Narrative:

On climbout from ZZZ passing thru approximately 6;000 feet; ATC called out traffic at our 1:00 above us and descending. I thought I had the traffic visually; but wasn't sure; so I responded 'searching.' I looked at the TCAS (traffic collision avoidance system) display; but there wasn't a target where I thought the traffic should be. At approximately 7;000 feet; we got a resolution advisory telling us to descend; followed by another telling us to increase descent rate. The first officer (first officer) complied with the RA (resolution advisory) and performed the descending maneuver. The TCAS display showed an aircraft right on our nose within 100 feet vertically. I called ATC and informed them we were descending in response to an RA. We descended approximately 1;000 feet during which time the TCAS continued to command a descent. After that 1;000 feet; the TCAS commanded climb/climb now. The first officer complied with the climb RA command. The target on the display continued to show an aircraft on our nose within 100 feet; and we began to realize that our TCAS system was more than likely malfunctioning. I switched the transponder xpndr and altitude source both to '1' (they had been both in '2') and the displays returned to normal. I informed ATC that it was likely an aircraft malfunction we were responding to; and he confirmed that he did not observe any traffic in our immediate vicinity and altitude. We continued the flight in system 1 with no other issues.

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Original NASA ASRS Text

Title: B737-800 flight crew experienced erroneous TCAS alerts during climbout due to apparent equipment malfunction.

Narrative: On climbout from ZZZ passing thru approximately 6;000 feet; ATC called out traffic at our 1:00 above us and descending. I thought I had the traffic visually; but wasn't sure; so I responded 'searching.' I looked at the TCAS (Traffic Collision Avoidance System) display; but there wasn't a target where I thought the traffic should be. At approximately 7;000 feet; we got a Resolution Advisory telling us to descend; followed by another telling us to increase descent rate. The FO (First Officer) complied with the RA (Resolution Advisory) and performed the descending maneuver. The TCAS display showed an aircraft right on our nose within 100 feet vertically. I called ATC and informed them we were descending in response to an RA. We descended approximately 1;000 feet during which time the TCAS continued to command a descent. After that 1;000 feet; the TCAS commanded Climb/Climb Now. The FO complied with the climb RA command. The target on the display continued to show an aircraft on our nose within 100 feet; and we began to realize that our TCAS system was more than likely malfunctioning. I switched the transponder XPNDR and ALT SOURCE both to '1' (they had been both in '2') and the displays returned to normal. I informed ATC that it was likely an aircraft malfunction we were responding to; and he confirmed that he did not observe any traffic in our immediate vicinity and altitude. We continued the flight in system 1 with no other issues.

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.