How do you reset the OBD system for 2000 Volvo s40?

To reset the OBD system on a 2000 Volvo S40, you can disconnect the vehicle's battery by removing the negative terminal for about 15 minutes. This will clear the diagnostic trouble codes stored in the system. Once you reconnect the battery, the OBD system will be reset. Alternatively, you can use an OBD-II scanner to clear the codes directly if you have access to one.