In the Universal Diagnostic Services (UDS) protocol, Service ID 0x85 (or hexadecimal 85) is known as Control DTC Settings. Its primary function is to enable or disable the detection of Diagnostic Trouble Codes (DTCs) within an Electronic Control Unit (ECU) or across the vehicle's network.
Understanding UDS and Service IDs
Universal Diagnostic Services (UDS), defined by the ISO 14229 standard, is a standardized diagnostic communication protocol used in automotive ECUs. It allows external test equipment, like diagnostic scanners, to communicate with and control various functions within a vehicle's electronic systems. Each specific command or function within UDS is assigned a unique identifier, known as a Service ID.
These Service IDs allow diagnostic tools to:
- Read data (e.g., sensor values, trouble codes)
- Write data (e.g., configuration parameters)
- Perform tests (e.g., actuator tests)
- Control specific behaviors of the ECU
The Role of Service ID 0x85: Control DTC Settings
The 0x85 service is crucial during specific diagnostic and development scenarios. When activated, it allows a technician or diagnostic tool to either enable or disable the ECU's ability to detect and store Diagnostic Trouble Codes.
Why Control DTC Settings?
Disabling DTC detection is particularly important when:
- Performing diagnostic work: Certain tests or procedures might intentionally cause an individual device or system to behave anomalously. If DTC detection were active during these procedures, it would generate numerous false fault codes, making actual fault diagnosis more difficult.
- Preventing false positives: For example, when a mechanic needs to temporarily disconnect a sensor, run a component without its usual input, or re-flash an ECU, disabling DTCs prevents the system from logging errors that are part of the testing process rather than genuine malfunctions.
- Software development and testing: During the development phase of new software or hardware, engineers might want to prevent DTCs from being stored while they are deliberately manipulating system behavior.
How It Works
The Control DTC Settings service typically includes sub-functions or parameters that specify what DTCs to control and how. For instance:
Enable DTC Setting: This sub-function instructs the ECU to resume its normal operation of monitoring systems and logging any detected faults. This is crucial after diagnostic work is complete to ensure the vehicle's diagnostic capabilities are fully restored.Disable DTC Setting: This sub-function commands the ECU to temporarily stop monitoring for and storing DTCs. Depending on the implementation, this could apply to all DTCs or specific groups of DTCs.
Practical Implications
Consider a scenario where a technician is performing a thorough check of an engine's fuel injector system. They might need to temporarily disable specific injectors or manipulate fuel pressure. If DTC detection were active, the ECU would immediately log multiple "Injector Circuit Malfunction" or "Fuel Pressure Too Low" codes. By using UDS Service 0x85 to disable DTC settings, the technician can perform their tests without cluttering the ECU's memory with irrelevant fault codes. Once the work is done, they would then use the Enable DTC Setting command to reactivate the diagnostic monitoring.
Key Aspects of Service 0x85
| Service ID (Hex) | Service Name | Purpose |
|---|---|---|
0x85 |
Control DTC Settings | Enable or disable the detection of any or all errors (DTCs). This prevents the storage of diagnostic trouble codes during specific scenarios like vehicle diagnostics, component testing, or software development, avoiding false positives. |
It's vital for anyone performing diagnostics or programming on an ECU to understand the implications of using this service. Mismanagement of DTC settings could potentially lead to:
- Undetected faults: If DTCs remain disabled after diagnostic work, actual faults that occur later might not be reported, compromising vehicle safety or reliability.
- Misinterpretation of vehicle status: If a vehicle is handed back to a customer with DTCs still disabled, a subsequent issue might go unnoticed by the onboard diagnostic system.
Therefore, best practices dictate that DTC settings should always be re-enabled once the specific task requiring their deactivation is complete. For more comprehensive information on UDS and its services, you can refer to resources on automotive diagnostic protocols like those based on ISO 14229.
