Tester Present (0x3E) Service in UDS Protocol: Purpose, Function, and Implementation Explained
Hello, automotive tech enthusiasts! In this blog post, I will introduce you to Tester Present (0x3E) Service in UDS Protocol – one of the key services in the
embsystech.com/uds-protocol/" target="_blank" rel="noreferrer noopener">UDS (Unified Diagnostic Services) protocol: the Tester Present (0x3E) service. This service plays a vital role in maintaining communication between the tester and the ECU during long diagnostic sessions. Without it, the ECU may assume the tester is inactive and terminate the session. In this post, I’ll explain what the Tester Present service is, why it’s important, how it works, and how it’s implemented in real-world diagnostic tools. By the end, you’ll have a solid understanding of this service and how it keeps diagnostic sessions alive. Let’s dive in!
Introduction to Tester Present (0x3E) Service in UDS Protocol
In the world of automotive diagnostics, maintaining an active session between the tester and the Electronic Control Unit (ECU) is crucial. One key service that ensures this connection stays alive is the Tester Present (0x3E) service in the UDS (Unified Diagnostic Services) protocol. This service is used to prevent session timeouts by informing the ECU that the diagnostic tool (tester) is still present and active. Without periodic Tester Present messages, the ECU may automatically terminate the diagnostic session, interrupting ongoing operations. In this article, we’ll explore the purpose of the Tester Present service, how it works, its different sub-functions, and best practices for implementation in real-world applications. Whether you’re a beginner or a professional in automotive diagnostics, understanding this service is essential for smooth and uninterrupted communication with the ECU.
What is Tester Present (0x3E) Service in UDS Protocol?
The Tester Present (0x3E) service is used to notify the ECU that the diagnostic tester is still active during a session. It prevents the ECU from closing the session due to inactivity. This is especially important during long operations like firmware updates, data logging, or coding procedures, where there may be no other diagnostic messages sent for a period of time.
In automotive diagnostics using UDS (Unified Diagnostic Services), it’s essential to maintain an active session between the tester (diagnostic tool) and the ECU (Electronic Control Unit). One key service designed to keep this communication alive is the Tester Present (0x3E) service. Without it, the ECU may assume the tester has gone inactive and terminate the session, interrupting diagnostic operations. This article explores the purpose, working, structure, sub-functions, and implementation of the Tester Present service in UDS.
Purpose of Tester Present Service 0x3E in UDS Protocol
Keeps the diagnostic session alive during long inactivity periods.
Prevents the ECU from timing out and reverting to the default session (e.g., Default Session 0x01).
Ensures that critical diagnostic procedures like reprogramming or coding are not interrupted.
Acts as a “heartbeat” signal from the tester to the ECU.
Avoids the need to re-establish diagnostic sessions repeatedly, saving time and effort.
Helps maintain security access levels during protected operations that require authentication.
Supports seamless execution of automated test sequences or scripts that involve delays between steps.
Prevents session-based configurations (like enabling certain DIDs or routines) from being lost.
Essential for maintaining ECU readiness in long-duration emissions or performance testing scenarios.
Reduces the risk of diagnostic errors caused by unintended session terminations.
Ensures stability and reliability during ECU flashing or bootloader communication.
Facilitates robust communication in multi-ECU systems where parallel diagnostics are performed.
Avoids system reset or fallback behavior that may be triggered by session timeout.
Supports compliance with OEM diagnostic standards and service tool specifications.
Enhances user experience by maintaining continuity in diagnostic applications or tools.
Why do we need Tester Present (0x3E) Service in UDS Protocol?
Below is a detailed explanation of each reason why we need the Tester Present (0x3E) Service in the UDS Protocol:
1. Prevents Session Timeout
When there’s no diagnostic activity for a certain period, the ECU assumes the tester has become inactive and automatically ends the session. This timeout mechanism is built-in to protect the system and conserve ECU resources. The Tester Present service helps avoid this by sending periodic signals to keep the session alive. This is especially critical during operations that involve waiting, such as between diagnostic steps or while the ECU is busy processing.
2. Maintains Access to Extended or Programming Sessions
Certain UDS operations require entering an extended session (0x03) or programming session (0x02). These sessions grant special access to perform sensitive tasks like writing memory, changing parameters, or updating software. If the ECU doesn’t receive Tester Present messages during these sessions, it may revert to the default session, cancelling all elevated permissions. Thus, the 0x3E service ensures continuous access to privileged diagnostic functions.
