Pete had lent someone an old Lima 0-4-0 diesel shunter to test some Dingham couplings on the O gauge layout. Putting it on the track, nothing happened. A quick check with a German loco showed there was power, so the problem was in the model.
Looking inside, it had once been fitted a DCC chip but this had been removed a blanking plug replacing the electronics. An unwired blanking plug.
Since this section of the layout uses God's own DC, all I needed to do was use the wires on the plug to joins the pickups and motor. Simple enough - just strip the ends of the wires and twist them together.
Result - the loco now works as intended. And I understand how a blanking plug works!