I have a Toolcat "D" and my service manual designates code 50-30 as "5.0 VOLT SENSOR 2 UNDERVOLTAGE". The troubleshooting steps are:
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1) Disconnect C453 from the Front Speed Sensor. Check for 5.0 volts on C453, pin 3, wire 1560 (yes, at least some of the wires are numbered) to chassis ground. (C453 means Connector 453.)
2a) The voltage reading is 4.1v or greater. System appears Ok. Replace Drive Controller.
2b) The voltage reading is 4.0v or less. Disconnect C350. Check continuity on C350, pin 30, wire 1530 to all other pins on C350. If continuity exists, repair or replace main frame harness. Otherwise (no continuity), the circuit appears Ok. Drive controller is possibly bad. Replace controller.
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Sorry about the bad grammer, what you see is pretty much what's in the electrical service manual. Apparently the manual writers don't know about the invention of flow charts.
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Also in the manual....NOTE: IF THE ALARM IS INTERMITTANT AND NOT ON AT THE TIME OF TESTING, THE CAUSE OF THE ALARM MAY NOT BE FOUND AT THIS TIME. PROBABLE CAUSES MAY INCLUDE: LOOSE CONNECTIONS, PUSHED BACK PINS, THE CONTROLLER, THE WIRING HARNESS OR A COMBINATION OF THESE PROBLEMS.
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Good Luck.