3. Supports Long-Running Operations
Processes like ECU reprogramming, flashing, or executing long-duration routines can take several seconds or even minutes. During this time, the tester may not send any other UDS requests. Without Tester Present messages, the ECU could terminate the session in the middle of the operation. Sending 0x3E at regular intervals ensures that such operations continue without session timeout interruptions.
4. Ensures Uninterrupted Execution of Routines
Some diagnostic routines, such as actuator testing, calibration, or system resets, require the ECU to stay in a specific session for proper execution. If the ECU times out, these routines can be disrupted or aborted, leading to incomplete testing or inconsistent results. Tester Present ensures the ECU doesn’t exit the session unexpectedly, allowing routines to complete as intended.
5. Improves Communication Reliability in Automated Testing
In automated test environments, scripts are often used to run a series of diagnostics with predefined delays or waiting periods. During such idle periods, the absence of Tester Present messages could result in the ECU terminating the session. Integrating the 0x3E service into test scripts helps maintain stability and reliability of the entire automated diagnostic workflow.
6. Avoids Unnecessary Reinitialization of Procedures
Once a session is closed due to timeout, the tester must re-establish the session, re-authenticate security access, and resend previous configurations. This process wastes time and can disrupt critical workflows. By sending periodic Tester Present messages, the diagnostic tool avoids the overhead of starting from scratch, improving overall efficiency during development or testing.
7. Helps Maintain Security Access During Sensitive Operations
Many UDS operations are protected by a security access mechanism, which grants temporary permissions to perform critical tasks like memory writes or configuration changes. Once security access is granted, it is tied to the current session. If the session times out due to inactivity, the security level is lost and must be reacquired. Sending Tester Present messages during sensitive operations ensures the session and its associated security access remains valid, preventing unnecessary delays and security re-negotiation.
Request Message Sub-Function Parameter of (0x3E) SID
Bits 6 – 0
Description
0x00
zeroSubFunction This parameter value indicates that no sub-function values other than the suppressPosRspMsgIndicationBit are supported by this service.
0x01 – 0x7F
ISOSAEReserved This range of values is reserved by this document.
Syntax of 0x3E SID Request Message Frame Format
Data byte
Parameter Name
Byte Value
#1
Tester Present Request SID
0x3E
#2
sub-function = [ zero Sub Function ]
0x00 / 0x80
Syntax of 0x3E SID Positive Response Message Frame Format
Data byte
Parameter Name
Byte Value
#1
Tester Present +Ve Response SID
0x7E
#2
sub-function = [ zero Sub Function ]
0x00
Response Message Data-Parameter
zeroSubFunction
This parameter echoes bits 6 to 0 of the sub-function parameter from the request message.
Syntax of 0x3E SID Negative Response Message Frame Format
Data byte
Parameter Name
Byte Value
#1
Tester Present –Ve Response SID
0x7F
#2
Requested SID = [ byte#1 ]
0x3E
#3
Negative Response Code [ byte#1 ]
NRC
Supported Sub-Function ID of (0x3E) SID
The Tester Present request can have an optional sub-function parameter. This parameter distinguishes between two types of Tester Present messages:
Bits
Description
0x00
Without a sub-function (default value), indicating that the ECU should remain in its current state and continue with the active diagnostic session. No sub-function value beside SuppressPosResMsgIndication (SPRMI) not supported
0x80
0x80: With the sub-function, indicating that the tester is present but does not require any response from the ECU. SuppressPosResMsgIndication (SPRMI) support (7th bit has been set – (SPRMIB))
Supported Negative Response Codes (NRCs) of (0x3E) SID
The following negative response codes shall be implemented for this service. The circumstances under which each response code would occur are documented in Table below. The listed negative responses shall be used if the error scenario applies to the server.
NRC
Description
0x12
sub-function Not Supported This NRC shall be sent if the sub-function parameter is not supported.
0x13
Incorrect Message Length Or Invalid Format This NRC shall be sent if the length of the message is wrong.
Example of Tester Present (0x3E) Service in UDS Protocol
Here are the Examples of Tester Present (0x3E) Service in UDS Protocol:
Example #1 – TesterPresent (suppressPosRspMsgIndicationBit = FALSE)
Example of Request Message:
Message direction
Client → Server
Message Type
Request
Data Byte
Description (all values are in hexadecimal)
Byte Value
#1
TesterPresent Request SID
0x3E
#2
zeroSubFunction, suppressPosRspMsgIndicationBit = FALSE Data
// Below example shows how to use the UDS commands over the CAN data field as per the DoCAN
// Example for Positive Response for SID-0x3E
Request: Client --> Server
02 3E 00
Positive Response: Server --> Client
02 7E 00
// Example for Negative Response for SID-0x3E
Request: Client --> Server
03 3E 00
Negative Response: Server --> Client
03 7F 3E 13
CAN Data Format for UDS (DoCAN Protocol)
UDS over CAN follows a specific format where the first byte indicates the length of the data, followed by the SID and sub-functions.
