There are 2 basic types of DTCs (codes). "Current" and "History". Current codes are exactly what you would think, they are occuring now during the "current" ignition cycle. "History" codes are ones that happened in a past ignition cycle and are NOT currently occuring. All CAN be cleared by use of the OBD system and/or removing power from the module, but to be honest, history codes clear themselves after several ignition cycles without reoccurance ( I thing 15 cycles but don't quote me on it).