Fault code: 3L?

My boiler keeps reverting to code: 3L, but on pressing the reset button, after approximately 10 times it restarts. But on using the hot tap it most of the time goes to Code: 3L.
However, if the heating (radiators) is in use the hot water tap does not fail. More or less as soon as the heating is turned off and first use of the hot water it fails and Code: 3L shows. Then I have to keep pressing the reset button at least 10 times before it will ignite again.