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

8.15.1 Service Description. 
This service keeps other diagnostic services active by resetting the diagnostic 
timer (TesterPresent_Timer) each time a request for this service is received. A portion of the ECU diagnostic 
application executes in the background that modifies and tests the value of the timer (modification based on 
the processing loop time). When the value of the TesterPresent_Timer meets or exceeds the value of the P3
C
application timer, a TesterPresent or P3
C
timeout occurs.
Note:
See the paragraphs within this specification on application timing requirements for more information on 
P3
C
. 
When a P3
C
timeout occurs, the node shall execute the same logic that would be executed if a 
ReturnToNormalMode ($20) service request was received. This includes the transmission of a 
ReturnToNormalMode positive response message. (See note below) Nodes are required to time out no sooner 
than P3
C
and no later than P3
Cmax
. 
Note:
The unsolicited service $20 positive response is only sent if programming mode was not active prior to 
the TesterPresent timeout. See service $A5 for more information on programming mode. 
There shall only be a response to a request for this service if the request is physically addressed. 
Note: 
Receiving a TesterPresent request message shall not put a node into a diagnostic mode (i.e., the 
TesterPresent_Timer_State shall not be set to ACTIVE by this message; see pseudo code). 
8.15.2 Request Message Definition (Table 153). 
Table 153: TesterPresent Request Message 
Data Byte Parameter Name 
Cvt 
Hex Value Mnemonic 
#1 
TesterPresent Request Service Id 
M 
3E 
SIDRQ 
8.15.2.1 Request Message Sub-Function $Level Parameter Definition. 
There are no sub-function 
parameters used by this service.