Diagnostic Trouble Code ( DTC ) Categories


Current DTCs, also known as "active" or "live" DTCs, represent the problems that are presently detected and confirmed by the vehicle's OBD system. These codes are stored in the OBD system's memory and will typically trigger the Malfunction Indicator Lamp (MIL), commonly referred to as the "Check Engine" or "Service Engine Soon" light on the vehicle's dashboard.

Significance of Current DTCs
  • Immediate Notification: When a Current DTC is set, it means the OBD system has detected a malfunction that is occurring at that moment. It is an immediate call to action to diagnose and repair the issue.
  • Potential Impact on Vehicle Performance: These codes can represent issues ranging from minor inefficiencies to critical malfunctions that can affect vehicle drivability, safety, and emissions.
  • Preventive Measure: Addressing Current DTCs promptly can prevent further damage to the vehicle, which might result from ignoring these alerts.
Examples of Current DTCs and Their Implications
  • P0171 - System Too Lean (Bank 1):
    • Implication: This code means that the oxygen sensor in bank 1 detected a lean condition (too much oxygen in the exhaust). This could be due to a vacuum leak, faulty fuel injector, insufficient fuel pressure, or a malfunctioning oxygen sensor.
    • Potential Impact: Ignoring this DTC could lead to engine misfiring, reduced fuel efficiency, and potentially damaging the catalytic converter.
  • P0300 - Random/Multiple Cylinder Misfire Detected:
    • Implication: This code indicates that multiple cylinders are misfiring, which could be caused by various issues, such as problems with the ignition system, fuel system, or internal engine failure.
    • Potential Impact: Continual misfires can lead to increased emissions, rough idling, poor acceleration, and damage to the engine or the catalytic converter.
  • P0420 - Catalyst System Efficiency Below Threshold (Bank 1):
    • Implication: This code suggests that the catalytic converter is not functioning as efficiently as it should, which could be due to a faulty catalytic converter, oxygen sensors, or an exhaust leak.
    • Potential Impact: Driving with a faulty catalytic converter can result in reduced engine performance and fuel economy, and failure to pass emissions tests in regions where they are required.
Responding to Current DTCs
  • Diagnostic Procedure: When a Current DTC is present, a technician should conduct a full diagnostic procedure using an OBD-II scanner to retrieve the DTCs and possibly access the freeze frame data, which captures the engine's condition at the time the code was set.
  • Repair and Verification: After the underlying issue is identified and repaired, a test drive or a specific drive cycle may be required to ensure the repair's effectiveness and to reset the OBD system's monitors.
  • Documentation: It is important to document all repairs associated with DTCs for future reference and to inform vehicle owners about the repairs carried out and the importance of timely attention to these codes.

In summary, Current DTCs play a critical role in vehicle maintenance and cannot be ignored without risking further complications. Properly diagnosing and addressing these codes is essential for ensuring the longevity and performance of the vehicle, as well as maintaining its compliance with emission standards.


Permanent Diagnostic Trouble Codes (DTCs) are critical to understanding the emissions system of a vehicle. These codes cannot be easily reset by disconnecting the battery or clearing codes with a scan tool. Instead, they are designed to be cleared only after the vehicle has gone through an appropriate repair action followed by a drive cycle to ensure the issue has been resolved.

Purpose and Significance of Permanent DTCs
  • Emissions Compliance: Permanent DTCs specifically relate to the emissions control systems of a vehicle, ensuring it adheres to environmental regulations.
  • Preventing Tampering: The permanent nature of these codes is intended to prevent the manipulation of emissions data, which is crucial for regions where passing an emissions test is required for vehicle registration.
  • Repair Verification: They act as an indicator that a vehicle has had an emissions-related issue that was serious enough to warrant a proper repair procedure.
Examples of Permanent DTCs and Their Implications
  • P0420 - Catalyst System Efficiency Below Threshold (Bank 1):
    • Implication: The catalytic converter is not operating as efficiently as it is supposed to, potentially due to its deterioration over time.
    • Resolution: Replacing the catalytic converter followed by a drive cycle to ensure the new converter is functioning properly and clears the code.
  • P0455 - Evaporative Emission Control System Leak Detected (gross leak):
    • Implication: There is a significant leak in the EVAP system, which could be due to loose, broken, or missing gas cap, or other components of the EVAP system.
    • Resolution: Repairing the leak source and conducting a drive cycle to verify that the repair has fixed the leak.
Responding to Permanent DTCs
  • Accurate Diagnosis: Technicians must use advanced diagnostic tools to accurately identify the cause of the permanent DTC.
  • Repair Procedure: Once identified, the faulty component must be repaired or replaced, and the repair must be documented.
  • Drive Cycle Completion: After the repair, the vehicle must undergo a specific OBD-II drive cycle. This drive cycle ensures that all the onboard diagnostics have run and verified that the repair is successful. If the system finds no further evidence of the problem, the permanent DTC will be cleared automatically.
  • Confirmation of Code Clearance: Technicians should confirm that the permanent DTC has been cleared by using an OBD-II scanner after completing the drive cycle.
Legal and Environmental Considerations
  • Mandatory Repair: Depending on the region, repairing issues related to permanent DTCs may not only be a matter of vehicle health but also a legal requirement for emissions testing.
  • Record-Keeping: It's crucial to maintain accurate records of the diagnosis, repair, and clearance of permanent DTCs for regulatory compliance and future reference.

In conclusion, permanent DTCs are an integral part of modern vehicle emissions control systems. They help ensure that emissions-related problems are properly addressed, thereby reducing environmental impact and helping vehicle owners comply with increasingly stringent emissions regulations. Understanding and managing these codes is essential for any automotive repair facility or technician.


