Here is how the CEL (check engine light) is best described...On a percentage scale from 0 to 100, 0 being total failure and 100 being a perfect sensor, that sensor will probably work and do its job from 10% all the way to 100%...however if that sensor signals the computer that it is operating at 80%, the computer will turn on the CEL letting you know to get it looked at.
Keep in mind this is not the case all the time. Sensors can go from 100% to total failure without warning.
