Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
Descending into ZZZ the aircraft locked onto a glideslope signal at around FL340 and started to climb. Took a couple of seconds to figure out what it was doing; but once I saw the G/S on the FMA was green (meaning it was locked on) it was clear the aircraft thought it should follow the glide slope so it started a climb. At that point the autopilot was disconnected and ATC was notified that we had a problem. All the automation was turned off and once the aircraft was stable in the descent the automation LNAV/VNAV etc.; was re-established. Aircraft landed safely in ZZZ and was turned over to Maintenance. They discovered that the FCC computer on the A side had tripped a fault code and placarded the autopilot A inoperative. Flew the Aircraft back to ZZZ on autopilot B with no issues.
B737-800 Captain reported that an FMS malfunction resulted in an uncommanded climb during approach.