Will a scan tool scan when the check engine light is off?

Question

Will a code reader read any codes if your check engine light is not on?

Answer 1

Yes, many codes will not set the engine light until the control unit verifies there is a problem. Some codes require a two-trip logic to set the engine light or the code, in other words, the problem has to occur twice for the control unit to turn the engine light ON, but the code will be stored in the control unit.