Pending Diagnostic Trouble Codes (DTCs) are a preemptive alert system in vehicles. They indicate that the vehicle's On-Board Diagnostics (OBD) system has detected an issue that may not have met the threshold to trigger a "current" DTC or illuminate the Malfunction Indicator Lamp (MIL), also known as the Check Engine Light.

Characteristics of Pending DTCs
  • Initial Detection: Pending DTCs are set when the OBD system first recognizes a potential issue. The system requires more data or occurrences of the issue to determine whether it will become a consistent problem that necessitates a Current DTC.
  • No MIL Activation: Unlike Current DTCs, Pending DTCs do not activate the MIL. However, they are stored in the vehicle’s computer system and can be read with an OBD-II scanner.
  • Monitoring: They are part of the OBD system's monitoring strategy, which continually checks for malfunctions related to emissions and other vehicle systems.
Examples of Pending DTCs and Their Significance
  • P0300 - Random/Multiple Cylinder Misfire Detected:
    • Situation: This code may be set as pending if the misfire is sporadic and not consistent enough to affect vehicle performance immediately.
    • Monitoring: The OBD system will continue to monitor the engine's operation, and if the misfire becomes frequent or consistent, the code will escalate to a Current DTC, and the MIL will be illuminated.
  • P0171 - System Too Lean (Bank 1):
    • Situation: If the OBD system detects a lean condition but cannot confirm the issue over multiple driving cycles, it may set this code as pending.
    • Monitoring: The vehicle's computer will continue to analyze the data from the oxygen sensors and fuel system to confirm if the lean condition persists or corrects itself.
Responding to Pending DTCs
  • Preventative Maintenance: Technicians and vehicle owners should view Pending DTCs as an early warning sign. Addressing these issues promptly can often prevent them from escalating into more serious problems.
  • Further Diagnosis: In some cases, additional diagnostic tests may be necessary to pinpoint the cause of the problem before it becomes a persistent fault.
  • Monitoring: If a Pending DTC is found during a routine scan, it is advisable to monitor the vehicle's performance closely and recheck the OBD system after a few driving cycles to see if the code escalates or clears.
Implications of Ignoring Pending DTCs
  • Potential for Escalation: If Pending DTCs are ignored, there is a risk that the fault will become more serious, leading to vehicle performance issues, increased emissions, or even system damage.
  • Cost-Efficiency: Addressing issues while they are still in the pending stage can be more cost-effective, as the repair may be simpler and less expensive than if the issue is allowed to progress.

In summary, while Pending DTCs do not immediately affect the drivability of a vehicle or trigger the MIL, they are crucial for early detection and prevention of potential issues. Automotive service professionals should use Pending DTCs as a proactive tool for vehicle maintenance and should educate vehicle owners on the importance of these early alerts to avoid future complications.


Historic Diagnostic Trouble Codes (DTCs), often referred to as "stored" or "history" DTCs, are essentially a log of past faults that the vehicle’s On-Board Diagnostics (OBD) system has detected and recorded. Even after the issue that triggered the DTC has been resolved and the code has been cleared, the record of the code remains in the system's memory.

Purpose of Historic DTCs
  • Diagnostic Reference: Historic DTCs serve as a reference for mechanics, providing a history of the issues the vehicle has experienced.
  • Troubleshooting: They are useful in identifying patterns of recurring issues or intermittent faults that may not be present during inspection but could reappear later.
  • Vehicle History: For used car assessments, historic DTCs can provide insight into the vehicle’s previous conditions and repairs.
Examples of Historic DTCs and Their Uses
  • P0302 - Cylinder 2 Misfire Detected:
    • Scenario: This code may have been set as current when a misfire was actively occurring. After the faulty spark plug was replaced, the code was cleared from current status but remains as a historic DTC.
    • Use: If the misfire reoccurs, the presence of a historic P0302 could indicate an underlying issue in Cylinder 2 that was not addressed by simply changing the spark plug.
  • P0442 - Evaporative Emission Control System Leak Detected (small leak):
    • Scenario: Suppose this code was triggered by a loose fuel cap, which has since been tightened, clearing the code. However, the code remains in the vehicle's history.
    • Use: Should the vehicle exhibit symptoms of an EVAP leak again, the historic P0442 could guide a technician to conduct a more thorough inspection of the EVAP system.
Managing Historic DTCs
  • Interpretation: Understanding the context of historic DTCs is essential. A technician must consider when the code was set and what repairs were done since then.
  • Diagnosis: A comprehensive diagnosis should involve checking for any unresolved or intermittent issues that could cause the DTC to resurface.
  • Clearing History: Some diagnostic tools allow for the clearing of historic DTCs. However, this should be done judiciously, as these codes provide valuable information about the vehicle's history.
Considerations with Historic DTCs
  • Potential Misinterpretation: It is important not to assume that a historic DTC is an indicator of a current problem without further diagnosis.
  • Vehicle Purchase: When purchasing a used vehicle, reviewing the historic DTCs can offer insight into the maintenance and issues the vehicle has had.
  • Warranty Claims: Historic DTCs can be significant for warranty claims, as they can show if the issue has been previously detected and whether it is part of a recurring problem.

In conclusion, historic DTCs are an important aspect of vehicle diagnostics, providing a record of past issues that can be instrumental in future troubleshooting. Proper interpretation and management of these codes are necessary skills for any automotive service professional. They ensure a comprehensive understanding of a vehicle's health over time and can inform both preventative maintenance and necessary repairs.