If the Toyota truck is a 1994 or newer, it might display an onboard diagnostic code of P1565, indicating a problem with the cruise control main switch circuit, or a P1120 signifying an issue with the accelerator pedal positioning sensor. Reading these codes when the check engine light illuminates requires the use of a scan tool or a cable that connects a computer to the data port of the vehicle.
Toyota was one of the first manufacturers to install the OBD2 system on its vehicles, including the 1994 T100 pickup. Manufacturers began installing the system on all vehicles after 1996. OBD2 readings include codes that are generic and apply to all vehicles and manufacturer specific codes, like the two above.
The check engine light, technically known as the malfunction indicator light, illuminates when the onboard diagnostic computer indicates an issue with the engine. Manufacturers divide these issues into several classes, but a type A event is the most serious and triggers the MIL with a single occurrence. The diagnostic computer stores the code, freeze frame data and failure record to help the mechanic diagnose the problem. With a type B event, the computer must sense the event on two successive trips to illuminate the lamp. Type C and D events are less serious and do not always trigger the malfunction indicator light.