What are DTC codes, and why do they matter?
The on-board computer diagnostic system integrated into all new vehicles today monitors countless in-vehicle systems, from the engine to the airbags. When the system detects an error or potential malfunction, it warns drivers with an indicator, such as the infamous check engine light. This tells the driver it’s time to take the vehicle to a specialist for attention. Behind the scenes, it also activates a specific diagnostic trouble code (DTC). This code, consisting of a series of letters and numbers, is only accessible by a technician or other professional, and it helps them quickly determine the problem and how to best solve it.
DTC codes came around as a result of the Clean Air Act of 1995, in order to monitor emission levels standardized by the Environmental Protection Agency (EPA). Of course, now they monitor much more than that. DTC codes have played a huge role in streamlining the automotive repair process.
Interpreting DTC Codes – Critical and Non-critical
There are three types of DTC codes:
- OBD-1, which are found in vehicles from before the 90s and are no longer used,
- OBD-2, the primary DTC codes used today, and
- J1939, for commercial vehicles and large trucks.
Within the standard type of DTC codes, OBD-2, there are critical and non-critical codes. Distinguishing between the two helps technicians quickly understand what they’re dealing with. Critical codes indicate that the vehicle is unsafe and requires immediate attention, such as brake failure. Non-critical codes are less urgent, indicating malfunctions with systems such as heating and cooling.
Decoding
The Society of Automotive Engineers created a standard DTC list that every manufacturer uses, so technicians everywhere are on the same page. However, manufacturer-specific DTC codes exist as well.
DTC codes are five characters long, starting with a letter that indicates the overall problem 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, with 0 meaning standard, and 1-3 representing a manufacturer-specific code. You are most likely to find 0s and 1s here.
The third letter determines the subsystem of the vehicle 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
Lastly are the fourth and fifth letter of the code, which are 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 would tell you there’s an issue with the powertrain. 0 indicates that it’s a standard code, so you know you don’t have to go searching through manufacturer-specific documents. 3 identifies the 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 DTC codes. All codes, whether standard or manufacturer-specific, can be easily looked up using your fleet management system. If you are not using a fleet management system, you can find a definition for the code by inserting an on-board diagnostic (OBD) scanner tool into the OBD port, usually located under the driver’s side dash area.
Emission Control and PDTCs
PDTCs refer to permanent diagnostic trouble codes, meaning they can’t be reset by simply disconnecting the battery or resetting the system, a shortcut sometimes used to avoid actually repairing the malfunctioning system. The only way to get rid of a PDTC is to repair the vehicle. When the monitor runs without identifying a problem, the code will clear itself.
In California, biennial vehicle checks must be performed in accordance with the Smog Check Program, focused on controlling vehicle emissions and improving overall air quality. In accordance with the California Code of Regulations, PDTCs must be included in the Smog Check Program as of 2019. The Bureau of Automotive Repair (BAR) determined that the presence of PDTCs will be considered in determining a vehicle’s Smog Check inspection result. If a vehicle shows a PDTC, it could indicate that its emission control systems aren’t functioning properly. This can have a dramatic impact on smog production and air quality.
Vehicles of model year 2010 and newer with a PDTC will fail the Smog Check inspection, regardless of whether an on-vehicle indicator is illuminated. The Consumer Assistance Program (CAP) offers repair assistance to income-eligible consumers should their vehicle fail the two-yearly Smog Check inspection due to an 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.
Conclusion
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 these emissions.
