NOTE
Be sure the battery is fully charged.
Main Harness [B] Vehicle-down Sensor [C]
Special Tool - Measuring Adapter: 57001-1700

Vehicle-down Sensor Input Voltage Connections to Adapter: Digital Meter (+) → R (sensor BL) lead Digital Meter (–) → BK (sensor BR/BK) lead

Input Voltage
Standard: DC 4.75
5.25 V
If the reading is within the standard, check the output voltage (see Vehicle-down Sensor Output Voltage Inspection).
If the reading is out of the standard, remove the ECU and check the wiring for continuity between main harness connectors.
Special Tool - Hand Tester: 57001-1394
Disconnect the ECU and sensor connectors.
Wiring Continuity Inspection ECU Connector [A] ←→ Vehicle-down Sensor Connector [B] BL lead (ECU terminal 9) [C] BR/BK lead (ECU terminal 13) [D]

If the wiring is good, check the ECU for its ground and power supply (see ECU Power Supply Inspection in the Fuel System (DFI) chapter).
If the ground and power supply are good, replace the ECU (see ECU Removal/Installation in the Fuel System (DFI) chapter).
Vehicle-down Sensor Installation
Vehicle-down Sensor Output Voltage InspectionCylinder (Upper Crankcase) Wear Inspection
Split the crankcase (see Crankcase Splitting).
Remove the pistons (see Piston Removal).
Assemble the cylinder (upper crankcase) [A], the cylinder
head [B] and the cylinder head gasket [C].
Tighten the cylinder head bolts (see Cylinder Head Installation
in the Engine Top End chapter).
...
Brake Light Switch Operation Inspection
Turn the ignition switch to ON.
The brake light (LED) [A] should go on when the brake
lever is applied or after the brake pedal is depressed
about 10 mm (0.39 in.).
If it does not, adjust the brake light switch.
Remove the front footpeg bracket (see Brake Pedal Removal
in the B ...
Stem, Stem Bearing Installation
Replace the bearing outer races with new ones.
Drive them into the head pipe at the same time.
Special Tools - Head Pipe Outer Race Press Shaft [A]:
57001-1075
Head Pipe Outer Race Driver,
55 [B]: 57001
-1446 or Bearing Driver Set: 57001-1129
Apply grease to the outer races.
...