The Engine Control Module (ECM) is capable of monitoring both input and output signals. When the ECM detects that an irregularity, it sets a Diagnostic Trouble Code (DTC) and outputs the signal to the self-diagnosis output terminal. There are 29, two digit codes including the normal state. DTC's can be read out by using the Maintenance Indicator Lamp (MIL) or a generic scan tool. DTC's stay in the ECM as long as battery power is maintained.