This error can be one of two things, the temperature supply sensor may have short circuited causing a low loss header, or the flow sensor has shorted out | To get both these issues fixed, you will need to call a boiler engineer. |
| 07 & 09 | The air pressure isn’t correct when it is in standby mode & the boiler has locked out. | Try re-pressurise the boiler and reset it. |
| 04, 05, 06 & 08 | The speed of the fan is either too low or high when the boiler starts | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| 0A | The speed of the fan is faulty | To get this issue fixed, you will need to call a boiler engineer. |
| 0B | The burner is blocked | This could be a multitude of different things so I would advise calling a boiler engineer. |
| 0C | There is an insufficient power supply or the flame signal was still present after switching the burner on | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| 0D | The controls have been locked out. | This can be because you’ve reset the boiler too many times, so give it a few minutes. |
| 0F | A service or maintenance is required | You will need to call a boiler engineer. |
| 10, 14 & F3, F4 | The flame signal is not present | Check the gas supply, try reset the boiler & if the issue persists you will need to call a boiler engineer. |
| 18 & D1 | The outdoor temperature sensor cable is broken | To get this issue fixed, you will need to call a boiler engineer. |
| 1A & 1B | The flow sensor is faulty | To get this issue fixed, you will need to call a boiler engineer. |
| 1F | The rate of the flow is exceeding the recommended | To get this issue fixed, you will need to call a boiler engineer. |
| F80, F81, F83, F88, F89 & 29, 41, 49 | The sensor for the return water temperature has an open circuit | To get this issue fixed, you will need to call a boiler engineer. |
| 40 & 44 | The temperature sensor has short circuited on circuit 2 | To get this issue fixed, you will need to call a boiler engineer. |
| 48 & 4C | On the supply temperature sensor there is a cable that is broken. | To get this issue fixed, you will need to call a boiler engineer. |
| 50-51 & 58-59 | There is an issue with the temperature sensor for the DHW tank, it’s either short circuited or has an open circuit. | To get this issue fixed, you will need to call a boiler engineer. |
| 55-57 | A service or maintenance is required. | You will need to call a boiler engineer. |
| 5A & B7, B8 | There’s a low loss header or lead break with the supply temperature sensor. | You will need to call a boiler engineer. |
| 5C, 5D, 5E & 5F | There is a fault with the communication internally within the boiler | You will need to call a boiler engineer. |
| BD, BE, BF & 60, 68, 70, 78 | The sensor for the temperature has an open circuited or has short circuited. | You will need to call a boiler engineer. |
| 80-83 & FD | The burner control unit or the sensor for the water temperature has short circuited. | You will need to call a boiler engineer. |
| 88-89 & E6 | This is a fault with the burner control unit. | You will need to call a boiler engineer. |
| 8C, 8D, 8E & 8F | There is a fault with the communication internally within the boiler | You will need to call a boiler engineer. |
| 90-94, 98-99 & 9A, 9B, 9C, 9E, 9F & AB | There has been a short circuit in the sensor for the temperature | You will need to call a boiler engineer. |
| A0-A2 & AA, AC | Safety equipment fault or a BUS fault | You will need to call a boiler engineer. |
| A2 | KM-BUS fault | You will need to call a boiler engineer. |
| A3, B0, F1 & FE1-FE2 | The gas burner is blocked, often because of the incorrect positioning of the flue. | You might be able to move the flue slightly, but if not you will need to call a boiler engineer. |
| A4 | In the variable speed heating circuit pump there is an open circuit with the BUS connection. | You will need to call a boiler engineer. |
| A5 | There isn’t too much information about this one, apart from the fact it’s an open or shorted circuit. | You will need to call a boiler engineer. |
| A7 | The programming unit is faulty – you typically only see this right after installation. | Get in touch with whoever installed the boiler. |
| A8 | There is either air lock in the circulation pump, or the minimum flow rate hasn’t been achieved. | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| A9, BA, BB, BC & FE0 | There is a fault with the communication internally within the boiler | You will need to call a boiler engineer. |
| AD | The isolation valve has an error with it’s configuration. | If you’ve recently had work done on your boiler, get back in touch with them. |
| B1 | The programming unit has a communication error. | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| B2 | The boiler isn’t reading the sensor inputs correctly. | You will need to call a boiler engineer. |
| B3 & E5 | There is an internal fault and the boiler hasn’t stored the data points | You usually only see this during a service. |
| B4 & B5 | There’s a fault with the electronics internally within the boiler | You will need to call a boiler engineer. |
| B6 | The hardware is invalid | Is it a recent installation? Contact the boiler engineer that installed the boiler. |
| B9 & FEE, FEC, FEF, FFD, FFF, F0, FE5 & ED | There’s a fault internally. | You will need to call a boiler engineer. |
| C1 | Safety equipment externally | No action |
| CD-CF, C2-C38 & DA-DF, DF, D2-D8 & E0 | The solar control unit & KM-BUS have an open circuit or there is a communication fault. | You will need to call a boiler engineer. |
| CA, CB & C9 | There has been an issue due to the maximum pressure. | Release some of the pressure in the boiler & reset it. |
| CC & F0 | There is an open circuit or a differential in the air pressure sensor. | You will need to call a boiler engineer. |
| D4 | The fixed high limit has responded of F2 has blown. | You will need to call a boiler engineer. |
| D5 | The cascade control unit is not being logged into when the boiler is started. | You will need to call a boiler engineer. |
| E1, E7-E9, EA, EB, EC & FF2 | The ionisation is too high during the calibration. | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| E2-E3 | During the calibration the water flow rate has been too low. | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| E4 | There’s a fault with the supply voltage. | You will need to call a boiler engineer. |
| EE & EF | The flame signal is missing or too weak. | Try reset the boiler, if the issue persists you will need to call a boiler engineer. |
| F5 | The switch for the air pressure is faulty. | You will need to call a boiler engineer. |
| F6, F9 & FE1-FE2, FF6 & 58 | This is an issue with the gas pressure switch, it’s showing there’s no gas pressure. | Is your gas supply working? Test other appliances to make sure. If not you will need to call a boiler engineer. |
| FA & FB | The burner is in fault mode. | You will need to call a boiler engineer. |
| FB7 & FF2, FF4, FF7-FF9 | Faulty or not inserted coding card | This is in relation to the coding card and burner control unit, contact whoever has worked on your boiler recently. |
| FC | Faulty gas valve. | You will need to call a boiler engineer. |
| FE | There is a fault with the main printed circuit board (PCB). | This can be quite an expensive one & may even mean replacement. You will need to call a boiler engineer. |
| FE4 | The voltage has been too low multiple times. | You will need to call a boiler engineer. |
| FFA, FFB & FR | There is a fault with the fan. | You will need to call a boiler engineer. |
