hydraulic indicator light? there isn't one on this truck
this is a list of the indicators and guages on the dash instrument cluster on this model truck
- Hourmeter
- Warning Light, Alternator
- Voltmeter
- Indicator Light, Coolant Temperature
- Coolant Temperature Gauge
- Indicator Light, Left-Hand Turn, H3.50-5.50XM (H70-120XM)
- Indicator Light, Service Engine Soon
(Lift Trucks with GM 4.3L Engine and Gas/LPG Fuel System)
- Warning Light, Coolant Level in the Radiator
- Indicator Light, Air Filter Restriction
- Operator Password Function
- Warning Light, Brake Fluid Level
- Indicator Light, Cold Start (Diesel Only)
- Warning Light, Fasten Seat Belt
- Indicator Light, Transmission 1st Gear, H3.50-5.50XM (H70-120XM)
- Warning Light, Powershift Transmission Oil Temperature
- Warning Light, Hydrostatic Transmission Oil Temperature, H3.50-5.50XM (H70-120XM)
- Warning Light, Hydrostatic Transmission Overspeed, H3.50-5.50XM (H70-120XM)
- Warning Light, Water Separator (Diesel Only)
- Warning Light, Hydrostatic Transmission Fault, H3.50-5.50XM (H70-120XM)
- Indicator Light, Right-Hand Turn, H3.50-5.50XM (H70-120XM)
- Fuel Gauge
- Indicator Light, Fuel Level
- Engine Oil Pressure Gauge
- Warning Light, Engine Oil Pressure

No mention of any hydraulic anything
You sure you're not seeing a transmission light?
If you have the operators manual which should be in the rear pouch on the back of the seat it will show you what all the instrument lights and guages are.
And usually if there is a sensor error of any sort it will show an error code in the hour meter LCD screen.