OBD-II Diagnostic Terms Explained: Complete DTC Glossary

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OBD-II Diagnostic Terms Explained: Complete DTC Glossary

The most important automotive diagnostic terms explained. This guide, prepared by OBD Spark, provides simple and accurate explanations of the most common OBD-II Diagnostic Trouble Code (DTC) terms used by major vehicle manufacturers. Whether you are a professional technician, automotive student, DIY mechanic, or vehicle owner, understanding these terms will help you interpret fault codes faster and diagnose vehicle problems more efficiently.

OBD-II Diagnostic Terms Explained: Complete DTC Glossary The most important automotive diagnostic terms explained. This guide, prepared by OBD Spark, provides simple and accurate explanations of the most common OBD-II Diagnostic Trouble Code (DTC) terms used by major vehicle manufacturers. Whether you are a professional technician, automotive student, DIY mechanic, or vehicle owner, understanding these terms will help you interpret fault codes faster and diagnose vehicle problems more efficiently. In this reference, you'll learn the meaning of common diagnostic terms such as Open Circuit, Short to Ground, Short to Voltage, Circuit High, Circuit Low, High Input, Low Input, Range/Performance, Intermittent, Signal Erratic, Lost Communication, CAN Bus Fault, LIN Bus Fault, Mechanical Failure, Sensor Circuit, Actuator Circuit, and many more. These terms appear in OBD-II fault codes for Toyota, Honda, Ford, Chevrolet, BMW, Mercedes-Benz, Volkswagen, Audi, Nissan, Hyundai, Kia, Mazda, Subaru, Lexus, Jeep, Dodge, RAM, GMC, Volvo, Tesla, and many other vehicle brands. This page is regularly updated by OBD Spark to provide a reliable, beginner-friendly, and technician-focused reference for understanding automotive diagnostic terminology used in modern vehicles worldwide. Common OBD-II Diagnostic Terms Explained Open Circuit The electrical circuit is broken, preventing current from flowing between components. Short to Ground The circuit is accidentally connected to ground, causing the voltage to drop abnormally. Short to Voltage The circuit is unintentionally connected to battery voltage or another power source. Circuit High The control module detects a voltage higher than the expected operating range. Circuit Low The control module detects a voltage lower than the expected operating range. High Input The sensor sends an input signal that is higher than the specified limit. Low Input The sensor sends an input signal that is lower than the specified limit. Range/Performance The component operates, but its performance is outside the expected range. Performance The system functions but does not meet the expected operating performance. Intermittent The fault appears and disappears randomly, making diagnosis more difficult. Signal Erratic The signal is unstable, noisy, or changes unexpectedly. No Signal No signal is detected from the sensor or actuator. Signal Missing The expected signal is completely absent during operation. Signal Invalid The received signal is outside the acceptable specification. Implausible Signal The signal is present but unrealistic compared with other sensor values. Rationality Failure The sensor value is illogical when compared with related system inputs. Correlation Two or more related sensors do not agree with each other. Stuck Open The valve, relay, or switch remains open when it should be closed. Stuck Closed The valve, relay, or switch remains closed when it should be open. Slow Response The component reacts slower than the manufacturer specification. Response Too Slow The response time exceeds the acceptable operating limit. Response Too Fast The component changes state faster than expected by the control module. Over Temperature The measured temperature is higher than the specified operating range. Under Temperature The measured temperature is lower than the specified operating range. Over Voltage The electrical system voltage exceeds the normal operating limit. Under Voltage The electrical system voltage is below the normal operating range. Over Current The current flowing through the circuit exceeds the specified limit. High Resistance The circuit resistance is higher than expected, often caused by corrosion or poor connections. Low Resistance The circuit resistance is lower than expected, which may indicate a short circuit. Frequency Incorrect The signal frequency is outside the manufacturer's specified range. Pressure Too High The measured pressure exceeds the acceptable operating limit. Pressure Too Low The measured pressure is lower than the acceptable operating limit. Voltage Reference Fault The reference voltage supplied to a sensor is incorrect or unstable. Sensor Circuit A fault has been detected in the electrical circuit of a sensor. Actuator Circuit A malfunction has been detected in the circuit controlling an actuator. Control Circuit The control circuit is not operating as intended. Feedback Circuit The feedback signal from the component is incorrect or missing. Mechanical Failure The component has failed mechanically rather than electrically. Electrical Failure An electrical fault has been detected in the component or its circuit. Internal Failure The module or component has experienced an