The F28 error code means the pressure in the system is close to zero. It can have different causes.
First, this may occur due to lack of ignition: insufficient spark, lack of gass supply, a faulty gas valve, poor or damaged ignition lead or electrode.
Check all other gas appliances that you have in your house and make sure they do not have gas supply problems. If they do, at least you will know it is not a gas supply or meter issue.
Other possible causes:
- Faulty gas meter or gas pressure relay
- Air in the piping
- Low gas pressure
- Poor gas fittings
- Improper gas supply setting
- The boiler is not grounded properly
- Faulty electronic board
Some forums say that adding water to the boiler using the boiler tap works in most cases. However, boiler maintenance and repair takes a bit of expertise, so use professional help.