• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 5 – 7
GM Historic (OBD I) Diagnostics
5
Read Codes
✓The Read Codes function is used to retrieve all stored DTCs from
the control module(s). This can be performed with KOEO or
KOER. Two types of codes were used by GM in the Historic
System: Current codes and History codes. Prior to 1986, all codes
were referred to as Current Codes. From 1986 through 1995, both
codes were used because the PCM was able to differentiate
between them.
✓The Scan Tool automatically displays the type of codes that pertain
to the vehicle under test. The DTC types are defined as follows:
❒History Codes — intermittent codes placed in the vehicle’s
memory when the trouble originally occurred, and will remain
there even if the trouble has been corrected. If no trouble after 50
engine warm-up cycles, the DTC erases.
❒Current Codes — codes transmitted through the PCMs data
stream when a trouble condition is active and cannot be erased.
The problem must be repaired to remove the DTC.
✓On some vehicles, all codes are Current Codes, because the PCM
cannot distinguish Current Codes from History Codes. The only
way to determine this is to erase the code and then drive the
vehicle to see if the code returns.
✓Perform the following:
1) Set parking brake.
2) KOEO or KOER.
3) Put transmission in park (P) or neutral (N).
From the Diagnostic Codes Menu:
1.Select Read Codes.
•Use
UP
or
DOWN
arrow
key until Read Codes is
highlighted.
•Press ENTER.
✓The Scan Tool retrieves the DTCs.
Read Codes
Erase Codes
Review Data
Print Data
Code Lookup
Tool Setup
Diagnostic Codes Menu
Read Codes