Unfortunately, that’s all it tells the driver. In some cases you can also feel a performance problem like rough running, or idling, or a lack of power, but many times you don’t notice anything – just that annoying yellow or red light on the dash. So you take it to a shop to get it fixed.
It’s a very large misconception that a “computer diagnostic” can provide a complete explanation for a problem related to a check engine light. In fact, all the check engine light indicates that the computer has set one or more codes related to sensor data that is outside a preset group of specifications. Depending on the type of vehicle, you could have well over 400 different codes that will cause this light to come on.
Because of that, it takes proper training and a specialized set of computer interfaces and software to not only read the codes, but see exactly what the computer is seeing from the sensors. Sometimes, an issue requires looking at short term and long term fuel trim data – other times, simple voltage readings from certain sensors are required. Other times the sensors will only work when the engine is performing a certain operation – for example, the trailing (sometimes referred to as the “Bank 2”) oxygen sensor only gives proper readings for diagnostic purposes when the engine is at cruise (typically 2,500 to 3,500 RPM’s).
As vehicles become even more and more sophisticated (and to be sure, they will), problems will also require more and more sophisticated diagnostic processes. In a very real sense, it’s not unlike the medical profession. Until your vehicle learns to “feel”, a trained automotive specialist and the latest technology will be instrumental to nurse it back to health.