If you are new to the automotive diagnostics industry, you must be very curious about DTC codes. When you use an OBD II code reader or OBD II scanner, you will always see some English and numeric codes, which may make you confused. Do you know what they mean?
Fortunately, most Autel scanners already have explanations of the meaning of DTC codes, which can help you better solve the diagnosed problems. However, I believe that some novices may still be confused by some explanations. After all, some professional terms are always obscure and difficult to understand.
Today, we'll take you deeper into DTC codes from multiple perspectives: what the different codes mean, how to read and understand them with an Autel scanner, and how to perform inspections and repairs when common fault codes appear. Let's learn together!
DTC codes are closely related to cars.
Many car owners have experienced this: the "check engine light" on the dashboard suddenly illuminates while driving, even though the vehicle continues to operate normally. This often leads to confusion: What went wrong? Is repair necessary immediately?
This brings us to the topic of DTC codes.
A DTC (Diagnostic Trouble Code) is a combination of letters and numbers stored by the vehicle's onboard diagnostics system (OBD-II) when a fault is detected. It acts like a "health language" for the vehicle, helping the owner or repair technician quickly identify the problem.
Simply put: a DTC code is a combination of a warning signal and a problem indicator.
In modern cars, DTCs are a crucial component of the OBD-II system. The OBD-II interface is standard on nearly all models after 1996, and any fault information is recorded in the vehicle's ECU (Engine Control Unit) as a DTC.
This means:
In short, DTCs are the vehicle's self-diagnosis.
Read the DTC code in the car ECU through the OBD port.
The DTC code has five characters. Each character corresponds to a specific problem with your vehicle. Below is an overview of what each character means.
The first DTC character is always a letter. There are four types of codes:
The second DTC character is a number and can be either "0" or "1":
The five characters of the DTC code have different meanings.
If the second DTC character is "0," the third character helps you determine which subsystem is malfunctioning. There are eight digits:
The fourth and fifth DTC codes are two-digit numbers from 0 to 99 called the "Specific Fault Index." They identify the specific malfunction in your vehicle.
You have an OBD II scanner and plug it into your vehicle's diagnostic port. It then displays DTC code P0420, but what does it actually mean?
Here's what this common code means:
Vehicles displaying the P0420 trouble code have a problem with their catalytic converter. Oxygen levels are below expected thresholds, and the system is leaking more pollutants into the air—definitely a problem you should address as soon as possible.
If you are interested in this fault code or have detected this fault code through the auto diagnostic tool but don't know how to deal with it, you can refer to the solution in the link below:
The following is a list of some DTC codes that are commonly encountered in car maintenance:
| DTC Codes | Meaning |
| P0171 | Lean Fuel Mixture (commonly caused by an oxygen sensor or intake system abnormality) |
| P0300 | Random/Multiple Cylinder Ignition Misfire |
| P0420 | Inefficient Three-Way Catalyst (usually related to the emissions system) |
| P0446 | EVAP System Ventilation Valve Circuit Malfunction |
| C0035 | Left Front Wheel Speed Sensor Circuit Malfunction |
| U0100 | Loss of Communication with the ECU |
| P0700 | Transmission control system malfunction |
| P0174 | Engine is running too lean on Bank 2 |
Among these, P0171 and P0420 are the most typical and frequently searched for by vehicle owners.
DTC Code P0171: Indicates a lean fuel mixture. Common causes include an intake system leak, insufficient fuel pump pressure, or a damaged oxygen sensor.
DTC Code P0420: Indicates decreased three-way catalytic converter efficiency. This may be caused by incomplete engine combustion, a faulty oxygen sensor, or damage to the three-way catalytic converter itself.
To retrieve fault codes, you need an OBD-II diagnostic tool. This is where the Autel scanner comes in.
Reading Process:
DTC Codes can be detected using Autel scanner.
Why is the Autel scanner recommended for reading and understanding DTC codes? The following are the main reasons:
DTC codes are the core of vehicle self-diagnosis. Understanding DTC codes and a list of common DTC codes, especially high-frequency codes like P0171 and P0420, can help drivers quickly understand their vehicle's condition.
With the Autel scanner, both DIY car owners and professional technicians can quickly read, interpret, and resolve DTCs, significantly improving repair efficiency and avoiding unnecessary costs.
If you're looking for a tool that meets everyday needs while also providing professional functionality, the Autel scanner is a worthwhile investment.
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