Interpreting DTC Codes

We previously detailed the basic backstory behind what a OBD-II diagnostics system is and how it works, but how does that system talk to repair technicians?

The OBD-II standard displays its findings in what are known as DTC’s: Diagnostic Trouble Codes, also known as “fault codes.” These codes are separated into four general categories: “B,” “C,” “P,” and “U.”

  • “B” codes refer to diagnostics related to the state of a vehicle’s body.
  • “C” codes pertain to the health of the vehicle’s chassis.
  • “P” codes relate to the vehicle’s powertrain.
  • “U” codes are connected to the integrity of the vehicle’s User Network, or interconnected computer systems

After the first character of any given fault code, a 0 or a 1 will be present. A 0 signals that the problem is with a generic part, system, or process common across all makes and models. If a 1 is displayed, it will refer to a problem in a manufacturer-specific component of the car.

From here, we’ll go into detail the most common type of DTC, which is the P code, which you’ll encounter when a vehicle’s engine or transmission encounters trouble, triggering the dreaded “Check Engine” light. The readout will display a third character, a digit in this case, pertaining to a more specific category of the trouble:

  • 1-Emission Management (Fuel/Air)
  • 2-Injector Circuit (Fuel/Air)
  • 3-Ignition/Misfire
  • 4-Emission Control
  • 5-Vehicle Speed & Idle Control
  • 6-Computer & Output Circuit
  • 7-Transmission
  • 8-Transmission
  • 9-ISO/SAE Reserved (Likely indicates malfunction within network of wiring & connectors)
  • 0-ISO/SAE Reserved

The last two digits of a P-code will then refer a technician to the exact sensor within the car’s OBD II network whose reading triggered the trouble code.

B, C, and U codes cover a different range of issues than P codes do, so their classification from the third character onward will have to do with different systems and their components than the powertrain.

Once repair technicians or ambitious at-home DIYers scan a car’s OBD II system for a specific P-code, they’ll know where to start inspecting the car to find the exact cause of the problem and determine a repair solution from there.

For consumers who want a greater connection to their vehicle’s maintenance and care, the technical nature of OBD-II and DTC’s does not have to be a barrier to gaining that new connection and insight. Autonet Mobile has made progress in bringing easy OBD II access to vehicle owners by way of the CarCure app, which wields the capability of sending vehicle diagnostic information from a special OBD II device directly to a consumer’s smartphone, allowing quick diagnoses and specialized matching to qualified repair shops experienced with that driver’s same year, make, and model of car or truck. Knowing what a car repair technician knows can inspire confidence in consumers that they are making the right repair choice, and can foster new, proactive engagement among vehicle owners that can lead to a more steady pattern of repeat business for repair shops.

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