As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
YYZ FLIGHT 3991 THE CREW REPORTED A AIR/GROUND MESSAGE WHILE POWERING UP THE AIRCRAFT AT THE GATE. REPORTED THE SENSORS APPEAR TO INDICATE THE AIRCRAFT WAS IN FLIGHT. MAINTENANCE REPLACED THE RIGHT MAIN GEAR AFT WOW PROXIMITY SENSOR. OPERATIONAL CHECKS WERE GOOD.