1. Open the hood of your Peugeot 405.
2. Locate the starter motor. It is usually cylindrical and has a heavy electrical cable connected to it.
3. Follow the heavy electrical cable from the battery to the starter motor. The starter solenoid is usually located at the end of this cable.
4. The starter solenoid is a small cylindrical device with two large electrical terminals and one smaller terminal. It may be covered with a rubber boot or plastic cover.
5. If you are still having trouble finding the starter solenoid, you may need to consult a repair manual or diagram specific to your Peugeot 405 model.