PIAGGIO 500 cc Service Manual page 215

Fuel injection
Connect the electromagnetic switch and check the
presence of tension between terminals 5 and 23 with
the key switch set to "ON".
5 - 23 = battery tension with panel set to "ON".
Repeat the control with the control unit connected and the engine cold.
5 - 23 = battery tension with panel set to "ON".
If the anomaly persists replace the control unit.
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