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.
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| OBD-II Diagnostic Terms Explained: Complete DTC Glossary |
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.
