As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IMT - THE CREW ELECTED AN AIR TURN BACK AFTER THE LT ENGINE TORQUE GAUGE WENT TO ZERO ON BOTH ANALOG AND DIGITAL. AN UNEVENTFUL LANDING WAS ACCOMPLISHED AFTER WHICH MAINTENANCE SWAPPED TORQUE PROBES LT TO RT. ON A TEST RUN-UP, THE SYSTEM FUNCTIONED NORMALLY.