What was found
CPDLC ALTERS ARE ABNORMAL. WHEN RECEIVING A MESSAGE FROM ATC INFLIGHT WE ONLY RECEIVE A SCRATCH PAD MESSAGE AND FMC P/RST LIGHT ON THE FORWARD PANEL. THERE IS NO AURAL HIGH/LOW ALTER TONE NOR IS. ATC DISPLAYED ON THE ENGINE DISPLAY. PRIOR TO DEPARTURE THE DCL CLEARANCE WAS RECEIVED NORMALLY WITH ALL VISUAL AND AUDIO ALTERS. CPDLC COMMUNICATION IS NORMAL. ATC MESSAGES ARE RECEIVED AND MAY BE ACKNOWLEDGE BY THE CREW. ATC REPORTED NO ISSUES ON THEIR END. GROUNDED - YES. PERFORMED FLIGHT MANAGEMENT COMPUTER SYSTEM-CPDLC FUNCTIONAL TEST AND OPS CHECK GOOD REF B737-800 AMM 34-61-00-720-899. OK FOR SVC.