How Vehicle Emissions Drove the Creation of Diagnostic Trouble Codes (DTCs)

How do vehicle emissions, clean air and Diagnostic Trouble Codes related to one another? For that matter, what are DTC codes, and why do they matter?

All new and currently-manufactured vehicles feature an integrated, on-board diagnostic system. These systems monitor countless in-vehicle components, from the engine to the airbags. When the diagnostic system detects an error, it flashes a warning indicator. (Just picture the infamous Check Engine light.) This lets the driver know it’s time to have the vehicle inspected by a specialist.

But behind the scenes, each one of these events corresponds to a specific diagnostic trouble code (DTC). This series of letters and numbers is only accessible through special equipment. Its purpose it to help a technician quickly identify and resolve the operational issue.

DTCs were developed in conjunction with 1995’s Clean Air Act. Their original purpose was to monitor the Environmental Protection Agency’s newly-established vehicle emissions standards. These days they monitor a far wider range of vehicle operations. In fact, DTCs have come to play a huge role in streamlining  automotive repair.

Interpreting DTCs – Critical and Non-critical

There are three types of DTCs:

  • OBD-1, no longer in use, and found in vehicles from made prior to the 1990s
  • OBD-2, the primary DTCs used today, and
  • J1939, for commercial vehicles and large trucks

The industry groups OBD-2 DTCs into two categories of critical and non-critical codes. This distinction helps technicians understand what they’re dealing with. Critical codes indicate the vehicle is unsafe and requires immediate attention—for example, brake failure. Non-critical codes are less urgent. Non-critical DTCs include those tied to heating and cooling systems.


The Society of Automotive Engineers created a standard DTC list now used by every manufacturer. However, manufacturer-specific DTCs do exist as well.

DTCs are five characters long, and begin with a letter indicating the overall area of the malfunction:

  • P refers to the powertrain (engine, transmission and fuel systems)
  • C refers to the chassis (steering, suspension and brakes)
  • B refers to the body (A/C)
  • U refers to the network (onboard computer systems)

The second letter indicates whether the code is standard or manufacturer-specific. 0 means standard, while 1-3 representing a manufacturer-specific code. 0s and 1s are most common here.

The third letter identifies the relevant vehicle subsystem, so the technician knows exactly what needs attention:

  • 1-2: Fuel or air metering
  • 3: Ignition
  • 4: Emissions system
  • 5: Vehicle control or speed
  • 6: Computer circuitry
  • 7-8: Transmission

Last are the fourth and fifth characters of the code, read together as a single two-digit number. These final digits get even more granular, pointing to the exact cause of the malfunction, such as a broken part.

For the code P0300, then, P indicates powertrain. 0 identifies this as a standard code, so the specialist knows they won’t need to search through manufacturer-specific documents. 3 flags ignition as the trouble area, and finally, 00 refers to the cylinders. The definition of DTC P0300 is “Random or multiple cylinder fires detected.”

Using an OBD Scanner

There is no need for technicians—and certainly not drivers—to memorize any DTCs. The fleet management system offers easy research for all codes, whether standard or manufacturer-specific. To find code definitions outside a fleet management system, technicians insert a scan tool into the on-board diagnostic (OBD) port. This port usually under the driver’s side dash.

VEHICLE EmissionS Control and PDTCs

PDTCs are permanent diagnostic trouble codes. These DTCs cannot be reset by disconnecting the battery or resetting the system (a shortcut sometimes used to avoid actually repairing the malfunctioning system). The only way to clear a PDTC is to repair the vehicle. When the monitor runs without identifying a problem, the code will clear itself.

California requires biennial vehicle checks in accordance with the Smog Check Program. If a vehicle shows a PDTC, its emission control systems may be malfunctioning, impacting smog production and air quality.

Any vehicle with a PDTC produced after 2009 will automatically fail its Smog Check inspection. The Consumer Assistance Program (CAP) offers repair assistance to income-eligible consumers should their vehicle fail the two-yearly Smog Check inspection due to a vehicle emissions-related PDTC. They may also receive up to $1,500 to retire their vehicle.

“During the first month of implementation, 3,124 vehicles failed for PDTCs that would have previously passed a Smog Check inspection. Including PDTCs … raised the failure rate on these vehicles from 4.2% to 5.6%.”

BAR then re-examined the test records for these vehicles. Of the 3,124 that failed inspection due to a PDTC, 2,385 passed a subsequent re-inspection.


Diagnostic trouble codes are a powerful tool in determining and correcting vehicle malfunctions. They save time and increase efficiency and accuracy for technicians. And believe it or not, they help the environment. The EPA’s Clean Air Act has cut pollutant emissions by 78%. Transportation emissions are detrimental to air quality, and DTC codes are one more way to keep tabs on vehicle emissions.

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