As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ATC COULD NOT READ XPONDER, NEITHER L NOR R. AC RETURNED TO GATE. MAINTENANCE PERFORMED TROUBLESHOOTING RESULTING IN REPLACEMENT OF THE ATC CONTROL PANEL IAW AMM 34-53-02. OPS TEST OF BOTH TRANSPONDERS GOOD. AC RVSM COMPLIANT. AC DEPARTED.