pcs is for counting a number of small parts. Depending on the scale, you put 10 or 50 on the tray and program in the number of units. Then the average weight of the items is used to calculate the number of pieces for the subsequent handfuls of pieces. In the Units mode, press Print Unit to the type of unit counting and follow the screen to tell the number of parts used to set the average mass of each one.
On the "old", is there a problem with the left most LCD elements (so it could be Hold)? This merely means that once you weigh an object if you remove it from the platform the weight will still be displayed. You can turn this off in the menu. Press On/Off (for > 5 seconds) and then Print Unit. If it says Setup, then press On/Off and the Print Unit repeatedly until you see Mode. Then press On/Off and then Print Unit until you see Off. Then press On/Off and Yes to save the mode.
The other things to check: is the scale connected to the AC with the adapter or is it on battery? Is the lockswitch in the locked position? First try removing the batteries or disconnecting the power adapter. (Consider replacing the batteries) Then unlock the lock switch (under the balance at the front) and the shipping lock (below the platform) if needed.
I hope this helps. I don't have this scale but a very similar one.
(the manual for the SP601 is here: http://www.oldwillknottscales.com/ohaus-scout-sp601.aspx
scroll down for the pdf link)