F70 Vaillant: Meaning, Symptoms, Actions, Costs, Causes, Prevention
The Vaillant F70 fault means the device-specific number (DSN) is invalid, usually due to an issue with the printed circuit board (PCB). Your boiler display will show the F70 error, and the system may stop providing heating or hot water. You need to contact a Gas Safe engineer to diagnose and resolve the issue, as it often requires reprogramming or replacing the PCB. Fixing the F70 fault costs between £100 and £400, depending on whether the engineer can reprogram the PCB or if a full replacement is necessary. This fault happens when the PCB is unprogrammed, incorrectly assigned to the boiler model, or faulty. Preventative measures include using genuine Vaillant parts, ensuring professional installation, and scheduling regular servicing. Only a Gas Safe registered engineer should work on your boiler to ensure safe repairs. The next section explains the meaning of the F70 fault in more detail.
Meaning of the F70 Vaillant Boiler Fault
The F70 fault on a Vaillant boiler means the device-specific number (DSN) is invalid or missing. The DSN is a unique identification code that allows the boiler’s printed circuit board (PCB) to recognise and communicate with the boiler model. When this code is incorrect or unreadable, the boiler cannot function properly.
This issue often occurs after a PCB replacement, where the new board has not been programmed or assigned correctly. In some cases, the original PCB develops a fault, losing its ability to store or read the DSN. The error prevents the boiler from operating because it does not recognise its internal components.
A Gas Safe engineer can resolve the fault by reprogramming the PCB or, if necessary, installing a correctly programmed replacement. The next section explains the symptoms of the F70 fault in more detail.
Is the F70 fault a serious issue?
The F70 fault prevents your boiler from working, but it does not pose an immediate safety risk. However, without heating or hot water, the fault causes major inconvenience. A Gas Safe engineer should inspect the boiler to restore normal operation.
Can I reset my boiler to fix the F70 fault?
A simple reset will not fix the F70 fault because the issue relates to the PCB’s programming. A qualified engineer must diagnose the problem and either reprogram or replace the PCB.
Does a new PCB always trigger the F70 fault?
A new PCB can trigger the F70 fault if it has not been programmed or assigned to the boiler model. If the board is correctly set up, the fault will not appear. Professional installation ensures the PCB is assigned the correct DSN.
The next section explains the symptoms of the F70 fault in more detail.
Symptoms of the F70 Vaillant Boiler Fault
The F70 Vaillant boiler fault prevents normal operation due to an invalid device-specific number (DSN). Your boiler display will show the F70 error code, indicating a communication failure between the printed circuit board (PCB) and the boiler model.
Your boiler will not provide heating or hot water because it cannot function without a valid DSN. The controls on the boiler may become unresponsive, preventing you from adjusting the settings. The display screen remains active, but the boiler does not ignite or respond to thermostat commands.
If the fault is caused by an incorrectly programmed PCB, the issue appears immediately after installation. If the fault is due to a failing PCB, the boiler may have operated normally before the error appeared. A Gas Safe engineer can diagnose whether reprogramming or replacement is necessary.
How To Recognise The F70 Vaillant Boiler Fault on Different Types of Boilers
The F70 fault manifests differently depending on the type of Vaillant boiler installed. Modern digital boilers display clear fault codes, while older models rely on indicator lights or error patterns.
Vaillant Combi Boilers
Vaillant combi boilers display the F70 fault code on the digital control screen. The boiler stops providing heating and hot water, and the display may remain unresponsive. If the PCB has failed completely, the screen may flicker or show random symbols.
Vaillant System Boilers
Vaillant system boilers show the F70 error on the LCD screen, and the boiler stops working. Your central heating system remains inactive, and the hot water cylinder does not receive heated water. The external pump and motorised valves remain functional, but no hot water circulates.
Older Non-Digital Boilers
Older Vaillant boilers without digital displays do not show the F70 code directly. Instead, the boiler may show a sequence of flashing lights or remain unresponsive. If the boiler fails to ignite or stops communicating with the thermostat, the PCB may be at fault. A Gas Safe engineer can confirm whether the issue is related to the F70 fault.
The next section suggests actions to take in the event of the F70 Vaillant Boiler Fault.
What to Do When Your Vaillant Boiler Shows the F70 Boiler Fault
When your Vaillant boiler displays the F70 fault code, the best course of action is to check the display screen and note the error. Your boiler will not provide heating or hot water until the issue is resolved, so immediate action is necessary.
First, reset your boiler using the manufacturer’s instructions. If the fault persists, check whether the issue appeared after recent maintenance or a PCB replacement. If no recent work has been done, the PCB may have developed a fault.
Do not attempt to repair, reprogram, or replace the PCB yourself. The PCB controls critical boiler functions and must be handled by a qualified Gas Safe engineer. Working on a boiler without proper certification is dangerous and may invalidate your boiler warranty.
If your boiler is unresponsive, check your home’s gas and water supply to ensure no leaks or visible damage. If you detect a gas smell or suspect a leak, turn off the gas supply at the meter and contact a Gas Safe engineer immediately.
