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GM WORLDWIDE ENGINEERING STANDARDS
GMW3110
8.2.2.1 Request Message Sub-Function Parameter $Level (LEV_) Definition. 
The sub-function parameter
is used by the InitiateDiagnosticOperation service to select the specific behavior of the ECU. Explanations and
usage of the possible levels are detailed below. The following sub-parameter values are specified in this
document (Table 49).
Table 49: Definition of Sub-Function Values
Hex Description
Cvt
02
disableAllDTCs
This level shall disable setting of all DTCs.
C
03
enableDTCsDuringDevCntrl
This level shall be used to allow DTC algorithms to continue to execute while the
DeviceControl ($AE) service is active. This request shall have to be made prior to
activating DeviceControl or the request shall be rejected. If this service and level are not
requested before entering DeviceControl, then DTCs shall be inhibited while
DeviceControl is active. (See the $AE service for further details).
Note: 
If another diagnostic service is requested which disables DTCs (after the request is
sent to allow DTCs to run during DeviceControl) then the DTCs shall become inhibited
and remain inhibited until after a TesterPresent timeout occurs or a $20 service is
requested.
C
04
wakeUpLinks
This level shall cause a gateway ECU to initiate the appropriate wake-up sequence on all
GMLAN subnets that it is connected to (provided that a given subnet has a wake-up
mechanism defined).
Note: 
The rules for sending a wake-up as defined in GMW 3104 - GMLAN
Communications Strategy Specification still apply (e.g., the strategy specification restricts
wake-up requests to have a minimum time interval between them. If a diagnostic request
is received to initiate a wake-up and the minimum interval has not expired, then the ECU
shall send the positive response message back to the tester without initiating another
wake-up).
If a GMLAN subnet uses a shared local input as a wake-up wire and the shared local
input has to remain asserted to keep communications active, then the gateway device
shall ensure that the wake-up wire is asserted while the gateways diagnostic VN is active.
Note: 
An example of the shared local input wake-up mechanism described above would
be a gateway that is connected to both the single wire CAN link and a dual wire CAN link.
In this example, the gateway uses a relay to switch power to the other devices on the dual
wire CAN subnet. For normal operations the gateway would receive the High Voltage
wake-up on the single wire CAN bus and then enable the relay to provide power to the
dual wire devices. If the ECU receives a request for this service with the wakeUpLinks
($04) sub-function parameter, then the ECU would ensure that the relay providing power
to the dual wire link ECUs remains enabled as long as the diagnostic VN is active in the
gateway (or longer if the ECU would otherwise keep the relay enabled for normal
functionality after the diagnostic VN is no longer active).
C
1