Equus 3160 - ABS/SRS + OBD2 Scan Tool - Manual (Page 17)

Onboard Diagnostics
DIAGNOSTIC TROUBLE CODES (DTCs)
OBD2 15
Diagnostic Trouble
Codes (DTCs) are
codes that identify a
specific problem area.
DIAGNOSTIC TROUBLE CODES (DTCs)
Diagnostic Trouble Codes (DTCs) are
meant to guide you to the proper
service procedure in the vehicle’s
service manual. DO NOT replace parts
based only on DTCs without first
consulting the vehicle’s service manual
for proper testing procedures for that
particular system, circuit or component.
DTCs are alphanumeric codes that are used to identify a
problem that is present in any of the systems that are
monitored by the on-board computer (PCM). Each trouble
code has an assigned message that identifies the circuit,
component or system area where the problem was found.
OBD2 diagnostic trouble codes are made up of five characters:
„ The 1st character is a letter. It identifies the “main system” where
the fault occurred (Body, Chassis, Powertrain, or Network).
„ The 2nd character is a numeric digit. It identifies the “type” of code
(Generic or Manufacturer-Specific).
Generic DTCs are codes that are used by all vehicle
manufacturers. The standards for generic DTCs, as well as
their definitions, are set by the Society of Automotive
Engineers (SAE).
Manufacturer-Specific DTCs are codes that are controlled by
the vehicle manufacturers. The Federal Government does not
require vehicle manufacturers to go beyond the standardized
generic DTCs in order to comply with the new OBD2
emissions standards. However, manufacturers are free to
expand beyond the standardized codes to make their systems
easier to diagnose.
„ The 3rd character is a numeric digit. It identifies the specific
system or sub-system where the problem is located.
„ The 4th and 5th characters are numeric digits. They identify the
section of the system that is malfunctioning.
DTCs and MIL Status
When the vehicle’s on-board computer detects
a failure in an emissions-related component or
system, the computer’s internal diagnostic
program assigns a diagnostic trouble code
(DTC) that points to the system (and subsystem)
where the fault was found. The diagnostic
program saves the code in the computer’s
memory. It records a “Freeze Frame” of
conditions present when the fault was found, and lights the Malfunction
Indicator Lamp (MIL). Some faults require detection for two trips in a row
before the MIL is turned on.