Common Check Engine Light Codes and What They May Mean

A Check Engine light is something that can almost immediately cause stress, fear, and confusion on the road. While many drivers understand that their Check Engine light represents a problem with one or more of their vehicle’s systems, most may not be sure what to do when their Check Engine light comes on, let alone how to accurately interpret diagnostic trouble codes, commonly called DTCs. Read on to learn what kinds of issues can most commonly cause your Check Engine light to illuminate, how to read Check Engine codes, and what some of the most common DTCs may mean.


What Can Cause Check Engine Light Codes?

Every modern vehicle is equipped with an On-Board Diagnostics (OBD) system, which is responsible for monitoring all of the electrical and diagnostic systems in the vehicle. Vehicles manufactured in 1996 or later are built with OBD-II diagnostic ports that are capable of communicating system problems by connecting an OBD-II code reader to the vehicle’s computer. OBD-II diagnostic systems are used to alert the driver of current and stored diagnostic trouble codes (DTCs), allowing for simpler diagnosis and helping narrow down the source of problems with your vehicle. These codes often indicate or help you diagnose the system or component that is causing problems in the vehicle's engine, emissions control system, and more.

Check Engine alerts can come in the form of illuminated images or icons of an engine, or may include text that says “Check Engine” or “Service Engine”. If your Check Engine light is on, your onboard diagnostic system is alerting you to problems occurring in an electrical component or sensor in one or more of your vehicle’s core systems, including the engine, transmission, or exhaust system. Typical OBD codes can be triggered by failed oxygen sensors, faulty spark plugs or spark plug wires, a bad mass air flow (MAF) sensor, failed or clogged catalytic converters or exhaust components, leaks in the air intake system, emissions system leaks, and loose, damaged, or missing gas caps.

The severity of the problem is indicated by either an illuminated or flashing Check Engine light. If the light is constant, it often means that a problem is present but likely does not require immediate attention or pose any instant threat to your vehicle if you continue driving. If the light is flashing, the problem poses a severe threat that could cause catastrophic failure and should be addressed immediately. Learn more about what to do if your Check Engine light comes on in our helpful How-To article.


Checking OBD-II codes with a scanner.

How to Use an On-Board Diagnostics (OBD) Scanner to Read Check Engine Light Codes

Seeing your Check Engine light appear on your dash can be both nerve-wracking and intimidating, especially if you’re not sure what types of issues can trigger system alerts. The first thing to do when your Check Engine light is on is to use an OBD-II scanner to accurately scan and read which system in your vehicle is triggering the Check Engine light. After the code or codes have been read, you can assess the potential needed repairs or continue narrowing down your diagnosis. OBD-II readers can also be used to read and display DTCs, but may not provide as much depth as OBD-II scanners.

To use an OBD-II scanner, you must plug it into the vehicle's OBD-II port. The port is typically located under the driver’s-side dash, near the pedals, and is shaped like a trapezoid. Plug the OBD-II scanner into the port, and follow the instructions that came with your scanner to locate and select the appropriate option to read codes. Then you can simply wait for the recorded codes to display on the screen. Write the codes down exactly as they are portrayed on the scanner screen, just in case you need to refer to them at the repair shop or when shopping for the parts needed to make your repair.

If you don’t have an OBD-II scanner, you can find one at O’Reilly Auto Parts. You can also visit O’Reilly Auto Parts and ask one of our parts professionals about our free Check Engine Light Testing with O'Reilly VeriScan to have your OBD-II codes scanned at your local store. You'll be provided with an O'Reilly VeriScan report to help you identify possible causes and repair solutions for your illuminated Check Engine light.
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Interpreting OBD Codes

If you don’t have experience reading OBD-II codes, they can look intimidating and confusing at first. But if you break down the code into sections, and you know what each number and letter in the sequence represents, it can be easier to identify what the code means and which problem it refers to in your vehicle.


Infographic explaining the meanings of the different parts of an OBD code.

Section 1: Code Category

The first character in the OBD-II code sequence represents the general vehicle area causing the code to appear. There are four main code categories and characters that represent them: Powertrain (P), Body (B), Chassis (C), and Network Communications (U).

