# Unified Diagnostic Services

Unified Diagnostic Services (UDS) is a diagnostic communication protocol used in electronic control units (ECUs) within automotive electronics, which is specified in the ISO 14229-1. It is derived from ISO 14230-3 (KWP2000) and the now obsolete ISO 15765-3 (Diagnostic Communication over Controller Area Network (DoCAN)). 'Unified' in this context means that it is an international and not a company-specific standard. By now this communication protocol is used in all new ECUs made by Tier 1 suppliers of original equipment manufacturer (OEM), and is incorporated into other standards, such as AUTOSAR. The ECUs in modern vehicles control nearly all functions, including electronic fuel injection (EFI), engine control, the transmission, anti-lock braking system, door locks, braking, window operation, and more.
Diagnostic tools are able to contact all ECUs installed in a vehicle which has UDS services enabled. In contrast to the CAN bus protocol, which only uses the first and second layers of the OSI model, UDS utilizes the fifth and seventh layers of the OSI model. The Service ID (SID) and the parameters associated with the services are contained in the payload of a message frame.
Modern vehicles have a diagnostic interface for off-board diagnostics, which makes it possible to connect a computer (client) or diagnostics tool, which is referred to as tester, to the communication system of the vehicle. Thus, UDS requests can be sent to the controllers which must provide a response (this may be positive or negative). This makes it possible to interrogate the fault memory of the individual control units, to update them with new firmware, have low-level interaction with their hardware (e.g. to turn a specific output on or off), or to make use of special functions (referred to as routines) to attempt to understand the environment and operating conditions of an ECU to be able to diagnose faulty or otherwise undesirable behavior.
UDS uses the ISO-TP transport layer (ISO 15765-2). The United States standard OBD-II also uses ISO-TP. Since OBD-II uses service numbers 0x01-0x0A, UDS uses service numbers starting with 0x10, in order to avoid overlap.

