1997 Toyota Avalon Engine check light

When the Check Engine light turns on it means the Engine Computer detected a problem, set a diagnostic fault code, and turned the light on to tell you. There's well over a thousand potential fault codes. Those that could result in an adverse affect on emissions are required to turn the light on. Many other codes do not turn it on. Guessing is useless. Many auto parts stores will read the codes for you for free. Those will indicate the circuit or system that needs further diagnosis.