What was found
DURING LINE OP CHECK JUST PRIOR TO DEPARTURE, CREW OBSERVED 'STEER-BY-WIRE FAIL' MESSAGE ON CASDU AND ALSO FOUND NR 1 AND NR 2 STEERING CONTROL CIRCUIT BREAKERS TRIPPED. BREAKERS WOULD NOT RESET. AIRCRAFT TAXIED BACK TO RAMP WITH DIFFERENTIAL BRAKING. MAINTENANCE FOUND STEERING SYSTEM NR 1 SOLENOID SHUT-OFF VALVE CAUSING FAULT AND REPLACED SAME AND RETURNED AIRCRAFT TO SERVICE.