## Services

SID (Service Identifier)
| Function group | Request SID | Response SID | Service | Description |
| Diagnostic and Communications Management | 0x10 | 0x50 | Diagnostic Session Control | UDS offers access levels called "sessions". Different sessions usually offer different levels of access to services and/or sub-functions. During normal use (on start), the session should be 0x01 Default Session. This "Diagnostic Session Control" service allows the user to switch between available sessions specific to the ECU. Some sessions may not have been implemented. Others may have access control measures, requiring authorization through services like 0x27 Security Access or 0x29 Authentication. Additionally, sessions may require certain operating conditions to be met, such as manufacturer specific "pre-production" or "boot modes", which are separate from UDS. The following sessions are standard, but vehicle manufacturer and supplier-specific sessions are often implemented in conjunction: 0x01 Default Session which typically has the lowest level of access to services and/or sub-functions. This will usually allow 0x14 Clear Diagnostic Information, certain 0x22 Read Data By Identifier "Data Identifiers", and more. 0x02 Programming Session which gives access to "Upload / Download" and "Data Transmission" services, and sometimes advanced access to other services and/or sub-functions like . 0x03 Extended Diagnostic Session which typically offers further "Diagnostic and Communications Management", "Data Transmission", "Input / Output Control", and "Remote Activation of Routine" services. 0x04 Safety System Diagnostic Session offers largely the same as 0x03 Extended Diagnostic Session, but does not always offer sensitive or unrelated services and/or sub-functions. It may offer further routines and data transmission sub-functions. |
| Diagnostic and Communications Management | 0x11 | 0x51 | ECU Reset | The service "ECU reset" is used to restart the control unit (ECU). Depending on the control unit hardware and implementation, different forms of reset can be used: 0x11 01 Hard Reset simulates a shutdown of the power supply. 0x11 02 key Off-On Reset simulates the drain and turn on the ignition with the key. 0x11 03 Soft Reset allows the initialization of certain program units and their storage structures. Again, there are reserved values that can be defined for vehicle manufacturers and vehicle suppliers specific use. |
| Diagnostic and Communications Management | 0x14 | 0x54 | Clear Diagnostic Information | Clears diagnostic trouble codes (DTC's) from ECU memory. DTC grouping is used to loosely identify fault locations in undocumented ECU's. 0x14 00 00 00 to FF FF FF clears specific powertrain DTC's 0x14 40 00 00 to 7F FF FF clears specific chassis DTC's 0x14 80 00 00 to BF FF FF clears specific body DTC's 0x14 C0 00 00 to FF FF FF clears specific network related DTC's. 0x14 FF FF 33 clears all emissions-related DTC's 0x14 FF FF FF clears all DTC's. Like other sub-functions in this list, there are manufacturer-specific reserved values that offer unique and/or specific functionality. |
| Diagnostic and Communications Management | 0x27 | 0x67 | Security Access | Security check is available to enable the most security-critical services. For this purpose a "Seed" is generated and sent to the client by the control unit. From this "Seed" the client has to compute a "Key" and send it back to the control unit to unlock the security-critical services. |
| Diagnostic and Communications Management | 0x28 | 0x68 | Communication Control | With this service, both the sending and receiving of messages can be turned off in the control unit. |
| Diagnostic and Communications Management | 0x29 | 0x69 | Authentication | An update (2020) of the standard added this service to provide a standardized approach to more modern methods of authentication than are permitted by the Security Access (0x27) service, including bidirectional authentication with PKI-based Certificate Exchange. |
| Diagnostic and Communications Management | 0x3E | 0x7E | Tester Present | If no communication is exchanged with the client for a long time, the control unit automatically exits the current session and returns to the "Default Session" back, and might go to sleep mode. Therefore, there is an extra service which purpose is to signal to the device that the client is still present. |
| Diagnostic and Communications Management | 0x83 | 0xC3 | Access Timing Parameters | In the communication between the controllers and the client, certain times must be observed. If these are exceeded, without a message being sent, it must be assumed that the connection was interrupted. These times can be called up and changed. |
| Diagnostic and Communications Management | 0x84 | 0xC4 | Secured Data Transmission | |
| Diagnostic and Communications Management | 0x85 | 0xC5 | Control DTC Settings | Enables, disables, or otherwise controls the transmission of DTC's. This is helpful when maintenance or modification work is being performed on a vehicle to avoid anomalous or annoying behavior, like beeping on an instrument cluster or safety systems activating in response to an external DTC. Like most UDS settings, this does not typically persist after a reboot. 0x01 On enables the transmission of DTC's. 0x02 Off disabled the transmission of DTC's. |
| Diagnostic and Communications Management | 0x86 | 0xC6 | Response On Event | |
| Diagnostic and Communications Management | 0x87 | 0xC7 | Link Control | The Service Link Control is used to set the baud rate of the diagnostic access. It is usually implemented only at the central gateway. |
| Data Transmission | 0x22 | 0x62 | Read Data By Identifier | With this service, it is possible to retrieve one or more values of a control unit. This can be information of all kinds and of different lengths such as Partnumber or the software version. Dynamic values such as the current state of the sensor can be queried. Each value is associated to a Data Identifier (DID) between 0 and 65535; for example, the VIN DID is 61840d (0xF190). Normal CAN signals are meant for information that some ECU uses in its functionality. DID data is sent on request only, and is for information that no ECU uses, but a service tool or a software tester can benefit from. |
| Data Transmission | 0x23 | 0x63 | Read Memory By Address | Read data from the physical memory at the provided address. This function can be used by a testing tool, in order to read the internal behaviour of the software. |
| Data Transmission | 0x24 | 0x64 | Read Scaling Data By Identifier | |
| Data Transmission | 0x2A | 0x6A | Read Data By Identifier Periodic | With this service, values are sent periodically by a control unit. The values to be sent must be defined to only using the "Dynamically Define Data Identifier". |
| Data Transmission | 0x2C | 0x6C | Dynamically Define Data Identifier | This service offers the possibility of a fix for a device specified Data Identifier (DID) pool to configure another Data Identifier. This is usually a combination of parts of different DIDs or simply a concatenation of complete DIDs. The requested data may be configured or grouped in the following manner: Source DID, position, length (in bytes), Sub-Function Byte: defineByIdentifier Memory address length (in bytes), Sub-Function Byte: defineByMemoryAddress Combinations of the two above methods through multiple requests. |
| Data Transmission | 0x2E | 0x6E | Write Data By Identifier | With the same Data Identifier (DID), values can also be changed. In addition to the identifier, the new value is sent along. |
| Data Transmission | 0x3D | 0x7D | Write Memory By Address | The “Write Memory By Address” service allows the external diagnostic tool to write information into the ECU at one or more contiguous memory locations. |
| Stored Data Transmission | 0x14 | 0x54 | Clear Diagnostic Information | Delete all stored DTC |
| Stored Data Transmission | 0x19 | 0x59 | Read DTC Information | DTC stands for "Diagnostic Trouble Codes". Each DTC handled by the control unit fault is stored with its own code in the error memory and can be read at any time. In addition to the error, additional information will be stored, which can also be read. |
| Input / Output Control | 0x2F | 0x6F | Input Output Control By Identifier | This service allows an external system intervention on internal / external signals via the diagnostic interface. By specifying a so-called option bytes additional conditions for a request can be specified, the following values are specified: ReturnControlToECU: The device must get back controls of the mentioned signals. ResetToDefault: The tester prompts to reset signals to the system wide default value. Freeze Current State: The device shall freeze the current signal value. ShortTermAdjustment: The device shall use the provided value for the signal |
| Remote Activation of Routine | 0x31 | 0x71 | Routine Control | Control routine services of all kinds can be performed. There are three different message types: With the start-message, a service can be initiated. It can be defined to confirm the beginning of the execution or to notify when the service is completed. With the Stop message, a running service can be interrupted at any time. The third option is a message to query the results of the service. The start and stop message parameters can be specified. This makes it possible to implement every possible project-specific service. |
| Upload / Download | 0x34 | 0x74 | Request Download | Downloading new software or other data into the control unit is introduced using the "Request Download". Here, the location and size of the data is specified. In turn, the tester specifies how large the data packets can be. |
| Upload / Download | 0x35 | 0x75 | Request Upload | The service "request upload" is almost identical to the service "Request Download". With this service, the software from the control unit is transferred to the tester. The location and size must be specified. Again, the size of the data blocks are specified by the tester. |
| Upload / Download | 0x36 | 0x76 | Transfer Data | For the actual transmission of data, the service "Transfer Data" is used. This service is used for both uploading and downloading data. The transfer direction is notified in advance by the service "Request Download" or "Upload Request". This service should try to send packets at maximum length, as specified in previous services. If the data set is larger than the maximum, the "Transfer Data" service must be used several times in succession until all data has arrived. |
| Upload / Download | 0x37 | 0x77 | Request Transfer Exit | A data transmission can be 'completed' when using the "Transfer Exit" service. This service is used for comparison between the control unit and the tester. When it is running, a control unit can answer negatively on this request to stop a data transfer request. This will be used when the amount of data (set in "Request Download" or "Upload Request") has not been transferred. |
| Upload / Download | 0x38 | 0x78 | Request File Transfer | This service is used to initiate a file download from the client to the server or upload from the server to the client. Additionally information about the file system are available by this service. |
| | | 0x7F | Negative Response | This response is given when a service request could not be performed, for example having a not supported Data Identifier. A Negative Response Code will be included. |

