Narrative:

On a long final. Captain's autopilot disengaged without his input. VOR/localizer mode was initially selected; but we were re-cleared the ILS (there had been a ship in the harbor). Neither autopilot would engage. Tried to engage the approach mode without success. Tried to re-engage the VOR/localizer mode without success. Captain elected to go around. On radar downwind; I tried to 'boot strap' the FMC to either side to see if we could get one of the autopilots back. We also reviewed the raw data approach checklist in the QRH. On final; we were slowed and configured; but not having any luck with selecting an approach mode. Approach control wanted us to speed up (they were sequencing in a airbus 330; I think). The captain elected to declare an emergency due to our decreasing fuel options and very odd instrumentation issues. We were using raw data; but the captain only had vertical command steering bars. I had vertical and lateral even though we never were able to select an approach mode. I asked if he would like for me to fly the approach and he gave me the airplane to land. The landing was uneventful. Throughout this entire situation; we did not have a master caution light; and no system degradation indications in the cockpit. In hindsight; the captain could have given me the aircraft earlier which would have let him have time to brief the passengers and flight attendants. This airplane had a previous history of pseu [proximity sensor electronic unit] (primarily radar altimeter) issue. The mechanic on the ground in ZZZ told us afterwards 18 faults showed in the system.

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Original NASA ASRS Text

Title: During approach; B737-800 flight crew experiences autopilot and flight director anomalies and the autopilot cannot be reconnected. The Captain elected to go-around. On the second approach autopilots and approach mode can still not be selected but the First Officer had a functioning flight director and flies the approach to landing.

Narrative: On a long final. Captain's Autopilot disengaged without his input. VOR/LOC mode was initially selected; but we were re-cleared the ILS (There had been a ship in the harbor). Neither Autopilot would engage. Tried to engage the Approach mode without success. Tried to re-engage the VOR/LOC mode without success. Captain elected to Go Around. On radar downwind; I tried to 'boot strap' the FMC to either side to see if we could get one of the Autopilots back. We also reviewed the Raw Data approach checklist in the QRH. On final; we were slowed and configured; but not having any luck with selecting an approach mode. Approach control wanted us to speed up (they were sequencing in a Airbus 330; I think). The Captain elected to declare an emergency due to our decreasing fuel options and very odd instrumentation issues. We were using raw data; but the Captain only had vertical command steering bars. I had vertical and lateral even though we never were able to select an approach mode. I asked if he would like for me to fly the approach and he gave me the airplane to land. The landing was uneventful. Throughout this entire situation; we did not have a master caution light; and no system degradation indications in the cockpit. In hindsight; The Captain could have given me the aircraft earlier which would have let him have time to brief the passengers and flight attendants. This airplane had a previous history of PSEU [Proximity Sensor Electronic Unit] (primarily Radar Altimeter) issue. The mechanic on the ground in ZZZ told us afterwards 18 faults showed in the system.

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.