A good drive can quickly go south when a check engine or any other warning light appears on your dashboard.
These dashboard lights can come on when your car registers a diagnostic trouble code (DTC), possibly due to a faulty oxygen sensor or an airbag problem.
But here’s the tricky part:
Without specialized diagnostic tools and repair experience, pinpointing the code’s trigger is next to impossible. This uncertainty can make your ride feel unreliable.
So, let’s break down the DTC codes — how they work, the steps to extract them, and ways to clear them effectively.
This Article Contains:
- What Is a DTC?
- How Do DTCs Work?
- How to Extract and Read Trouble Codes Step-by-Step
- How to Interpret OBD-II Codes
- What Are Some Common DTC Codes?
- How Do You Evaluate the Severity of an OBD Code?
- What’s the Difference Between Critical and Non-Critical Codes?
- Is It Necessary to Clear a Diagnostic Trouble Code?
- How to Clear Diagnostic Trouble Codes in 3 Simple Steps
- 4 FAQs on DTC Codes
What Is a DTC?
DTC stands for Diagnostic Trouble Code.
Your vehicle’s onboard diagnostics (or OBD system) uses these 5-digit alphanumeric codes to alert you about a failing part or performance problem.
Initially, several versions of onboard diagnostics interfaces were available, and different guidelines were used to resolve the problem. However, on January 1, 1996, the Society of Automotive Engineers (SAE) introduced a standard list of common codes for every vehicle manufacturer.
They also provided two primary protocols for reading a DTC code:
- OBD-II: For light and medium-duty vehicles (weighing 6,000 to 26,000lbs)
- J1939: For heavy-duty vehicles and equipment (weighing 26,001 to 33,000+ lbs)
Now that you know about OBD-II and J1939 DTC codes, let’s see how they activate.
How Do DTCs Work?
Modern cars have various control modules to monitor key systems. While the engine control module (ECM) keeps your car’s performance in check, other modules manage the transmission, brake, HVAC (heating, ventilation, and air conditioning) system, etc.
However, if one of these modules detects an irregularity — such as a misfiring cylinder or faulty ABS sensor — it logs an error code. As this code registers, the onboard diagnostic system illuminates the CEL or the associated dashboard light to alert you about the issue.
But how do you determine what these warning lights are signaling?
Let’s take a look.
How to Extract and Read Trouble Codes Step-by-Step
You need an OBD scanner or code reader to retrieve the fault codes and understand why the dashboard light is on. Here’s how you can extract codes with an OBD scanner:
- Plug the code reader into your car’s 16-pin onboard diagnostic connector. It’s located under the dashboard on the left of the steering column. If not, refer to the owner’s manual for the precise location.
- Turn your car’s key to the ‘ON’ position with the engine off to power the diagnostic tool. This will help the tool connect with the ECM.
- Access the scanner’s menu and select the option “read codes.” Your OBD-II scanner will automatically detect your vehicle model and year. If it doesn’t, you’ll have to enter the specifics manually. The scanner will then retrieve the registered codes.
- Check the scanner’s screen — it’ll display the code definition or a one-line DTC meaning to help you understand the issue. A modern OBD-II scanner will also let you transfer this information to a smartphone via Bluetooth or Wi-Fi for added convenience.
The DTCs or manufacturer-specific codes you extract can be one of thousands. If this feels overwhelming, we’ll break it down one character at a time.
How to Interpret OBD-II Codes
The alphanumeric codes have five characters, and here’s what they mean:
1. First Character: Alphabet (System Type)
An OBD-II diagnostic trouble code starts with an alphabet denoting the system with a fault, for example:
- P (powertrain): Covers transmission, engine, fuel system, and other associated accessories
- C (chassis): Includes mechanical systems and functions outside the passenger compartment like steering, suspension, and braking
- B (body): Car parts primarily found in the passenger compartment region, such as airbags
- U (network and vehicle integration): Includes functions or communication controlled by the OBD system
2. Second DTC Character: Number (Code Type)
The second DTC character is a number that determines if the diagnostic trouble code is standardized or not. It’s either 0 or 1, where:
- 0 denotes a standardized SAE international code. It’s known as a generic code or global code
- 1 represents a manufacturer-specific code, also known as an enhanced code
3. Third Character: Number (Subsystem Indicator)
This number helps you identify the subsystem that triggers the fault code, such as:
- 0 indicates fuel and air metering as well as auxiliary emission control
- 1 denotes fuel and air metering
- 2 represents fuel and air metering (injector circuit)
- 3 denotes the ignition system or a misfire
- 4 indicates auxiliary emission control
- 5 denotes vehicle speed control, idle control system, and auxiliary inputs
- 6 indicates the computer and output circuit
- 7, 8, and 9 represent transmission and gearbox issues
NOTE: You can also encounter codes with A, B, and C as the third character, where A stands for hybrid propulsion systems, B for body systems, and C for chassis systems. For example, the ECM logs the P0A80 when it detects an issue in the battery management system. This common code often registers in hybrid vehicles.