NRC 0x13 indicates that the format of the request is incorrect, such as an invalid length or an unsupported sub-function.
Advantages of Tester Present (0x3E) Service in UDS Protocol
Here are the Advantages of Tester Present (0x3E) Service in UDS Protocol:
Prevents Diagnostic Session Timeout: The ECU has a timer that resets the session after a period of inactivity. The Tester Present service prevents this by acting as a keep-alive signal. It ensures that the session remains active, even during idle periods in the communication. This is crucial during lengthy operations like calibration or memory reading. Without this service, important tasks may get interrupted due to session timeout.
Enables Continuous Access to Special Sessions: Special sessions like programming or extended mode offer advanced access levels. These sessions are time-sensitive and reset if no messages are received. Tester Present messages help maintain these sessions without requiring re-authentication. This enables smooth execution of tasks that demand elevated privileges. It saves time and effort during complex diagnostic procedures.
Supports Long-Running Flashing or Coding Operations: Flashing or coding the ECU can take several minutes where no diagnostic requests are sent. During this time, the session could expire if not maintained. Tester Present serves as a periodic confirmation of activity, keeping the ECU from timing out. It ensures the flashing process completes without disruption or data corruption. This makes reprogramming safer and more reliable.
Ensures Smooth Execution of Diagnostic Routines: Certain diagnostic routines like system tests or calibrations require a stable session to run properly. If the ECU times out mid-process, it can halt the operation or provide incorrect results. Sending Tester Present messages keeps the session alive during these routines. This guarantees uninterrupted diagnostics and ensures accurate testing outcomes. It’s a critical component of routine-based services.
Minimizes Risk of Data Corruption or Loss: Operations like memory writes or configuration changes must not be interrupted. If the session ends in the middle, data can become corrupted or incomplete. Tester Present ensures the session stays active until all changes are completed. This reduces the risk of ECU malfunctions due to partial data. It’s a safety measure during critical data-handling tasks.
Reduces the Need for Session Reinitialization: If the session times out, the tester must reinitiate the session, re-unlock security, and resend configuration commands. This adds unnecessary delay and complexity. Using Tester Present maintains the session, eliminating the need for repetition. It makes the diagnostic process smoother and more efficient. This is especially useful during batch or scripted operations.
Improves Stability in Automated Test Environments: Automated testing systems often involve delays between requests. These delays can trigger a session timeout if not managed. Including Tester Present in test scripts keeps the ECU session alive throughout the test. It ensures reliable and uninterrupted test execution. This improves the accuracy and success rate of automated diagnostics.
Maintains Security Access During Protected Tasks: Once security access is granted, it remains valid only for the current session. If the session times out, the tester must perform security unlocking again. Tester Present helps in maintaining both the session and its security level. This allows seamless execution of secure tasks like memory access or variant coding. It saves time and avoids redundant security operations.
Enhances Compatibility with OEM Standards: OEMs require adherence to ISO 14229 and internal specifications. Using the Tester Present service is often part of these standards. Including it ensures the diagnostic tool behaves as expected across different ECUs. This improves tool compatibility and acceptance during validation. It also supports easier compliance and certification processes.
Boosts Diagnostic Tool Performance and Reliability: When the diagnostic session is stable, tool performance naturally improves. Tester Present avoids session drops, disconnections, and retries. This leads to faster, smoother, and more predictable tool behavior. It enhances the confidence of technicians using the tool. Overall, it makes diagnostics more professional and dependable.
Disadvantages of Tester Present (0x3E) Service in UDS Protocol
Here are the Disadvantages of Tester Present (0x3E) Service in UDS Protocol:
Increases Bus Load During Idle Periods: Sending Tester Present messages repeatedly, even when no actual diagnostic data is exchanged, adds unnecessary traffic to the communication bus. In systems with multiple ECUs or heavy communication, this can slightly increase latency or affect time-sensitive messages, especially on a CAN bus with limited bandwidth.
Can Mask Real Communication Issues: Since Tester Present messages are simple and may continue to succeed even if other diagnostic requests are failing, they can hide underlying communication problems. This makes troubleshooting harder, as it might appear that the session is stable when in reality, other important messages are being lost or corrupted.
