Narrative:

On climb out; we had a left elevator probe light illuminate and it was determined that it was safe to continue the flight. On the ZZZ ILS xxl approach in IMC conditions with the glide slope and localizer captured and after passing the final approach fix the localizer rapidly went full scale deflection. I disconnected the autopilot and we immediately went missed approach. On the missed trying to regain control of heading and altitude; the caution bank angle sounded and I immediately corrected. Once stabilized on the missed and being vectored we double checked everything. We couldn't find anything abnormal and decided to try another approach to runway xxl. On the second approach I coupled the second autopilot as the ceilings were lowering. Basically the same thing happened again. We entered a hold this time and again nothing was found to be wrong. We attempted to talk to maintenance and dispatch but the conversation was broken and couldn't be completed. Meanwhile our fuel was getting to the reserve quantity and the weather had deteriorated tremendously with thunderstorms over the field and low ceilings. It was then decided for safety to go to our alternate. The rest of the flight was uneventful ending with a normal ILS in ZZZ1. The return flight later in the day back to ZZZ was also uneventful with a normal ILS runway xxl approach.we weren't able to determine what caused the localizer malfunction. The first officer said he did see the elevator feel light flickering. In hindsight we probably should have gone to the alternate for better weather after the first go around.

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Original NASA ASRS Text

Title: B737-800 Captain reported a navigation error that resulted in a missed approach.

Narrative: On climb out; we had a left elevator probe light illuminate and it was determined that it was safe to continue the flight. On the ZZZ ILS XXL approach in IMC conditions with the glide slope and localizer captured and after passing the final approach fix the localizer rapidly went full scale deflection. I disconnected the autopilot and we immediately went missed approach. On the missed trying to regain control of heading and altitude; the caution bank angle sounded and I immediately corrected. Once stabilized on the missed and being vectored we double checked everything. We couldn't find anything abnormal and decided to try another approach to Runway XXL. On the second approach I coupled the second autopilot as the ceilings were lowering. Basically the same thing happened again. We entered a hold this time and again nothing was found to be wrong. We attempted to talk to Maintenance and Dispatch but the conversation was broken and couldn't be completed. Meanwhile our fuel was getting to the reserve quantity and the weather had deteriorated tremendously with thunderstorms over the field and low ceilings. It was then decided for safety to go to our alternate. The rest of the flight was uneventful ending with a normal ILS in ZZZ1. The return flight later in the day back to ZZZ was also uneventful with a normal ILS Runway XXL approach.We weren't able to determine what caused the localizer malfunction. The First Officer said he did see the elevator feel light flickering. In hindsight we probably should have gone to the alternate for better weather after the first go around.

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.