Whether drivability issues, a check engine light. or any other warning light, we are fully capable of diagnosing today's American and Asian vehicles.
Today's cars are amazing works of manufacturing. When a MIL [malfunction indicator lamp] is activated, your car is speaking to you in the only language it knows. Something's wrong. It may still operate in seemingly normal fashion, but trust me, there is an issue. Continuing to operate the vehicle WILL cause damage to components that
may not be damaged at the moment. Example: an 02 sensor can malfunction, causing the check engine light to activate. The 02 sensor communicates with the PCM [performance control module] or ECM [electronic control module], measuring the condition of the exhaust and prompting the computer to increase or decrease the amount of fuel to the air/fuel ratio. If the 02 sensor is completely dead, the computer will default to a rich/start up mixture. Your vehicle will seem to run fine, but the fuel economy will suffer, and the rich fuel mixture will cause premature failure of the catalytic converter. Those units can easily exceed $1000, and could have been prevented with a fix of less than $100.
We wouldn't ignore a child telling us they were not feeling well, and when a MIL activates, the car is telling us the same thing. I don't feel well.
A diagnosis costs $75. Unfortunately, our OBD II reader, [pretty expensive, it's what is used to talk with the car] will not tell us exactly what's wrong. It will give us information that must be deciphered to know what part of the system is not well. Then a series of test for the most likely culprit is performed. Sometimes it's easy, sometimes not. A good technician will systematically test each part of a system until he finds the defective part, then inform the service writer, who then determines the cost of parts and availability, adds the labor time, and then offers the estimate to you. Under the best of circumstances, it will take an hour or so, as well as 3 people, for the service writer to inform you of the issue and cost. Just as you don't get a doctor's advice for free, we can't perform all of this work for free.