Diagnostic Trouble Code Type Definition

Government regulations based on SAE standards, have mandated the automotive industry to adopt a common numbering system for emission diagnostic trouble codes (DTCs). The numbering system requires one alpha followed by four numeric numbers to make up the code (Pxxxx). The "P" indicates a powertrain diagnostic trouble code. This means that a DTC number for a throttle position sensor will be the same for Saturn, as for any other manufacturer. (P0 before the code means industry standard and P1 before the code means manufacturer specific.)
Along with the change in the numbering system is the ability to know if a diagnostic test has been performed on each system. This means that it can be determined if a test has been run on a subsystem and whether the result was passed, failed, or pass & fail.
Since the MIL (SERVICE ENGINE SOON) telltale is restricted to being illuminated only for emission-related faults, there have been four code types assigned. These types will be referenced to as type A, B, C and D. Type A DTCs are emission-related faults that will turn On the MIL at the first occurrence of a fail condition.
Type B DTCs are emission-related faults that turn On the MIL if a fault occurs in two consecutive ignition cycles.
Type C DTCs are non-emission faults that may cause the SERVICE telltale to be illuminated. If a test passes after a fault is detected the SERVICE telltale will turn Off after about 3 seconds, but the fault remains stored in history and a failure record is stored.
Type D DTCs are used to detect non-emission faults, and aid in diagnosis, but do not turn On any telltales.