try
Menu -> setup -> service searching -> tuner configuration -> tuner C and then change configuration to nothing connected, Green button. Exit back to Service searching and select Automatic scan. If tuner B or C (DVB-T) is set to yes change this to no and leave Scan tuner (DVB-C) to yes and then green button to scan.
Does it find any channels?
Now return to tuner configuration, enable tuner C and disable tuner B and then do an automatic scan and see if any channels are found.
This will tell you which tuner, if any is at fault.