Equus 3120 - OBD 2&1 Tool Kit - Manual (Page 69)

OBD2 & 1 67
E
GM OBD1 Systems
DTCs - CODE RETRIEVAL PROCEDURE
DIAGNOSTIC TROUBLE CODES (DTC's)
The service codes are also called "fault codes", "Diagnostic Trouble
Codes" (DTC's) or "trouble codes". These codes can be used to iden-
tify systems or components that are malfunctioning. The computer
records codes for two types of problems:
■ "Hard" Diagnostic Trouble Codes
"Hard" DTCs represent problems that are occurring now and cause the
instrument panel Malfunction Indicator Lamp (MIL) or Check Engine
light to illuminate and remain on until the failure is repaired. A DTC is
stored in the vehicle's computer memory for each fault detected. A Tool
or Scanner can be used to retrieve DTCs that are stored in the vehi-
cle's computer memory.
■ Intermittent/History DTCs
Intermittent/History DTCs are stored in the computer's memory for
problems that occur intermittently, or for problems that happened in the
past but are not currently present. Intermittent DTCs may cause the
Malfunction Indicator light to flicker or stay on until the intermittent mal-
function goes away. However, the corresponding fault code will be
stored in memory as a history DTC. If the malfunction that caused the
history DTC to set does not recur within a predetermined length of time
(usually within 40-80 ignition key start cycles), the computer will auto-
matically erase the related fault code from its memory.
CODE RETRIEVAL PROCEDURE
Never replace a part based only on the DTC definition.
Each DTC has a set of testing procedures, instructions and
flow charts that must be followed to confirm the location of
the problem. This information is found in the vehicle's service
manual. Always refer to the vehicle's service manual for
detailed testing instructions.
Check your vehicle thoroughly before performing any
test. See Before You Begin on page 17 for details.
ALWAYS observe safety precautions whenever working on a
vehicle. See Safety Precautions on page 3 for more infor-
mation.
1. Locate the vehicle's Data Link Connector (DLC). See Data Link
Connector (DLC) on page 66 for connector location.
Retrieving and using Diagnostic Trouble Codes (DTCs) for
troubleshooting vehicle operation is only one part
of an overall diagnostic strategy.