Switch engine off.
Locate the grey diagnostic connector in the top left (looking from the front of the car) of the engine bay. Using a paperclip, link the TE1 and E1 sockets (pinouts are listed on the diag connector cap).
Turn the engine "on".. but do not start the car. The engine light in the dash will start flashing, and you read out codes from it.
Do a search in Google for 3S-GE ECU Codes, for a listing. This will tell you if a sensor is faulty or not.