internal malfunction. Internal Control Module Fault An internal hardware or software fault has been detected in the control module. Processor Fault The control module processor has detected an internal error. Memory Error The module cannot correctly read or write stored data. EEPROM Error An error has occurred in the EEPROM memory used by the control module. ROM Error The control module has detected a fault in its read-only memory. RAM Error A problem has been detected in the module's random access memory. Calibration Required The component must be calibrated or relearned to operate correctly. Programming Required The module or component must be programmed using a compatible diagnostic scan tool. Configuration Error The module configuration does not match the vehicle specifications. Initialization Failure The system failed to complete its startup or initialization process. Learned Values Lost The adaptive memory has been erased and the system must relearn operating values. Checksum Error The stored or transmitted data failed the integrity verification check. Timeout The expected response was not received within the specified time. Lost Communication Communication with another control module has been interrupted or lost. Bus Off The CAN controller has disconnected from the network because of excessive communication errors. CAN Bus Fault A fault has been detected in the Controller Area Network (CAN) communication system. LIN Bus Fault A malfunction has been detected in the Local Interconnect Network (LIN). FlexRay Bus Fault A communication problem has occurred on the FlexRay network. Ethernet Communication Fault A fault has been detected in the vehicle's Automotive Ethernet communication system. PWM Signal Fault The pulse-width modulation (PWM) signal is missing or outside the expected range. Serial Data Error An error has been detected while transmitting or receiving serial data. Invalid Data Received The received communication data is invalid or corrupted. Message Counter Incorrect The message sequence counter does not match the expected value. Signal Plausibility Failure The signal does not logically agree with related sensor or system data. Component Slipping The component is unable to maintain proper mechanical engagement or operation. Component Stuck The component is mechanically stuck and cannot move freely. Circuit/Open An open circuit has been detected in the monitored electrical system. Circuit Shorted The circuit is shorted to power, ground, or another circuit. Ground Fault An abnormal ground connection has been detected in the electrical circuit. Battery Voltage Low The battery voltage has dropped below the manufacturer's specified limit. Battery Voltage High The battery or charging system voltage exceeds the specified operating range. Sensor Out of Calibration The sensor requires recalibration because its readings are no longer accurate. Actuator Jammed The actuator is mechanically jammed and cannot move to the commanded position. Torque Performance The measured torque does not match the expected value under current operating conditions. Position Sensor Fault The position sensor signal is missing, inaccurate, or outside the expected range. Position Correlation Two position sensors report values that do not agree with each other. Position Not Learned The control module has not completed the required position learning procedure. Adaptation Limit Reached The control module has reached its maximum correction limit and can no longer compensate. System Too Rich The engine is operating with an excessive amount of fuel compared to air. System Too Lean The engine is operating with insufficient fuel compared to air. Efficiency Below Threshold The system efficiency has dropped below the manufacturer's acceptable limit. Pressure Sensor Performance The pressure sensor is operating outside its expected performance range. Temperature Sensor Performance The temperature sensor is responding incorrectly or outside specifications. Oxygen Sensor Slow Response The oxygen sensor is switching slower than expected, affecting fuel control. Heater Circuit Fault A malfunction has been detected in the heater circuit of a sensor. Relay Circuit Fault The relay control circuit is open, shorted, or operating incorrectly. Solenoid Circuit Fault The solenoid control circuit has an electrical malfunction. Injector Circuit Fault An electrical problem has been detected in the fuel injector circuit. Ignition Coil Circuit Fault The ignition coil circuit is not operating correctly. Glow Plug Circuit Fault A malfunction has been detected in the diesel glow plug circuit. Fan Control Circuit Fault The engine cooling fan control circuit has an electrical fault. Pump Control Circuit Fault The control circuit for a fuel, oil, vacuum, or water pump is malfunctioning. Valve Control Circuit Fault The valve actuator control circuit is not operating within specifications. Open Load The control module detects that the electrical load is disconnected or open. Shorted Load The electrical load is short-circuited, causing excessive current flow. Out of Range The measured value is outside the manufacturer's acceptable operating limits. Failure A general malfunction has been detected that requires diagnosis and repair.
OBD-II Diagnostic Terms Explained: Complete DTC Glossary

