You can pull the computer codes from your Toyota Previa right from your home garage, saving yourself time and money. To read the codes you will need to buy an OBD code scanner. These handheld computerised devices are available at most auto parts stores. The scanner reads computer codes which are stored in the vehicle's On-Board Diagnostics (OBD) computer. Each code tells you information about the Toyota to help you troubleshoot the problems.
Find a port on the driver's side of the vehicle near the steering column. This port looks the same as the connective end on the OBD code scanner. Plug the OBD code scanner into the port.
Put the key into the ignition and turn it to the "II" position, taking care not to start the engine. This should power on your scanner. If not, find an "on/off" switch on the scanner and press it on.
Select the "Retrieve Codes" command on the scanner. Wait for the scanner to show you the trouble codes and write them down on a piece of paper. Look up these alphanumeric codes in the scanner's manual.
Unplug the OBD code scanner from the port. Take the Previa to be serviced or repaired based on the trouble codes you retrieved.
Most auto parts stores will loan their customers a scan tool for free to diagnose the trouble codes.