Captain reported an unsafe landing gear indication after gear retraction on takeoff. The flight crew was unable to extinguish the unsafe indication after troubleshooting and running the check lists and QRH. The crew performed an air turn back and precautionary landing at the departure airport.

Narrative

This pairing leg was from ZZZ toZZZ1 which was to conduct right seat training for LCA (Line Check Airmen) upgrade. For all intent and purposes; this was a routine flight segment. We completed all required preflight checklist items and departed from the gate. Tower cleared us for takeoff on runway XYR [for an intersection takeoff] as planned and after liftoff; I called for gear and flap retraction on schedule and everything felt and sounded normal. I called for the after-takeoff checklist and at that point we noticed that the green landing gear lights had gone out; but not the red indicators. We leveled off the aircraft and discussed the indications that we were observing. After determining that it was a gear disagree indication; we divided duties. I continued to fly the aircraft and assumed control of the radios while the Captain would run the checklist. I informed ATC that we had an issue and requested that we stay in the local radar area while non-normal checklists were being conducted. The Captain checked the Manual gear extension door and circuit breaker panel and ran the gear disagree checklist from the flight manual. After completion of the checklist; the gear disagree indication still existed. Captain called maintenance control on radio #2 since we were still in the local area. Maintenance was informed of the steps that we had conducted and suggested that we cycle the landing down and then up. Knowing that this was going to feel and sound abnormal; Captain called the flight attendants to inform them of our issue and also to make a PA to the passengers that we were working an indication issue that required us to put the gear down. Once that was completed; I slowed the aircraft down to 225 KIAS for the retraction limitation of 235 KIAS. The gear handle was placed in the down position and all indications were normal. Captain retracted the gear and we received the same indications once again. The gear felt normal as it made its cycle both down and up. At this point we reported back to maintenance and they had no further suggestions. Captain called dispatch on AIRINC and had maintenance control phone patched into the call. After discussion with maintenance control; it was determined that we had exhausted all troubleshooting steps that could be conducted inflight. The decision was made to return to ZZZ and discontinue the flight in the interest of safety. We [requested priority handling] with ATC and had dispatch send landing data to the aircraft. We discussed the overweight landing (Approximately 4K) and felt safe that ZZZ was an appropriate destination. Through dispatch; we had station operations and maintenance informed of our return. The flight attendants and passengers were informed of our return to the ZZZ airport. We requested that airport crash fire rescue meet the aircraft to check the brake temperatures as we were above the normal landing weight. I planned a flaps 40 approach to bring the aircraft in as slow as possible to minimize the chance of overheating the brakes. We completed the rest of the required briefs and checklists; then informed ATC of our plan to return to ZZZ. We were vectored to the ILS XYL approach and landed without incident. The fire fighters met us to assess brake temperatures. They informed us that we were not showing any indications of hot brakes and followed us back to the gate. We arrived back at [the gate] and were met with maintenance; gate agents; and [company] personnel to debrief the incident.