B737 NG Captain reported the landing gear extension and indicator lights had problems and was observed to be acting abnormally upon landing gear extension. The flight crew performed a go-around to troubleshoot and a decision was made to divert to another airport to do a fly-by for Tower to confirm landing gear position. After the landing gear was confirmed to be down; the flight crew returned to the original airport and safely landed.

Narrative

All flight events and procedures that occurred prior to calling for gear extension were normal. The descent and approach were briefed per normal procedure. The FO (First Officer) was the pilot flying; PF; Captain was pilot monitoring; PM; there was no Jumpseater or other flight deck crew members present. The pre-departure briefing included duties in case of abnormal operations or emergency - PF would continue to fly the aircraft and take the radios; PM would run any required checklists. Go-around procedures were briefed during the descent briefing. The approach was planned as a normal ZZZ ILS XXR in visual; gusty-wind conditions.At approximately XA:00 on Day 0; the flight was approaching ZZZZZ - ZZZ ILS XXR final approach fix - at approximately 1;800 ft. AGL. As briefed; the FO called for gear extension and setting Flaps 15. The gear extension sounded normal at first. However; both pilots observed abnormal landing gear position light indications. The following indications were noted: All three green landing gear indicator lights on the instrument panel were illuminated. All three green landing gear indicator lights on the aft overhead panel were illuminated. All three red landing gear indicator lights on the instrument panel were illuminated. A single; unfamiliar; tone - beep - sounded briefly.Unsure of what may have caused the abnormal landing gear position indications and the unfamiliar tone - neither the PM nor the PF had heard this tone before - the PM suggested a go-around from the current altitude and the PF called for the go-around and started the procedures as briefed. ATC stated; Aircraft X; go around; runway heading; climb and maintain 3;000 ft." The first "half" - i.e.; TOGA engagement; raising flaps to 15; which we were already positioned at; checking thrust; raising gear; [and] setting missed approach altitude - of the go-around was conducted normally. At this time; ATC started calling to ask about the reason for the go-around. This interrupted the go-around procedure. Approximately 30 seconds later as the aircraft was accelerating; the remaining procedures were accomplished - heading select; level change; setting clean maneuvering speed; raising flaps to 0; After-Takeoff Checklist. Allowing the ATC interruption and subsequent delay was pilot/crew error. However; the procedures were accomplished fully and safely.After go-around procedures were accomplished and the aircraft was overflying the ZZZ field; during the field overflight; the Tower reported that the aircraft's left main gear appeared to be extended while all other gear appeared retracted. All landing gear indicators on the flight deck appeared normal; i.e.; there were no red or green gear lights illuminated. The crew had no way to confirm whether or not the left main gear was extended. ATC queried the crew as to what airspace was required to resolve the issue. The PM requested vectors to remain in the local area and ATC vectored the flight west and directed a climb to 5;000 ft. MSL. The PF engaged the autopilot and maintained 230 KIAS via the MCP (Mode Control Panel) speed window.During the vector; the PF selected VNAV instead of continuing the LVL CHG climb. This caused the aircraft to start a brief descent while it attempted to recapture the cruise altitude in the FMC. The issue was identified by the PF; who disconnected the Autopilot and set the desired pitch attitude for the climb; followed by requesting re-engagement of LVL CHG; which was accomplished by the PM. The PF then re-engaged the Autopilot.The PM started to run NG Gear Disagree Checklist. The PF coordinated vectors with ATC and confirmed altitude and heading changes with the PM. While running the checklist; the PM: Step 1 - Landing gear lever is OFF. Step 2 - Landing gear lever moved to the UP position after takeoff; go-around. Step 3 - 235 KIAS maximum; landing gear lever: UP. Step 4 - All red and green landing gear indicator lights are extinguished. End of the checklist.PMthen determined that no additional resolution was available. We as a crew discussed if we should request priority handling; but it was decided that we had time and fuel 14;400 lb. left to continue to work on the problem. So we opted not to request priority handling at that time. However; ATC then asked us souls on board and fuel remaining; so we provided them with the information of SOB and 14;400 lb. of fuel remaining; or 144 minutes of flight time available. They essentially provided priority handling for us. Since we had no indication if our left main gear was still extended or not like the ZZZ Tower had told us it was; the crew then decided to contact a Company Ground Agency for any potential assistance or information they could provide. The PM contacted Company ZZZ Operations; who then directed us to contact Company ZZZ Maintenance. The Maintenance personnel suggested cycling the Manual Gear Extension Door located behind the FO on the floor open and closed as well as check for any popped circuit breakers. No additional resolution was gained as all landing gear indicator lights were still extinguished. While the PM was communicating with Maintenance; ATC contacted the PF and suggested the flight overfly ZZZ1 Airport; where Tower personnel could examine the gear positions with binoculars. The PF agreed and accepted vectors toward ZZZ1. When the PM was available; the PF briefed the PM on the plan to have ZZZ1 personnel visually inspect the gear. The PM agreed; then briefed the Purser; and made a short announcement to the passengers.Once the PM confirmed checklist usage complete; Maintenance agreed with the plan to overfly ZZZ1 and extend the gear. The PF accepted a descent to 2;000 ft. MSL and continued toward ZZZ1. Approaching ZZZ1; the PF extended the gear with PM concurrence. The aircraft was at 230 KIAS. The gear extension appeared normal; both in sound and landing gear light indicator illumination sequence. When complete; there were three green landing gear indicator lights illuminated on the instrument panel; three green landing gear indicator lights illuminated on the aft overhead panel; and zero red landing gear indicator lights illuminated. The aircraft overflew ZZZ1 at 2;000 ft. MSL and in a right turn toward the south. Through ZZZ Approach Control; ZZZ1 Tower personnel reported that all three landing gear appeared to be down. At this time Ground / Maintenance communications were called complete.With normal gear indications on the flight deck and visual confirmation from ZZZ1 Tower; the PF and PM decided that the gear would remain down for the remainder of the flight. At this point; the crew discussed and determined the following: All applicable checklists were run. Both flight crew members were comfortable moving into the approach / landing phase. The PF would continue to fly the aircraft. Call to Purser to rebrief the flight attendants on the situation and to NOT plan for an evacuation.The PF then requested and accepted vectors for the ZZZ ILS XXR. The PM reloaded the approach into the FMC and the PF called for the descent checklist. The localizer was intercepted between ZZZZZ1 and ZZZZZ. The remainder of the flight; including approach; landing; and taxi-in; were uneventful. During the approach; it was apparent that ATC; most likely Tower personnel; [provided us priority handling] and made Airport Rescue and Firefighting (ARFF) vehicles available. Additionally; on short final; ZZZ Tower personnel confirmed that all three gear appeared to be down. ARFF vehicles and equipment followed us to the gate area.After parking; shutting down the aircraft and conducting a short crew-only debrief - which focused on the go-around procedures and checklist usage - the flight crew was met by two ZZZ assistant chief pilots; Person A and Person B; and one Company Maintenance Representative. The flight crew was fully debriefed by Maintenance and [other] personnel. The PF entered an electronic logbook entry to document theevent."