General Specification Electrical Function gmw3110 General Motors Local Area Network Enhanced Diagnostic Test Mode Specification

W
2
=
 
A node shall not allow the value of the MEC to change once it becomes $00 unless SecurityAccess 
($27) is successfully initiated (Security_Access_Unlocked is set to TRUE). Whether or not an 
ECU is allowed to write to this DID after it becomes $00 shall be agreed upon by the DRE, GM 
service, and GM manufacturing representatives. Agreement to support modifying this DID after it 
becomes $00 must be documented in the CTS/SSTS or other ECU specific diagnostic document 
referenced by the CTS or SSTS
.
The Security_Access_Unlocked flag is not set to FALSE when 
the MEC value is written to $00. The Security Status changes upon evaluation of the MEC and 
Vulnerability Flag. The Security_Access_Unlocked flag is evaluated upon receipt of a 
SecurityAccess ($27) request. The Security_Access_Unlocked flag is set to FALSE upon exiting 
the SecurityAccess mode via a ReturnToNormalMode ($20) request or a P3
C
timeout. 
Position 
1 
Description 
MEC 
127 
Hex value 
7F 
Table C12: Definition of ECU Configuration or Customization Data dataIdentifier 
DID Description 
R/W 
Data Type 
Len 
$A1 
thru 
ECUConfigurationOrCustomizationData (ECUCOCGD) 
R/W
3
User 
optional 
≥ 1 
$A8 DIDs ranging from $A1 thru $A8 contain data specific to the configuration of an ECU (e.g., option 
content data) or operating parameters (e.g., Engine Oil Life Remaining, Long Term Fuel Trim values, 
Transmission Adaptive Learn values, etc.) which would need to be read from an original ECU and 
transferred to a replacement ECU. It is the responsibility of system supplier(s) and vehicle manufacturer 
to define the data that can be read and written via these DIDs. The main usage of these DIDs is to 
provide a common set of identifiers to be used to retrieve configuration data from an ECU 
DID numbers in this range shall be assigned to an ECU in ascending order. This allows a tool to retrieve 
all ECU configuration data by first requesting DID $A1, and continuing until either DID $A8 is retrieved, 
or a negative response is sent to the tool with response code $31 (requestOutOfRange).
The overall size of the ECU Configuration Data (maximum of 8 DIDs) for a single ECU shall not exceed 
512 Bytes in total.