4. Fourth and Fifth Characters: Number (Exact Fault Description)
The fourth and fifth characters of the DTC are digits indicating the specific problem or malfunction your car is experiencing.
It can be a number between 0 and 99.
To better understand, let’s take the example of fault code P0573:
- P = “powertrain,” the system with the fault code
- 0 = indicates it’s a generic code
- 5 = relates to vehicle speed control, idle control system, and auxiliary inputs (the subsystem affected)
- 73 = specifies the fault “Cruise Control/Brake Switch A Circuit High”
Wondering which codes your car can potentially trigger?
Keep scrolling to familiarize yourself with a few common ones.
What Are Some Common DTC Codes?
Your car can malfunction for various reasons, depending on its condition and the weather you drive in. Here are some codes that you may encounter:
A. Powertrain (P-code)
- P0300: Random or multiple cylinders misfire detected
- P0130: O2 sensor (oxygen sensor) circuit malfunction (Bank 1, Sensor 1)
- P0141: O2 sensor heater circuit malfunction (Bank 1, Sensor 2)
- P0442: Small system leak within the evaporative emission control system
- P0352: Ignition coil B primary or secondary circuit malfunction
B. Chassis (C-code)
- C0035: Left front wheel speed circuit malfunction
- C0040: Right front wheel speed circuit malfunction
- C0225: Left front wheel speed sensor circuit open
- C0221: Right front wheel speed sensor circuit open
- C0045: Left rear wheel speed circuit malfunction
C. Body (B-code)
- B1203: Fuel sender circuit short to battery
- B0012: Passenger frontal deployment loop circuit
- B0022: Driver frontal deployment loop resistance low
- B1927: Faulty passenger side airbag
- B0092: Passenger presence system (PPS) detection error
D. Network and Vehicle Integration (U-code)
- U0100: Lost communication with ECM/PCM “A”
- U0073: Control module communication bus “A” off
- U0121: Lost communication with ABS control module
- U0155: Lost communication with instrument panel cluster (IPC) control module
- U0140: Lost communication with body control module
The list may look daunting, but not all trouble codes are serious.
Here’s how you can verify.
How Do You Evaluate the Severity of an OBD Code?
You can determine the criticality of the codes with these steps:
- Assess the dashboard lights: A steady CEL usually signals non-critical issues, while a flashing one means something serious, like an engine misfire. These misfires can trigger critical codes like P0301 — “Cylinder 1 Misfire Detected”.
- Scan the code: Use a scanner to retrieve the fault code, which helps pinpoint the system with the DTC. For example, C-code (chassis) relates to steering, suspension, and braking systems, which tend to be serious.
- Check the DTC meaning online: Once you retrieve the code, search for its meaning and symptoms or refer to your owner’s manual. A failed emissions test may be harmless, but signs like odd noises or engine overheating require urgent investigation.
- Evaluate the symptoms: Rough idling, stalling, and braking issues often accompany critical codes that need attention. Meanwhile, DTC related to a vacuum leak may affect your fuel economy but doesn’t warrant an emergency repair.
Based on the code and the symptoms you identify, you may either have a critical or non-critical code.
What’s the Difference Between Critical and Non-Critical Codes?
The main difference lies in how promptly they require attention and their impact on your safety.
Let’s read further to understand better:
1. Critical Codes
A critical DTC needs urgent attention because it can severely damage your car. For example, the C-code C0110 indicates an issue with the ABS pump motor’s circuit. This code is critical as it directly affects your car’s braking ability.
2. Non-Critical Codes
A non-critical DTC code isn’t urgent but still needs a proper diagnosis.
For example, P0463 is a common code where your fuel sensor is sending a high signal to the ECM, resulting in inaccurate fuel gauge readings. It doesn’t affect your driving safety or the vehicle’s stability, so you can still drive to the nearest garage for repair.
Think you can skip clearing the codes?
