If you push the Function side button and then click the rotary control and then repeat again it will show the data error on the display.
If you have a good signal, 3 bars or more, you can use auto for the threshold and it will adjust to best decode performance. Note what value it ends up with and set the site to manual threshold and the value that auto found where the best. If the signal are weak the auto mode doesn't work and that mentioned 12 value are probably a good start. The lower the value the easier the decode function starts to work but the bit error would not be optimal.
If the LCN are programmed wrong it will try and monitor a TG from the wrong frequency and would return to the control channel slot and then repeat again. It might look on the display as if it stays on a voice slot even when it isn't actually receiving anything from the slot, or it's just the control channels slot you are seeing.
/Ubbe