Typically, professional-grade OBD-II scanners are the only tools that can read codes outside of powertrain codes. However, more often than not, powertrain problems are the most common cause of an illuminated Check Engine light.


Section 2: Code Types

The second character in the OBD-II code sequence represents the code type and will be listed as either “1” or “0’. There are two code types: Enhanced or manufacturer-specific codes (1), which are codes only recognized by specific vehicle types based on manufacturer settings, and generic or global codes (0), which are globally-recognized codes in every vehicle’s OBD sensor system.


Section 3: System Being Represented

The third number in the OBD-II code sequence will be a number from 1 to 9 and represents which specific system in your vehicle is triggering the Check Engine light. The systems are represented as followed:

  • 1 and 2 - Fuel and air metering and injection systems
  • 3 - Ignition systems
  • 4 - Emissions systems
  • 5 - Speed and idle control systems
  • 6 - Computer output circuit systems
  • 7 and 8 - Transmission system

Section 4: Trouble Code Description

The final two numbers in the code sequence list the exact problem description for the present code.


What are the Most Common OBD-II Codes?

While we aren’t able to list manufacturer-specific codes for your vehicle, we can tell you about some of the most common universal OBD-II codes that might be triggering your Check Engine light, and the symptoms that typically accompany them. There are a few systems in your vehicle that contain sensors and other components that are recognized by your vehicle's OBD system, and problems with these parts or sensor parameters that are outside of specification will illuminate your Check Engine light. Misfire codes, oxygen (O2) sensor codes, mass air flow (MAF) system codes, manifold absolute pressure (MAP) system codes, evaporative emissions (EVAP) system codes and throttle position sensor (TPS) codes are all examples of universal vehicle systems that could be the cause of your illuminated Check Engine light. If your Check Engine light turns on, and you have scanned and checked for OBD-II codes, inspecting these core systems and components is a great place to start while trying to narrow down the cause of the problem.

Below is a list of the most common OBD-II codes, and the symptoms you may experience if your vehicle is alerting you of these specific codes.


