Diagnostic Trouble Codes (DTC)

OBD II Diagnostic Trouble Codes (DTC)

The Society of Automotive Engineers (SAE) established the Diagnostic Trouble Codes used for OBD II systems (SAE J2012). The DTC's are designed to be identified by their alpha/numeric structure. The SAE has designated the emission related DTC's to start with the letter "P" for Powertrain related systems, hence their nickname "P-code".

For example:







- DTC's are stored whenever the Check Engine Light (MIL) is illuminated.
- A requirement of CARB/EPA is providing universal diagnostic access to DTC's via a standardized Diagnostic Link Connector (DLC) using a standardized tester (scan tool).
- DTC's only provide one set of environmental operating conditions when a fault is stored. This single "Freeze Frame" or snapshot refers to a block of the vehicles environmental conditions for a specific time when the fault first occurred. The information which is stored is defined by SAE and is limited in scope. This information may not even be specific to the type of fault.

DTC Storage:

Bosch Systems Aftermarket Scan Tool
initial fault SAE defined freeze frame conditions
2nd occurrence n/a
3rd occurrence n/a
last occurrence n/a
Siemens Systems Aftermarket Scan Tool
initial fault SAE defined freeze frame conditions

The table represents the stored information that would be available via an aftermarket scan tool if the same fault occurred 5 times.

Scan Tool Connection:

Starting with the 1995 750iL, and soon after on all 1996 model year BMW vehicles, a separate OBD II Diagnostic Link Connector (DLC) was added.

The DLC provides access for an aftermarket scan tool to all emission related control systems (DME, AGS/EGS and EML). This diagnostic communication link uses the existing TXD II circuit in the vehicle through a separate circuit on the DLC when the 20 pin cap is installed.

Scan Tool Display

Example: A fault was induced into a 1998 750iL by removing the wire connector from Air Mass Meter. Using an aftermarket scan tool the following information can be displayed:

SCREEN ONE
DIAG. TROUBLE CODES
ECU: $12 (Engine)
Number of DTCs: 1
*P0100 Manufacturer controlled fuel and air metering
ENTER = FREEZE FRAME

SCREEN TWO
DTC P0100
ENGINE SPD 905 RPM
ECT 160 F
VEHICLE SPD 0 MPH
ENGINE LOAD 3.9%
FUEL STAT 1 OL
FUEL STAT 2 OL
ST FT 1 0.0%
LT FT 1 1.6%
ST FT 2 0.0%
LT FT 2 3.1%