Narrative:

Aircraft X was climbing to FL230. Aircraft Y was descending to cross osogy at FL240. We observed aircraft Z's mode C climbing through FL238 converging with aircraft Y. The r-side immediately advised him to stop his climb and descend to FL230. There was no response; therefore he proceeded to tell aircraft Y to stop his descent and climb to FL260.we then saw aircraft X climbing to FL242 so we turned aircraft Y 20 degrees left. When we finally got a hold of aircraft X; the pilot advised that they had been level at FL230 the whole time. We still turned him to a 360 heading because mode C was showing FL240. Later the mode C changed to FL230. We asked him his altimeter; and he responded 29.92. We asked aircraft Y if he saw anything on TCAS; and he responded that he saw someone 1;900 feet below him. At that point the conflict was resolved but we don't know what caused the discrepancy on the mode C.I believe that when the pilot called in he said that his mode C was set to something that was not the normal setting. I don't know the terminology but he told the operations manager that it may have something to do with that; but he was not sure.I would like the radar sort box to be looked at in case there was a discrepancy. I would also like to investigate further so that this does not happen again. It would be good to talk to the pilots of both aircraft to understand what they were seeing in the flight deck.

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Original NASA ASRS Text

Title: ZMA controllers and flight crew reported ATC display indicating erroneous mode C of a CL300. ATC issued control instructions for separation based on their display information.

Narrative: Aircraft X was climbing to FL230. Aircraft Y was descending to cross OSOGY at FL240. We observed Aircraft Z's mode C climbing through FL238 converging with Aircraft Y. The R-side immediately advised him to stop his climb and descend to FL230. There was no response; therefore he proceeded to tell Aircraft Y to stop his descent and climb to FL260.We then saw Aircraft X climbing to FL242 so we turned Aircraft Y 20 degrees left. When we finally got a hold of Aircraft X; the pilot advised that they had been level at FL230 the whole time. We still turned him to a 360 heading because mode C was showing FL240. Later the mode C changed to FL230. We asked him his altimeter; and he responded 29.92. We asked Aircraft Y if he saw anything on TCAS; and he responded that he saw someone 1;900 feet below him. At that point the conflict was resolved but we don't know what caused the discrepancy on the mode C.I believe that when the pilot called in he said that his mode C was set to something that was not the normal setting. I don't know the terminology but he told the operations manager that it may have something to do with that; but he was not sure.I would like the radar sort box to be looked at in case there was a discrepancy. I would also like to investigate further so that this does not happen again. It would be good to talk to the pilots of both aircraft to understand what they were seeing in the flight deck.

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.