  • P0300 - Random or Multiple Cylinder Misfire Code
    If this code appears on your OBD-II reader or scanner, this means more than one of your engine’s cylinders is misfiring. In addition to an illuminated Check Engine light, you will likely notice frequent shaking in your steering wheel or in the body of your vehicle while the engine is idling. If your engine has multiple misfiring cylinders, it can be caused by a number of factors, including a clogged fuel filter, a bad fuel pump, a fuel supply issue, worn or damaged ignition components, or a failing fuel pressure regulator. P0300 codes are often accompanied by a group of other codes that specifically pinpoint each problem cylinder.
  • P0301 through P0308 - Single-Cylinder Misfire Codes
    These codes specify which engine cylinder is misfiring. If cylinder 1 is misfiring, P0301 will appear, if cylinder 2, P0302, and so on and so forth as the final digit in the code increases. This allows you to locate individual problem cylinders faster, hopefully reducing diagnostic and repair time. The most common causes of cylinder misfires are damaged or worn ignition coils, spark plugs, or plug wires, clogged or damaged fuel delivery systems or fuel injectors, or problems with compression or valve sealing. These components are a good place to start when searching for the cause of cylinder misfires.
  • P0446 - Evaporative Emissions Control System Codes
    This code occurs when your vehicle’s OBD-II system senses a problem with the evaporative emissions (EVAP) system experiencing insufficient or excessive fuel system venting. If this OBD-II code appears on your reader, we suggest inspecting your fuel system for any clogs that may be caused by dust, dirt, or debris. These clogs can prevent your vehicle’s EVAP vent solenoid from circulating gasoline vapors or emissions from your fuel back into your engine and regulating the system's pressure levels. If your EVAP system is not able to vent properly, it can trigger this OBD-II code.
  • P0455 - Evaporative Emissions Control System Codes - Leaks
    Another OBD-II code related to the EVAP system is P0455, which suggests an apparent leak somewhere in your vehicle’s emissions system. Your vehicle may have fuel vapors escaping from the fuel tank or elsewhere in the fuel system and leaking into the atmosphere. Symptoms accompanying this code might include a strong smell of gasoline near the fuel tank of your vehicle or coming from your engine or fuel lines. The first thing to check for if this code comes across your OBD-II scanner is a loose gas cap. If your gas cap is not loose or damaged, you might have a leaking or cracked EVAP hose, a loose or open solenoid purge valve, or a damaged charcoal canister.
  • P0420 or P0430 - Catalyst Efficiency Codes
    These OBD-II codes will appear if the system senses a problem with your vehicle’s catalytic converter. Which code you will get will depend on which bank of the engine is producing the code. A 4-cylinder engine will result in a P0420 code, while a 6-cylinder engine will result in a P0430 code. Catalytic converters are important components in your exhaust system that are responsible for converting harmful pollutants in exhaust fumes into less toxic gasses which are not as harmful to the environment. If your catalytic converter is clogged or damaged, the airflow through your exhaust system needed for adequate performance can be reduced, resulting in decreased system performance. A damaged catalytic converter can also cause a slight sulfur smell, reduced engine power, poor fuel economy, or occasional engine misfires.
  • P0141 - Oxygen (O2) Sensor Codes
    Damaged or failing oxygen sensors are a common cause of an illuminated Check Engine light. If your O2 sensor is damaged, you might experience poor or reduced fuel economy or poor engine performance. If your Check Engine light is on and you receive P0141 as a code, you should check your O2 sensors for any loose or damaged electrical connections. These codes also often indicate the bank and sensor number of the oxygen sensor that is out of specification, and this indicates the relative position of the sensor that requires attention. In V-style engines, bank 1 would indicate the side of the engine on which cylinder 1 is located, and bank two would be the opposite side. From there, sensor 1 is the upstream oxygen sensor on that side of the engine, meaning it is in front of the catalytic converter, and sensor 2 would be downstream, or behind the catalytic converter. With many modern vehicles having up to four oxygen sensors, this can be very helpful to understand before beginning your diagnosis or repair.
  • P0171 or P0174 - Lean Code
    These OBD-II codes are triggered by a lean-running engine. If your engine is running lean, there is an excessive amount of air in the fuel mixture used to start and keep the engine running. Vehicles have a specific air-to-fuel ratio specification, and too little fuel or too much air can disrupt this balance and cause poor performance. If your engine is receiving more air than fuel, it might be running lean. Loss of power and stalling are two common signs that accompany lean OBD-II codes. This can be caused by a damaged fuel pump, a clogged fuel filter, leaking or disconnected hoses, or a bad fuel injector. If you receive these OBD-II codes on your scanner or code reader, you may need to perform a further diagnosis to determine the source of the problem.
  • C0221 - Wheel Speed Sensor
    A C0221 OBD-II code is triggered when you have a bad wheel speed sensor. Your wheel speed sensors are an important component of your vehicle’s anti-lock braking system (ABS). These sensors monitor tire rotation to determine vehicle speed, and this also allows the computer to determine if there is a loss of traction at one or more wheels to engage the ABS or traction control systems. If a wheel speed sensor fails, you will likely have a warning ABS light along with a Check Engine light, which can disable your ABS system and make your brakes less effective on wet or slippery surfaces. If your ABS light is on, and your Check Engine light is on, check your wheel speed sensors for any visible damage, and check electrical connections for any sign of corrosion or damage.

What To Do If Your Check Engine Light Comes On While Driving

If your Check Engine light comes on while you’re on the road, find a safe place to pull over and assess any potential causes of the light. If you have an OBD-II scanner or reader with you, connect the tool to the OBD-II port under your steering wheel to scan for problem codes. If the Check Engine light is steady and constant, and you don’t have a scanner or reader handy, find the nearest O’Reilly Auto Parts store and stop in to have your Check Engine light tested for free with O'Reilly VeriScan. If your Check Engine light begins flashing while on the road, find a safe place to pull over immediately, as a flashing warning light generally indicates a more serious problem that could lead to catastrophic engine failure if you continue to operate your vehicle. O’Reilly Auto Parts has a variety of OBD-II scanners and readers for every type of mechanic, as well as all of the parts and tools you need to make any repairs.