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Check Engine Light Codes and OBD II Codes Explained

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If you drive a car, you’ve probably heard about the dreaded check engine light and check engine light codes. It’s the orange icon that lights up on the dash to inform you there’s something wrong with the car and that your wallet is likely to take a hit. Every time the CEL is triggered, your car logs an error code. These codes can be extremely useful in figuring out what went wrong quickly and efficiently.

In this guide, we’ll give you an in-depth explanation of what check engine light codes (also known as OBD II, or Onboard Diagnostics II codes) are, how to read them, and generally how to go about fixing them. You’ll also find extensive explanations for some of the most common OBD II codes and what they mean. Grab a cup of coffee and settle in. We’ve got plenty of ground to cover.

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The dreaded check engine light

The History of Check Engine Light Codes

Check engine light codes as we know them today are a fairly recent thing. In fact, it wasn’t until 1990 that all cars manufactured in the United States had to be fitted with an OBD II system. That being said, there were other means of running onboard diagnostics on cars prior to OBD II. Some of the first diagnostic systems go back to the invention of the computer-controlled fuel injection. For context, Bosch was one of the first to do control injection using electronics with their Bosch Jetronic system back in the mid ’60s.

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Regulating fuel injection using electronics meant that cars were far more efficient, far more environmentally friendly, and easier to optimize. It wasn’t long before various car manufacturers decided to use the new electronic engine management systems as a diagnostic tool.

First, companies like General Motors used assembly line diagnostic links, or ALDL diagnostics, to test their ECUs before the vehicle left the factory. That happened in the early ’80s. By the mid-’80s, many companies used their own proprietary diagnostic systems to monitor ECU performance. None of these were standardized and each came with its own interface. This entire group of competing diagnostic standards is known as OBD I.

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A true revolution in car diagnostics happened in the mid-’90s when California’s Air Resource Board made it mandatory that all cars sold within the state had to have to use the newly established and mandated OBD II diagnostic standard. The federal government followed suit, and what was once state legislation became a federal norm.

Needless to say, just about every car manufacturer looking to sell cars in the US was running OBD II in their cars by the end of the decade. Soon enough OBD II became the norm all over the world, giving birth to a global, uniform standard in automotive diagnostics.

How Does Onboard Diagnostics Work?

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Running diagnostics using a professional OBD II scanner

Digital engine management was only the beginning. As electronics became more widespread in the automotive industry, car manufacturers began putting all kinds of sensors all over their cars. Today, your average car features around 70 sensors in total, with approximately 15-20 of them monitoring the engine alone.

The OBD II diagnostic standard allows repair technicians to access the data provided by all of these sensors both in real-time and in the form of logs. When something goes wrong on your car, and one of the sensors logs an error, that event log remains until the issue is fixed or until the error has been cleared.

Do All Errors Trigger the Check Engine Light?

Not all errors or issues will trigger your CEL. This is especially true for low-priority codes triggered by intermittent issues. Any mechanic will tell you stories of cars that looked clean at a first glance but came with a log full of misfire codes that were never cleared.

The fact that not all check engine light codes trigger the CEL also means that the ones that do should not be ignored. There is a reason why car manufacturers designed their vehicle to notify you that there is a problem. Although some CEL-causing codes are benign, most of them require your immediate attention.

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When it comes to addressing those pesky check engine light codes, eEuroparts.com stands as your reliable partner. With a history spanning two decades, we’ve become more than just an auto parts provider – we’re your go-to destination for seamless solutions and unmatched prices.

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Reliable Support — Our quality customer support isn’t just a promise; it’s a cornerstone of our service. We’re here to ensure your experience is smooth and your check engine light woes become a thing of the past.

Who Can Read OBD II Codes?

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Hand-held OBD II scanner

Back in the day, the only people who could read check engine light codes and the rest of the OBD II codes were diagnostics technicians. And it made perfect sense. Computers were massive 30 years ago, and ⅔ of the hardware necessary to read these errors was too niche for the consumer market.

Needless to say, that has changed over the years. You can now get OBD II scanners that will work on just about any car. The cheaper ones will generally give you insight into the core systems and their respective errors, all while allowing you to execute basic operations such as clearing said errors. Affordable OBD II solutions come in the form of standalone wired or wireless scanners while there are also Bluetooth devices that you can control via a phone app.

That being said, most professionals use far more advanced diagnostic scanners that can read all the basic codes, but also the more obscure errors in non-essential systems. For example, an affordable scanner will detect a cylinder 1 misfire, but it won’t might have trouble detecting a higher emissions output or some other non-critical error.

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At the end of the day, selecting an OBD scanner comes down to your needs and how much you’re willing to pay to have them met. It’s a good idea to have at least some kind of OBD II reader in your arsenal because you never know when you might need it.

How to Decipher Check Engine Light Codes?

