Reading Self-Test Service Codes

PURPOSE
The Electronic Engine Control (EEC) system communicates service through the self test codes. These codes are three digit numbers representing the results of the self test.

Key On Engine OFF (KOEO) Diagnostic Trouble Code Format:

Key On Engine Running (KOER) Diagnostic Trouble Code Format:

OPERATION
The Diagnostic Trouble Codes (DTC) are transmitted on the Self Test Output (STO) wire located on the vehicle self test connector. The codes are in the form of timed pulses, and are read by the technician using a scan tool, Analog Volt/Ohm Meter (VOM) or by monitoring the CHECK ENGINE light.

TYPES OF DIAGNOSTIC TROUBLE CODES

Fast Codes
Fast codes are given prior to regular DTCs. These codes contain the identical information as the regular DTCs but are transmitted at 100 times the normal rate. These codes are used at the factory and by scan tools. If a VOM is used to retrieve codes the fast codes should be ignored. Some VOMs in service may detect these fast codes as a short burst (meter deflection).

Hard Codes
The hard (on-demand) codes are displayed in both the Key On Engine Off (KOEO) and Key On Engine Running (KOER) self tests. If a hard code is displayed it means that there is a fault present in the system at the time the test was run. In the KOEO self test, the hard codes are displayed first. The hard codes are the codes up to, but not including the separator code (10). In the KOER self test the hard codes are the last codes displayed and will follow the "goose" code.

Dynamic
The dynamic response (goose) code is displayed during the KOER test only. It is displayed as single pulse (10 on a scanner) and is displayed after the engine ID code. When the "goose" code is displayed, the operator should briefly go to wide open throttle. This input is used to test the operation of the Throttle Position sensor, Mass Air Flow sensor, Manifold Absolute Pressure sensor, and Knock Sensor.

Separator
A single 1/2 second separator pulse is issued 6-9 seconds after the last functional test code (KOEO only). Then 6-9 seconds after the single 1/2 second separator pulse, the continuous codes will be issued.

Continuous
The continuous codes are issued as a result of information stored (memory codes) during continuous self-test, while the vehicle was in normal operation. These codes must be retrieved within 80 (40 on some models) warm-up cycles. On the 81st (41st on some models) warm-up cycle these codes will be erased. These codes can be manually erased during the KOEO self test. These codes are displayed only during the KOEO testing after the separator code.

Engine Identification Codes
Engine identification codes are issued at the beginning of the Key On Engine Running test and are one-digit numbers represented by the number of pulses sent out. The engine ID code is equal to 1/2 the number of engine cylinders (i.e. 2 pulses = 4 cylinders). These codes are used to verify the proper Powertrain Control Module (PCM) is installed and that the self-test has been entered.

NOTE: All trouble codes should be diagnosed in the order they are received.