When you first turn your car on, your vehicle goes into test mode. It runs through a series of diagnostic tests to ensure everything operates smoothly and up to the manufacturer’s guidelines. 

You’ll see the lights flash on your dashboard. They’ll stay lit for a second or two before turning back off. 

But what if the check engine lights don’t turn off? What are the different codes trying to tell you? Paying attention could be the difference between a minor repair and a significant malfunction. 

The History of Check Engine Light Codes

The Check Engine Light is a warning light on the dashboard of cars and trucks that alerts the driver to potential engine issues. When the Check Engine Light illuminates, the vehicle’s onboard diagnostic system (OBD) detects a problem and generates a code corresponding to the issue. This code can be read by a mechanic or an OBD scanner and is used to diagnose the problem and determine what repairs are needed.

The history of check engine light codes goes back several decades, and has evolved alongside advances in automotive technology. 

In the 1970s, the US government began implementing emissions standards for vehicles in an effort to reduce air pollution. As part of this effort, car manufacturers began developing onboard diagnostic systems to monitor engine performance and emissions levels. These early diagnostic systems were relatively primitive and could only detect a few issues.

In the 1980s, automotive technology advanced rapidly, and diagnostic systems became more sophisticated. This led to the development of the OBD-I system, used by car manufacturers from the late 1980s through the mid-1990s. OBD-I systems could detect a broader range of issues than earlier systems but were still limited in their diagnostic capabilities.

In 1994, the US government implemented new vehicle emissions standards, which required car manufacturers to use a more advanced onboard diagnostic system. This system, known as OBD-II, was more sophisticated than previous systems and could detect a broader range of issues more accurately.

One of the key features of OBD-II was the standardization of diagnostic trouble codes (DTCs). DTCs are five-digit codes that correspond to specific engine issues, and are used to diagnose problems with the vehicle. Before OBD-II, each car manufacturer used its own proprietary codes, which made it difficult for mechanics to diagnose and repair issues on different cars.

The standardization of DTCs made it easier for mechanics to diagnose and repair engine issues and allowed for the development of more advanced diagnostic tools. Today, most OBD-II scanners can read and interpret DTCs, which makes it easier for drivers to understand what is wrong with their vehicles

Types of DTCs

There are two types of DTCs: generic and manufacturer-specific. Generic DTCs are standardized across all car manufacturers, and are used to diagnose common engine issues. For example, code P0300 indicates a misfire in one or more cylinders, while code P0171 means a lean air/fuel mixture.

Manufacturer-specific DTCs are unique to each car manufacturer, and are used to diagnose issues specific to a particular vehicle make and model. For example, the code P0101 on a Toyota may indicate an issue with the mass air flow sensor, while the same code on a Ford may indicate a different problem.

Evolution of DTCs

Over the years, the number of DTCs has grown as engine technology has become more complex. The original OBD-II standard included around 400 generic DTCs, which has since grown to over 5,000 codes. These codes cover various engine issues, from minor problems like a loose gas cap to more severe issues like a faulty catalytic converter.

In recent years, there has been a push toward making diagnostic codes more user-friendly for drivers. Some car manufacturers have begun developing systems that can display simplified codes or plain-language descriptions of engine issues on the dashboard, rather than requiring drivers to decipher complex codes.

Triggering the check engine light

Over the years, cars have become complex technological systems that work and function through a variety of sensors. When a technician runs an OBD II diagnostic test, it provides a wealth of information that helps a mechanic pinpoint a problem. They consider real-time data and any information available from past performance issues to determine where the problem lies. 

Not all errors or issues will trigger the check engine light. This is especially true for low-priority codes that don’t have huge repercussions. This is why bringing your car in as soon as possible is essential if the check engine light does illuminate. If it’s enough to trigger the light, it’s serious enough to be a problem for your car’s engine. The manufacturer designed the OBD II system to be your guide and help you fix potential issues long before they worsen.  

What are the most common check engine light codes?

OBD II codes have been standardized to make it easier for mechanics to decipher where the problem lies. Every code starts with a letter:

P – powertrain
C – chassis
B – body
U – network

These letters will be followed by three or four digits that go further into which system is experiencing an issue. It can alert you to what part is involved. For example, a P0301 is used to state there is a problem with the ignition system and it’s with cylinder 1. P0302 would tell you the problem is with cylinder 2. 

Now that you know hundreds of codes can provide information on engine problems, you might think you need an owner’s manual just for the OBD II codes. It turns out that some check engine lights are much more common than others, shortening the list considerably. The top check engine light codes include:

P0010 – intake camshaft actuator circuit p[em
P0016 – crankshaft position / camshaft position sensor
P0102 – mass air flow circuit low input
P0113 – intake air temperature sensor
P0128 – engine coolant temperature below thermostat regulating temperature
P0133 – oxygen sensor slow response 
P0135 – oxygen sensor
P0141 – oxygen sensor heater
P0171 – fuel trim system lean 
P0172 – system too rich
P0174 – system too lean
P0218 – transmission over temperature condition
P0300 – engine misfire detected
P0301 – cylinder 1 misfire detected
P0302 – cylinder 2 misfire detected
P0303 – cylinder 3 misfire detected
P0304 – cylinder 4 misfire detected
P0325 – PCM knock sensor circuit
P0401 – exhaust gas recirculation flow insufficient
P0411 – EVAP system control incorrect purge flow
P0420 – catalyst system low efficiency
P0430 – catalyst system low efficiency 
P0440 – evaporative emission system
P0442 – evaporative emission system small leak detected
P0446 – EVAP vent solenoid valve control system
P0455 – evaporative emission system leak detected
U0101 – lost communication with TCM

Your car’s check engine light is illuminated and you have an OBD II code. Now what?

While having the code may alert you to where the problem lies, it does little to pinpoint the problem and make the correct repair. 

That’s where a relationship with a reliable mechanic comes into play. 

They can take the information the diagnostic codes are telling them, and use it to focus in on where the problem lies. A P0304 code will tell you there’s a misfire in cylinder 4, but it will take focus and critical thinking to determine the problem. Is it getting too much fuel? Is the spark reaching the cylinder? 

Once the problem is discovered, then the proper fix can be made. 

Is your car’s check engine light illuminating? Whether you know what OBD code it’s referring to or not, now is the time to bring your car in and get to the root of the problem. 

We’re here to help.