The check engine light comes on when the on-board computer detects a fault in engine driveability or performance, transmission control system or transmission, or the emissions system, and a fault code is set and stored in the computer. Faults that can set the check engine light can range from a vaccuum leak, engine mis-fire, low engine compression, faulty sensor such as O2 sensor or other engine sensor, hydraulic pressure loss in transmission, transmission shift solenoid, open/short in wiring harness, emission evaporative system leak, or one of over a hundred different reasons.
The only way to know what set a fault code and turned the check engine light on is to have a diagnostic scanner hooked up to the on-board computer and read it for stored diagnostic fault codes. Many auto parts stores will do this service for free- check with you local parts stores. If none in your area do, then you will have to take it to a shop and pay to have the computer read for stored fault codes.
Many of the other web site entries on this topic refer to Toyota's poor record of these sensor lights actually providing any useful information to the owner. I appreciate your tip on checking with the local parts store for obtaining a free diagnositic check. It would seem Toyota has built a problem into is warning light systems just to increase useage of repair service centers.
If I can not get the free diagnostic check from a parts store, I don't like the idea of going to a dealership and paying $100 + just to tell me that the warning lights were false reads or, worse yet, the dealership fixes something that really doesn't need to be fixed or doesn't actually take care of the warning light problem. As you can tell by this response, I am very sceptical of auto repair services.
What do you suggest I do to determine that the auto repair service is doing what actually needs to be done to fix the problem?