Let’s see if that’s really an option.
Is It Necessary to Clear a Diagnostic Trouble Code?
Yes, it’s necessary.
If you have limited knowledge of cars and repairs, you may fail to determine the trouble code’s severity. This incomplete information can be risky, especially if the active codes are related to the engine or transmission systems.
Driving with such critical system codes can be unsafe, so consult a local expert or mobile mechanic for help ASAP.
That said, there are a few ways to clear the codes.
Read on to find out what they are.
How to Clear Diagnostic Trouble Codes in 3 Simple Steps
The easiest way to clear a code is to fix the specific problem triggering it. However, if the codes are active after repairs, your mechanic can perform one or all of these steps:
- Use a code reader: Connect the scanner to your car’s OBD-II port. Then, press the “erase” or “clear” button to reset the code.
- Reset the battery: Remove the negative cable and drain the remaining power by turning on the lights. Then, reconnect the battery after 15 minutes. Your car’s systems will reset and clear the error code by this time.
- Drive your car multiple times: If the CEL stays on after a repair, your vehicle may not have completed its driving cycle. Drive it several times to let the computer reset and clear the code.
Now that you have a general overview of DTC codes, check out some FAQs.
4 FAQs on DTC Codes
Here are the answers to some questions you may have about DTC codes:
1. What Are Permanent DTC Codes?
Permanent diagnostic trouble codes (PDTC) are similar to other codes, except you can’t clear them by simply using a diagnostic tool or resetting the battery.
The only way to clear them is by fixing the underlying issue and resetting the code. Examples of PDTCs include:
- P0133: Oxygen sensor circuit slow response (Bank 1, Sensor 1)
- P0421: Warm-up catalyst efficiency below threshold (Bank 1)
2. How to Know If Your Vehicle Is OBD-II or J1939 Compliant?
If your car or light truck was manufactured in the US after January 1, 1996, it’s OBD-II compliant. Whereas heavy-duty trucks, ambulances, and military vehicles are J1939 compliant.
Here’s how to confirm it:
- For OBD-II compliance:
- Look under the hood near the engine for a label that says “OBD-II Compliant.”
- Search for a 16-pin diagnostic connector in the cabin under the dashboard.
- Check the dashboard for the “check engine” light symbol, which mainly appears in OBD-II-compliant vehicles.
- For J1939 compliance:
- Look for diagnostic connectors with pins between 6 and 14.
If you’re still unsure, check your owner’s manual or contact your dealership to verify.
3. Can You Monitor the DTCs of a Fleet?
Yes, and it plays a significant role in fleet management.
Instead of scanners designed for individual cars and light vehicles, fleet managers can use telematics systems to monitor their fleet. These systems help with code reading and combine GPS tracking, fuel monitoring, as well as maintenance alert data to prevent costly repairs.
With telematics systems in place, fleet managers can:
- Keep up with the routine vehicle diagnostics of their fleet
- Access real-time reports of active DTC fault codes
- Evaluate historical data and use features like an occurrence counter to see how often specific codes appear
Using this data, managers can quickly collaborate with fleet mechanics to fix their vehicles.
4. What Are the Benefits of Monitoring Diagnostic Trouble Codes?
Routine DTC monitoring helps:
- Improve driver safety: Illuminated dashboard lights alert you to visit a mechanic for prompt inspection and repair, keeping you safe in the future.
- Execute data-driven decisions: Mechanics can export freeze frame and real-time DTC data from your car’s computer to pinpoint the root issue. This specific information helps them diagnose problems systematically.
- Increase savings on repairs: Fixing issues like a faulty coolant sensor early can help you save money on extensive damage from engine overheating.
- Stay compliant with environmental standards: Regular vehicle diagnostics and fixes ensure modern cars meet the emission benchmarks set by the Environmental Protection Agency and pass required inspections.
- Reduce fleet vehicle downtime: Appropriate fleet management software can help stakeholders stay on top of vehicle maintenance, ensuring their business runs smoothly.
Get Fast and Convenient DTC Repairs with AutoNation Mobile Service
An illuminated dashboard light can be stressful, especially when you don’t know what’s causing it.
If you’re worried it might be serious, get it checked by AutoNation Mobile Service mechanics and put your mind at ease.
We’re a mobile auto repair solution that offers after-hours support and affordable repairs. Our trusted experts can help you decipher DTC codes and resolve underlying issues right from your driveway.
Contact us to get effortless auto repair and maintenance services.