This error code denotes poor pressure change detection at the launch stage. This is not always easy to solve and often requires professional help. This may be caused by:
It should be noted that Vaillanf Eco-Tec boileers, especially early ones (made in 2005 to 2010), often have poor quality pressure sensors. Newer ones boast better quality parts and are less likely to have this issue.
Most likely, the sensor damage was cause by debris (lime scale, sludge, etc.), which forms as the device is used and may block the sensor. A magnetic filter, scale reducer and/or an inhibitor can be decent solutions.
Older Vaillant boilers featured inferior Wilo pumps. They fell into disrepute because of numerous customer complaints about them failing too soon. If you have an old boiler equipped with a Wilo pump, it is not unlikely that the error has occurred due to a pump failure. Newer models feature much more reliable and affordable Grundfos pumps.
Check the pump
Launch the boiler and listen to the pump. It starts right after the ignition. As it force water to circulate the piping, it produces a characteristic humming noise and may vibrate slightly. So, if the noise and vibration are there, the pump is all right. Then you should check the pressure sensor.
If the pump is not humming or vibrating, it means that it is simply not working. You may need to repair or replace it, depending on the degree of damage. It is highly advisable to call a technician.
If you have detected a pressure sensor problem, it may be caused by debris. This problem can be fixed by mere cleaning. However, if blocked severely, this can be very time-taking and costly. Some users say that getting a new sensoor easier and more affordable.