The check engine light is controlled by a diagnostic computer that monitors sensors located throughout a vehicle's mechanical systems. In case of an abnormal reading, this warning light alerts you that the vehicle should be serviced.
On-board diagnostic computers began appearing in vehicles in the late 1980s. Starting in 1996, an OBD-II (on-board diagnostic, 2nd generation) system has been required on all vehicles sold in the United States. All OBD-II-compliant vehicles share a basic set of codes, which must be read by a scan tool. Many vehicles also have manufacturer-specific codes as well.
OBD-I (first-generation on-board-diagnostic) codes were not standardized from maker to maker, though a few post-1994 vehicles are equipped with an OBD-II-compliant diagnostic computer. To retrieve an OBD-I code, a mechanic must jump two terminals in the check engine connector and observe the check engine light to decode a pattern of blinks.
All post-1995 vehicles, including Toyotas, share a common set of diagnostic codes. These are five-character codes, one letter followed by four numbers in which the first number is a 0 (zero), such as P0171. Toyota also has a subset of manufacturer-specific codes, in which the first numeric character is a 1 (one), such as P1406.