In this reference, you'll learn the meaning of common diagnostic terms such as Open Circuit, Short to Ground, Short to Voltage, Circuit High, Circuit Low, High Input, Low Input, Range/Performance, Intermittent, Signal Erratic, Lost Communication, CAN Bus Fault, LIN Bus Fault, Mechanical Failure, Sensor Circuit, Actuator Circuit, and many more. These terms appear in OBD-II fault codes for Toyota, Honda, Ford, Chevrolet, BMW, Mercedes-Benz, Volkswagen, Audi, Nissan, Hyundai, Kia, Mazda, Subaru, Lexus, Jeep, Dodge, RAM, GMC, Volvo, Tesla, and many other vehicle brands.

This page is regularly updated by OBD Spark to provide a reliable, beginner-friendly, and technician-focused reference for understanding automotive diagnostic terminology used in modern vehicles worldwide.

Common OBD-II Diagnostic Terms Explained

Open Circuit

The electrical circuit is broken, preventing current from flowing between components.

Short to Ground

The circuit is accidentally connected to ground, causing the voltage to drop abnormally.

Short to Voltage

The circuit is unintentionally connected to battery voltage or another power source.

Circuit High

The control module detects a voltage higher than the expected operating range.

Circuit Low

The control module detects a voltage lower than the expected operating range.

High Input

The sensor sends an input signal that is higher than the specified limit.

Low Input

The sensor sends an input signal that is lower than the specified limit.

Range/Performance

The component operates, but its performance is outside the expected range.

Performance

The system functions but does not meet the expected operating performance.

Intermittent

The fault appears and disappears randomly, making diagnosis more difficult.

Signal Erratic

The signal is unstable, noisy, or changes unexpectedly.

No Signal

No signal is detected from the sensor or actuator.

Signal Missing

The expected signal is completely absent during operation.

Signal Invalid

The received signal is outside the acceptable specification.

Implausible Signal

The signal is present but unrealistic compared with other sensor values.

Rationality Failure

The sensor value is illogical when compared with related system inputs.

Correlation

Two or more related sensors do not agree with each other.

Stuck Open

The valve, relay, or switch remains open when it should be closed.

Stuck Closed

The valve, relay, or switch remains closed when it should be open.

Slow Response

The component reacts slower than the manufacturer specification.

Response Too Slow

The response time exceeds the acceptable operating limit.

Response Too Fast

The component changes state faster than expected by the control module.

Over Temperature

The measured temperature is higher than the specified operating range.

Under Temperature

The measured temperature is lower than the specified operating range.

Over Voltage

The electrical system voltage exceeds the normal operating limit.

Under Voltage

The electrical system voltage is below the normal operating range.

Over Current

The current flowing through the circuit exceeds the specified limit.

High Resistance

The circuit resistance is higher than expected, often caused by corrosion or poor connections.

Low Resistance

The circuit resistance is lower than expected, which may indicate a short circuit.

Frequency Incorrect

The signal frequency is outside the manufacturer's specified range.

Pressure Too High

The measured pressure exceeds the acceptable operating limit.

Pressure Too Low

The measured pressure is lower than the acceptable operating limit.

Voltage Reference Fault

The reference voltage supplied to a sensor is incorrect or unstable.

Sensor Circuit

A fault has been detected in the electrical circuit of a sensor.

Actuator Circuit

A malfunction has been detected in the circuit controlling an actuator.

Control Circuit

The control circuit is not operating as intended.

Feedback Circuit

The feedback signal from the component is incorrect or missing.

Mechanical Failure

The component has failed mechanically rather than electrically.

Electrical Failure

An electrical fault has been detected in the component or its circuit.

Internal Failure

The module or component has experienced an internal malfunction.

Internal Control Module Fault

An internal hardware or software fault has been detected in the control module.

Processor Fault

The control module processor has detected an internal error.

Memory Error

The module cannot correctly read or write stored data.