## Negative response codes

Negative response from ECU contains SID 0x7F and two payload bytes: request's SID and error code. These codes can be found in freely available software (for example, BusMaster) as well as in the ISO itself.
| NRC | Description |
| 0x10 | General reject |
| 0x11 | Service not supported |
| 0x12 | Subfunction not supported |
| 0x13 | Incorrect message length or invalid format |
| 0x14 | Response too long |
| 0x21 | Busy, repeat request |
| 0x22 | Conditions not correct |
| 0x24 | Request sequence error |
| 0x25 | No response from subnet component |
| 0x26 | Failure prevents execution of requested action |
| 0x31 | Request out of range |
| 0x33 | Security access denied |
| 0x34 | Authentication failed |
| 0x35 | Invalid key |
| 0x36 | Exceeded number of attempts |
| 0x37 | Required time delay not expired |
| 0x38 | Secure data transmission required |
| 0x39 | Secure data transmission not allowed |
| 0x3A | Secure data verification failed |
| 0x50 | Certificate validation failed, invalid time period |
| 0x51 | Certificate validation failed, invalid signature |
| 0x52 | Certificate validation failed, invalid chain of trust |
| 0x53 | Certificate validation failed, invalid type |
| 0x54 | Certificate validation failed, invalid format |
| 0x55 | Certificate validation failed, invalid content |
| 0x56 | Certificate validation failed, invalid scope |
| 0x57 | Certificate validation failed, invalid certificate |
| 0x58 | Ownership verification failed |
| 0x59 | Challenge calculation failed |
| 0x5A | Setting access right failed |
| 0x5B | Session key creation/derivation failed |
| 0x5C | Configuration data usage failed |
| 0x5D | Deauthentication failed |
| 0x70 | Upload download not accepted |
| 0x71 | Transfer data suspended |
| 0x72 | General programming failure |
| 0x73 | Wrong block sequence number |
| 0x78 | Request correctly received, response pending |
| 0x7E | Subfunction not supported in active session |
| 0x7F | Service not supported in active session |
| 0x81 | RPM too high |
| 0x82 | RPM too low |
| 0x83 | Engine is running |
| 0x84 | Engine is not running |
| 0x85 | Engine run time too low |
| 0x86 | Temperature too high |
| 0x87 | Temperature too low |
| 0x88 | Vehicle speed too high |
| 0x89 | Vehicle speed too low |
| 0x8A | Throttle/pedal too high |
| 0x8B | Throttle/pedal too low |
| 0x8C | Transmission range not in neutral |
| 0x8D | Transmission range not in gear |
| 0x8F | Brake switch not closed |
| 0x90 | Shifter lever not in park |
| 0x91 | Torque converter clutch locked |
| 0x92 | Voltage too high |
| 0x93 | Voltage too low |
| 0x94 | Resource temporary unavailable |