Safety Disclaimer
Always follow the instructions in your boiler’s manual before taking any action. Never open the boiler casing or attempt electrical repairs. A Gas Safe engineer should diagnose and fix the fault to ensure safety and compliance with regulations.
How to reset my Vaillant boiler after an F70 fault?
Locate the reset button on the boiler’s control panel and hold it for a few seconds. The boiler will attempt to restart, but if the F70 fault remains, further action is required.
Should I turn off my boiler if it shows the F70 error?
Yes, if the boiler is unresponsive or not working correctly, switching it off prevents further issues. If you detect a gas smell or hear unusual noises, turn off the gas supply at the meter and contact a professional.
Where can I find a Gas Safe engineer to fix an F70 fault?
You can find a Gas Safe registered engineer by checking the official Gas Safe Register website. Vaillant also provides approved service engineers who specialise in their boilers.
How to check if my boiler has a gas or water leak after an F70 error?
Look around the boiler and nearby pipework for signs of water leaks, such as damp patches or dripping water. If you smell gas, turn off the gas supply immediately and call the emergency gas helpline.
Should I replace my boiler if it keeps showing the F70 fault?
If the F70 error persists after reprogramming or replacing the PCB, your boiler may be experiencing multiple failures. A Gas Safe engineer can assess whether a new PCB or a complete boiler replacement is the best option.
The next section describes the major causes of the F70 fault in more detail.
Cost to Repair F70 Vaillant Boiler Fault
The cost to repair the F70 fault on a Vaillant boiler depends on whether the printed circuit board (PCB) requires reprogramming or replacement. If a Gas Safe engineer can reprogram the existing PCB, you may pay between £100 and £200, including call-out and labour costs.
If the PCB is faulty and needs replacing, the cost increases significantly. A new PCB for a Vaillant boiler costs between £200 and £400, depending on the model. With labour included, you can expect to pay £300 to £600 for a full PCB replacement.
The most expensive repairs occur when additional components need replacement. If the engineer finds further issues, such as wiring faults or display failures, the total repair cost may exceed £600.
You should always use a Gas Safe engineer to ensure the PCB is correctly programmed and installed. Attempting DIY repairs can lead to further damage, invalidating your boiler warranty.
The 4 Major Causes of the F70 Vaillant Boiler Fault
The F70 fault on a Vaillant boiler happens when the device-specific number (DSN) is missing, incorrect, or unreadable. This issue prevents the printed circuit board (PCB) from identifying the boiler model, leading to system failure. The most common causes include an unprogrammed PCB, an incorrectly assigned PCB, a faulty or damaged PCB, and corrupted boiler firmware.
1. Unprogrammed PCB
A new PCB must be programmed with the correct DSN to communicate with the boiler. If the engineer installs a replacement PCB without assigning it to the boiler model, the F70 fault appears. Manufacturers supply PCBs in a default state, requiring configuration before use. Without programming, the boiler does not recognise the PCB, preventing normal operation.
2. Incorrectly Assigned PCB
Each Vaillant boiler model has a unique DSN that must match the installed PCB. If a replacement PCB is assigned the wrong DSN, the boiler will display the F70 fault. This mistake often happens when engineers use a PCB from another model or fail to select the correct settings during installation. A misassigned PCB may function temporarily but will eventually trigger the fault as the boiler attempts verification.
3. Faulty or Damaged PCB
A malfunctioning PCB can lose its ability to read or store the DSN, causing the F70 fault. Electrical surges, moisture damage, or general wear and tear can lead to PCB failure. Over time, solder joints and circuits degrade, reducing the PCB’s ability to process commands. If the PCB develops an internal fault, replacing it with a correctly programmed unit is the only solution.
4. Corrupted Boiler Firmware
Firmware controls the boiler’s internal operations, including communication with the PCB. If a software update fails or the firmware becomes corrupted, the boiler may lose its ability to verify the DSN. Corrupted firmware can result from power interruptions, incorrect updates, or prolonged system faults. A Gas Safe engineer can attempt a firmware reset, but in severe cases, a new PCB may be necessary.
Preventing Future F70 Vaillant Boiler Fault
Regular boiler maintenance reduces the risk of an F70 fault by ensuring the printed circuit board (PCB) functions correctly. An annual boiler service allows a Gas Safe engineer to check for early signs of PCB failure and confirm that all components communicate properly. Servicing also prevents moisture buildup and electrical faults, which can lead to PCB damage.
A boiler service contract provides scheduled maintenance and priority repairs, reducing unexpected breakdowns. Using only manufacturer-approved parts for repairs and replacements ensures compatibility with the boiler’s software. Keeping the boiler clean, dry, and free from dust also helps prevent damage to sensitive electronic components.
Disclaimer
Any work beyond basic operation described in the user manual should only be performed by a Gas Safe registered engineer. Opening the boiler casing, reprogramming the PCB, or replacing internal components without proper certification is unsafe and may void the manufacturer’s warranty. Always seek professional assistance for diagnosing and repairing boiler faults.