[jdsully]

The check engine light is only for emissions related failures which is why in most cases the car is still driveable. In serious cases like this most cars will flash the CEL but AFAIK that is not legally required. The whole point of the CEL is that when the emission system is compromised its not immediately obvious, in fact the car may even perform better depending on the fault.

[bsagdiyev]

So just a nitpick, but the CEL is not exclusively for emissions related codes. There are a wide range of things that can fail that do not affect the emissions output of the car, but will still trigger a code and light. All of them can fail you in an emissions test, but not all of them are emissions related.

Edit: All of them will fail you -> All of them can fail you. Not all codes will trigger an immediate fail in CA smog testing anyways, depending on if they are manufacturer specific, or what they effect. I passed my most recent smog with a few codes.