May Interfere with Normal ECU Operations: Some ECUs may reduce internal activity or enter low-power modes during inactivity. Sending frequent Tester Present messages can prevent these power-saving modes from triggering, affecting overall system efficiency, especially in vehicles where power management is critical.
Increases Development and Testing Complexity: Implementing the logic to manage Tester Present timing correctly in tools or scripts adds complexity. Developers need to ensure these messages are sent periodically without fail. Missing even one can cause the session to time out, leading to test failure or flashing errors during development.
Risk of Overuse in Poorly Designed Tools: Diagnostic tools that send Tester Present too frequently or without proper logic may flood the network or cause confusion in ECUs. Overuse can also make ECUs behave abnormally, especially if the tool sends these messages without a valid diagnostic context, leading to performance degradation.
May Lead to ECU Malfunctions if Misused: Not all ECUs interpret the Tester Present message the same way. Improper use or incorrect message format can lead to unexpected ECU behavior or session resets. This could halt diagnostics or force the ECU back into default mode, requiring a restart of the process.
Adds to Power Consumption: Continuously sending and processing Tester Present messages prevents ECUs from going into sleep mode. In battery-powered environments, such as electric vehicles or during off-board diagnostics, this can lead to faster battery drain and increased energy usage.
Not Useful Outside Active Diagnostics: The Tester Present message is only meaningful when a diagnostic session is active. If used outside of this context, it serves no real purpose and only adds overhead. In automated systems or during background operations, unnecessary use can clutter communication logs and affect tool performance.
Potential for Misconfiguration: The UDS protocol specifies timing and format for Tester Present messages, but different OEMs may have unique requirements. Misconfiguring the timing or sub-function byte can lead to the ECU ignoring the message or timing out, which disrupts critical diagnostic or flashing processes.
Can Trigger Security Measures If Misused: Some ECUs interpret excessive or incorrectly timed Tester Present messages as a possible attack or misuse. This can trigger security responses like session lockout or forced reboot. Such false positives can delay diagnostics and require manual intervention to reset the system.
Future Development and Enhancement of Tester Present (0x3E) Service in UDS Protocol
These are the Future Development and Enhancement of Tester Present (0x3E) Service in UDS Protocol:
Adaptive Timing Mechanism: Future versions may implement adaptive intervals for sending Tester Present messages based on session activity. Instead of fixed intervals, the timing could be dynamically adjusted to reduce bus load while still maintaining session stability, improving both performance and efficiency.
Smart Session Monitoring: Advanced diagnostic tools may include intelligent monitoring to determine whether a Tester Present message is truly required. This reduces unnecessary traffic and helps avoid redundant communication, especially in idle diagnostic states or automated testing environments.
Secure Tester Present Handling: As vehicle cybersecurity becomes more critical, future ECUs may require secure authentication before accepting Tester Present messages. This enhancement will prevent unauthorized tools from maintaining sessions and protect sensitive operations from malicious interference.
Integration with Over-the-Air (OTA) Updates: In the era of remote diagnostics and firmware updates, Tester Present could be integrated with OTA systems to ensure that diagnostic sessions remain active during long-duration wireless updates, enhancing the reliability of remote ECU reprogramming.
Enhanced Logging and Traceability: Future diagnostic standards may require that every Tester Present message be logged with timestamps and context. This would improve traceability during debugging and compliance audits, helping developers and OEMs ensure proper session management.
Customizable Behavior per OEM Requirements: Upcoming UDS extensions may allow customization of Tester Present behavior per OEM. This includes defining unique timing, allowed sub-functions, or special response handling, improving flexibility and compatibility across different automotive platforms.
Context-Aware Messaging: Instead of sending generic Tester Present messages, future tools could embed contextual information (e.g., tool status or operation stage). This would help ECUs better understand the reason for the message and optimize internal behavior accordingly.
Reduced Power Consumption Strategies: To address energy concerns, newer implementations might allow low-power Tester Present modes. These would use minimal signals to maintain session activity while allowing the ECU to remain in semi-sleep states, particularly useful for electric or hybrid vehicles.
Standardization Across OEMs and ECUs: Currently, Tester Present handling can vary between manufacturers. Future developments may push for tighter standardization under ISO 14229 or extended protocols, simplifying tool design and ensuring consistent behavior across ECUs.
AI-Powered Diagnostic Management: AI-integrated diagnostic platforms might learn optimal timing and usage of Tester Present messages based on real-world ECU response patterns. This would allow predictive session maintenance, reducing communication overhead and improving reliability in complex systems.
