What does diagnostic code 13 on a Lexus sc400 mean?

RPM signal. Can be cam position sensors. Common for the drivers side cam position sensor wires to get chaffed from contact from the drive belt when they are incorrectly routed. The other possibilities are the crank position sensor which is on the drivers side front of engine by the crank pulley inboard of the oil filter. Third could be an incorrectly installed timing belt that is off a tooth. Also check all wiring on the cam position sensors and crank position sensor. Those are the most common causes of code 13 on 92 SC400 OBD 1.