OBD II codes follow a standardized pattern. Every code features a letter followed by 3 to 4 digits. The letters you can encounter are the following:

P — Powertrain
C — Chassis
B — Body
U — Network

As far as letters go, you are most likely to deal with P codes as that’s where things usually go wrong. The first digit following the letter is binary. If you get a 1, that means it’s a manufacturer-specific code, while a 0 means it’s a generic code.

The second digit tells you which system is experiencing an issue, and includes the following:

1 — Emissions management
2 — Injection control
3 — Misfire or ignition issue
4 — Emission Control
5 — Vehicle Speed & Idle Control
6 — Computer & Output Circuit
7 — Transmission
8 — Transmission
9 — SAE Reserved
0 — SAE Reserved

The third and fourth digits further define the issue. For instance, P0301 means that you’re having a cylinder 1 misfire while P0302 tells that the second cylinder is misfiring. Each of the systems has its own set of variables, and this is very much car-dependent.

Cars with V engines will have two banks instead of one, as is common for inline engines. Because of that, they’ll have a different set of fault codes. Fortunately, you can easily find all of these codes online, so there’s no need to worry if you have an oddball engine in your car.

Most Common Check Engine Light Codes

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Example of OBD II connector location

If you’re now convinced that you’ll need to carry around a booklet full of OBD II codes, you’ve nothing to worry about. As it turns out, some codes are far more common than others. So much so that we’ve put together a list of the most common codes that car owners are dealing with. Keep in mind that this list is a work in progress, so expect it to grow over time.

P0010 — Intake Camshaft Actuator Circuit Open/Short on Bank 1
P0016 — Crankshaft Position Camshaft Position Sensor Bank 1
P0102 — Mass Air Flow Circuit Low Input
P0113 — Intake Air Temperature Sensor 1 Circuit High Input
P0128 — Engine Coolant Temperature Below Thermostat Regulating Temperature
P0172 — System Too Rich Bank 1
P0174 — System Too Lean Bank 2

P0218 — Transmission Over Temperature Condition

P0300 — Random/Multiple Cylinder Misfire Detected
P0301 — Cylinder 1 Misfire Detected
P0320 — Ignition/Distributor Engine Speed Input Circuit Malfunction
P0325 — Knock Sensor Malfunction (Sensor 1 Bank 1)
P0340 — Camshaft Position Sensor Circuit Malfunction

P0401 — Exhaust Gas Recirculation (EGR) System Flow Insufficient
P0430 — Catalyst System Efficiency Below Threshold Bank 2
P0446 — Evaporative Emission Control System Vent Control Circuit Malfunction
P0455 — Evaporative Emissions System Large Leak Detected
P0456 — Evaporative Emissions System Small Leak Detected

P0613 — TCM processor Issues
P0706 — Transmission Range Sensor Circuit Range & Performance
P0715 — Input/Turbine Speed Sensor Issues
P0720 — Output Speed Sensor Circuit Malfunction
P0740 — Torque Converter Clutch Circuit Malfunction
P0780 — Shift Malfunction
U0101 — Lost Communication with TCM

You’ve Read the Check Engine Light Codes, Now What?

As nifty as OBD II fault codes are, they aren’t always painting the whole picture. An OBD II fault code will sometimes point you to a component that is unmistakably faulty and in need of replacement. However, more often than not, a fault code will only represent the first piece of a much larger puzzle you have to solve.

If we go back to our P0301 code example. The ECU is telling you that there’s a misfire in cylinder one, but it won’t tell you what’s causing the misfire. With that information, we can now focus our attention on cylinder one and start troubleshooting it. Is it getting fuel? Is there a spark in the cylinder? Is there compression? Eventually, using deduction, you’ll find the real culprit and get the problem sorted out.

To confirm that you’ve applied the right fix, wipe the errors and start the car while reading live data from your OBD II reader. If you get no new errors and if your CEL remains dark, you’ve most likely done the job well.

Use Quality Parts and Stay On Top of Maintenance

The best way to avoid dealing with any DTCs is to stay on top of your maintenance schedule and use quality parts while doing so. Here at eEuroparts.com, we carry a wide range of genuine, OEM and quality aftermarket parts for all kinds of European vehicles. To find what you need, simply head over to our store, select your vehicle and search for the part in question. Following these simple steps will give you a list of results that are a guaranteed fit for your vehicle!

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2 thoughts on “Check Engine Light Codes and OBD II Codes Explained

  1. Good morning I just bought a 2006 Dodge caravan since I got it the tire pressure signal and the check gas signal is on my dashboard I don’t know much about vehicle’s but I am leaving qwickly these days so I am low on funds right now is there a liquid sealer I can possibly use temporarily any suggestions it’s my only transportation right now and the internal system has wiring electral system is going haywire soon as you put the key in the eniction

    1. Hi Tristana,

      Make sure to check your tire pressure on all four tires. If the pressures in all of your tires are okay and the light is still popping up, you might have a bad tire pressure sensor on one of your wheels. As for the check gas signal, you’ll need to be a bit more specific. Is it the light next to the fuel gauge that turns on when you’re low on fuel?

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