EEPROM Error

An error has occurred in the EEPROM memory used by the control module.

ROM Error

The control module has detected a fault in its read-only memory.

RAM Error

A problem has been detected in the module's random access memory.

Calibration Required

The component must be calibrated or relearned to operate correctly.

Programming Required

The module or component must be programmed using a compatible diagnostic scan tool.

Configuration Error

The module configuration does not match the vehicle specifications.

Initialization Failure

The system failed to complete its startup or initialization process.

Learned Values Lost

The adaptive memory has been erased and the system must relearn operating values.

Checksum Error

The stored or transmitted data failed the integrity verification check.

Timeout

The expected response was not received within the specified time.

Lost Communication

Communication with another control module has been interrupted or lost.

Bus Off

The CAN controller has disconnected from the network because of excessive communication errors.

CAN Bus Fault

A fault has been detected in the Controller Area Network (CAN) communication system.

LIN Bus Fault

A malfunction has been detected in the Local Interconnect Network (LIN).

FlexRay Bus Fault

A communication problem has occurred on the FlexRay network.

Ethernet Communication Fault

A fault has been detected in the vehicle's Automotive Ethernet communication system.

PWM Signal Fault

The pulse-width modulation (PWM) signal is missing or outside the expected range.

Serial Data Error

An error has been detected while transmitting or receiving serial data.

Invalid Data Received

The received communication data is invalid or corrupted.

Message Counter Incorrect

The message sequence counter does not match the expected value.

Signal Plausibility Failure

The signal does not logically agree with related sensor or system data.

Component Slipping

The component is unable to maintain proper mechanical engagement or operation.

Component Stuck

The component is mechanically stuck and cannot move freely.

Circuit/Open

An open circuit has been detected in the monitored electrical system.

Circuit Shorted

The circuit is shorted to power, ground, or another circuit.

Ground Fault

An abnormal ground connection has been detected in the electrical circuit.

Battery Voltage Low

The battery voltage has dropped below the manufacturer's specified limit.

Battery Voltage High

The battery or charging system voltage exceeds the specified operating range.

Sensor Out of Calibration

The sensor requires recalibration because its readings are no longer accurate.

Actuator Jammed

The actuator is mechanically jammed and cannot move to the commanded position.

Torque Performance

The measured torque does not match the expected value under current operating conditions.

Position Sensor Fault

The position sensor signal is missing, inaccurate, or outside the expected range.

Position Correlation

Two position sensors report values that do not agree with each other.

Position Not Learned

The control module has not completed the required position learning procedure.

Adaptation Limit Reached

The control module has reached its maximum correction limit and can no longer compensate.

System Too Rich

The engine is operating with an excessive amount of fuel compared to air.

System Too Lean

The engine is operating with insufficient fuel compared to air.

Efficiency Below Threshold

The system efficiency has dropped below the manufacturer's acceptable limit.

Pressure Sensor Performance

The pressure sensor is operating outside its expected performance range.

Temperature Sensor Performance

The temperature sensor is responding incorrectly or outside specifications.

Oxygen Sensor Slow Response

The oxygen sensor is switching slower than expected, affecting fuel control.

Heater Circuit Fault

A malfunction has been detected in the heater circuit of a sensor.

Relay Circuit Fault

The relay control circuit is open, shorted, or operating incorrectly.

Solenoid Circuit Fault

The solenoid control circuit has an electrical malfunction.

Injector Circuit Fault

An electrical problem has been detected in the fuel injector circuit.

Ignition Coil Circuit Fault

The ignition coil circuit is not operating correctly.

Glow Plug Circuit Fault

A malfunction has been detected in the diesel glow plug circuit.

Fan Control Circuit Fault

The engine cooling fan control circuit has an electrical fault.

Pump Control Circuit Fault

The control circuit for a fuel, oil, vacuum, or water pump is malfunctioning.

Valve Control Circuit Fault

The valve actuator control circuit is not operating within specifications.

Open Load

The control module detects that the electrical load is disconnected or open.

Shorted Load

The electrical load is short-circuited, causing excessive current flow.

Out of Range

The measured value is outside the manufacturer's acceptable operating limits.

Failure

A general malfunction has been detected that requires diagnosis and repair.

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