What was found
AIR TURNBACK, AT LEVELING OFF AT CRUISE THE LEFT FMC SCREEN WENT BLANK. LEFT CDU IS INOP. WHEN NAV SELECT SWITCH SET TO CDU-L RECEIVED A VTK FLAG ON NAV DISPLAY LT CDU HAD NO SCREEN, RT CDU OPS NORMAL WHEN STARTING T/S. FOUND 2 OHMS AT PIN 41 AND 115VAC AT PIN 40. SWAPPED RT AND LT CDUS FOR T/S. FAULT FOLLOWED LT CDU TO RT SIDE. BELIEVE ITS A BAD UNIT. R & R LT CDU, RETURNED RT CDU TO ORIGINAL LOCATION. IAW AMM 34-61-02/401. OPS CHK GOOD.