Types of Diagnostic Trouble Codes (DTCs)

The Diagnostic Executive classifies Diagnostic Trouble Codes (DTCs) into certain categories. Each type has different requirements to set the code, and the Diagnostic Executive will only illuminate the Malfunction Indicator Lamp (MIL) for emissions- related DTCs. DTCs fall into four categories: A, B, C, and D; only types A & B are emission-related The following descriptions define these categories:

TYPE A - Will store the DTC and turn on the MIL ("Check Engine" lamp) on the first trip in which an emission-related diagnostic test has run and reported a "test failed" to the Diagnostic Executive.

TYPE B - Will store the DTC and turn on the MIL on the second consecutive trip in which an emission- related diagnostic test has run and reported a "test failed" to the Diagnostic Executive. One failed test will arm a type B DTC. After one fail, the type B DTC is "armed," or prepared to store a history code and turn on the MIL if a second failure occurs. One passed test will disarm a type B DTC. Some special conditions apply to misfire and fuel trim DTCs. For a type B DTC to store and turn on the MIL, two ignition cycles are required.

TYPE C - Will store the DTC and turn on a "SERVICE" lamp ("Check Trans" lamp) on the first trip that a non-emission-related diagnostic test has run and reported a "test failed" to the Diagnostic Executive. This type of DTC will be used in future applications.

TYPE D - Will store a DTC but will not turn on the MIL on the first trip that a non-emission-related diagnostic test has run and reported a "test failed" to the Diagnostic Executive. These codes can be very helpful for vehicle service when the driver may comment about a condition, but the MIL did not turn on.