B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 115
Aircraft Equipment Problem Less Severe
Flight Crew Returned To Departure Airport
Aircraft
At 400 feet we were unable to get LNAV to engage and lost both flight directors. At 1;000 feet we were unable to set speed above 189 knots without selecting Level Change. I asked the FO to inform ATC that we were unable RNAV and needed a vector. I'm not certain if we missed a turn on the SID; but I don't think we did. After the turn we briefly troubleshot the issue and still could not get LNAV or VNAV to work. After assessing the situation we decided we could navigate direct without actually engaging LNAV; because the HSI was indicating normal.We decided to continue climbing and would reassess and troubleshoot at altitude or at lower task loading. Climbing through FL234 we got the Cabin Altitude Warning horn and light. We immediately donned our masks and established communication. We ran the abnormal pressurization checklist and were able to prevent the cabin from climbing above 11;000 feet. We [advised ATC of our situation and] descended to 10;000 feet and informed ATC we wanted to return to [our departure airport]. Other added conditions were; when we pulled the thrust levers to idle we would get a low idle light; also the FMC thought the aircraft was still on the ground.We determined that there was a failure of the air/ground sensor; even though we were able to get the gear up and no configuration warning horn when flaps were retracted. We decided not to arm the speedbrake or use autobrakes for landing. Upon landing; the speedbrakes deployed automatically. We taxied to the gate uneventfully. We did try to get a voice contact with Dispatch during the event; but were unable so we had Operations inform Dispatch that we were returning to [departure airport].
[Report Narrative Contained No Additional Information]
B737 flight crew reported they returned to departure airport when the air/ground switch failed in the ground position after takeoff.