1. If the starter does not work, check the condition of the battery by turning on the headlights. If they shine brightly for a few seconds, and then their light gradually fades, then the battery is low.
2. If the headlights remain bright and the battery is clearly in good condition, check the battery wires are secure (and especially the ground wire from the terminal to the car body). If, when trying to start the engine, the positive terminal of the battery gets very hot, then it is on it that the wire has a bad contact. Disconnect the wire, clean the contacts and reconnect the wire. Check that the solenoid and starter wires are secure and visually inspect the wires for damage.
3. If the starter does not start after this, listen for a loud click when the ignition key is turned to the start position. If there is no click, then either the solenoid is faulty or it is not receiving power. Disconnect the power cable (N 46) from the solenoid and connect a voltmeter or 12 volt lamp between the end of the wire and "earth". Turn the key back to the start position and check if current is flowing through the wire. If so, then the solenoid is faulty, if not, you need to find the gap.
4. If the battery is in good condition, the wiring has no breaks, the solenoid works, and at the same time the starter refuses to work, the latter must be removed from the machine for inspection.