Narrative:

At 6000 ft on the zzzzz arrival into ZZZ; we were cleared direct ZZZ1. SOP was followed by the pm (pilot monitoring) moving ZZZ1 to top line. A magenta line appeared and an execution light. I verified. ZZZ1 was executed with LNAV. The plane started the turn direct. About half way through the turn; the plane rolled out and began a turn right reversing course. I turned the autopilot off and turned left towards ZZZ1. I then looked at the first officer's (first officer's) FMC (flight management computer) because he noticed the error as well and was trying to correct the FMC. There was no magenta line but ZZZ1 had disappeared. The first officer was trying to insert ZZZ1 back in the box and the FMC would not accept ZZZ1. There was a moment of confusion on my part where the entire 80 degree turn to ZZZ1 was delayed due to the confusion. Approach re-cleared us to ZZZ1 wondering why we had not completed the turn timely being vectored for the approach. There was a high workload with me hand flying trying to figure out what was going on; turning and descending while trying to verify a possible mechanical problem. I asked the first officer to re-engage my autopilot but it would not come on. We reset our flight directors then tried to turn on either autopilot and neither would engage. I asked him if we had an electrical issue or if there were any overhead lights on. We did a recall of the master caution system and no mechanical issues were noted. Both fmcs seemed to be working normally. I had some concern if we had a navigational issue and would be able to shoot an ILS but we verified we looked good for an approach. I wrote up the problem on the ground and briefed the outgoing crew and spoke with maintenance. All in all; approach had to call us again to complete the turn to ZZZ1 which was delayed due to the above. I do not believe there was any loss of separation.

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Original NASA ASRS Text

Title: B737 Captain reported an anomaly with the Flight Management Computer during approach.

Narrative: At 6000 ft on the ZZZZZ arrival into ZZZ; we were cleared direct ZZZ1. SOP was followed by the PM (Pilot Monitoring) moving ZZZ1 to top line. A magenta line appeared and an execution light. I verified. ZZZ1 was executed with LNAV. The plane started the turn direct. About half way through the turn; the plane rolled out and began a turn right reversing course. I turned the autopilot off and turned left towards ZZZ1. I then looked at the FO's (First Officer's) FMC (Flight Management Computer) because he noticed the error as well and was trying to correct the FMC. There was no magenta line but ZZZ1 had disappeared. The FO was trying to insert ZZZ1 back in the box and the FMC would not accept ZZZ1. There was a moment of confusion on my part where the entire 80 degree turn to ZZZ1 was delayed due to the confusion. Approach re-cleared us to ZZZ1 wondering why we had not completed the turn timely being vectored for the approach. There was a high workload with me hand flying trying to figure out what was going on; turning and descending while trying to verify a possible mechanical problem. I asked the FO to re-engage my autopilot but it would not come on. We reset our flight directors then tried to turn on either autopilot and neither would engage. I asked him if we had an electrical issue or if there were any overhead lights on. We did a recall of the master caution system and no mechanical issues were noted. Both FMCs seemed to be working normally. I had some concern if we had a navigational issue and would be able to shoot an ILS but we verified we looked good for an approach. I wrote up the problem on the ground and briefed the outgoing crew and spoke with Maintenance. All in all; Approach had to call us again to complete the turn to ZZZ1 which was delayed due to the above. I do not believe there was any loss of separation.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.