How to DIAGNOSE check engine light YOURSELF

how to read check engine light codes

How to Use the OBD-II Scanner

These instructions are useful regardless of what type of unit you have.

1.  Plug the unit into the port under the steering wheel, it's typically near where the driver's knees are when they're sitting in the driver's seat.

2.  Turn the unit's power on. Some run on the vehicle's power, some do not. Regardless, usually to pull the data from the vehicle's computer, the ignition needs to be in the on position.

3.  Next, press scan and follow the device's prompts. After it finishes scanning, press READ and it will retrieve the error code(s).

4.  There may be a long list of menu options. Select trouble codes from the menu. Follow the device's prompts and it will display the code along with the explanation if it is the type of device that does this.

For example, if the code is P0141 - Exhaust Gas Recirculation Flow Insufficient followed by the additional codes P0171 - system too lean (Bank 1) and P0174 - system too lean (bank 2), it means the oxygen sensors on both sides of the engine are reading too lean. This could indicate a problem with the fuel system and indicates that the sensor is incorrectly monitoring the amount of oxygen in the air/fuel mixture for the engine.

Tip: Doing your own detective work can be helpful in trying to diagnose and fix the specific problem that your vehicle has. For the most accurate results, use the internet to research the problem codes that your scanner is retrieving with the make and model of your vehicle.

Even though the problem code may indicate that the problem is with your oxygen sensor, replacing the sensor may not fix the problem. It may be something unrelated or a different part altogether. The only way to know is to research the code with your specific make and model of vehicle and perhaps seek out the advice of a mechanic that you trust.

What is an OBD-II Scan Tool?

An OBD-II scan tool, also sometimes called an OBD-II scanner, is a hand-held device that plugs into your vehicle's OBD-II port and connects to the vehicle's electronic control unit (ECU), which is essentially a computer. All vehicles manufactured in 1996 or later have OBD-II ports. When the scan tool is connected, it will retrieve any error codes that have been stored in the ECU memory.

When Do I Use the OBD-II Scan Tool?

When an error code (its formal name is Diagnostic Trouble Code or DTC) is stored in the memory of your vehicle's ECU, it will cause your check engine light to illuminate on your dashboard. Usually, it is an orange light in the shape of a car engine that might say Check Engine, Check, MIL, CEL, or Service Engine Soon. It is also known as the Malfunction Indicator Light (MIL). It means that something is wrong with your vehicle?s operation. It could be a simple fix or a more complicated problem. The answer to finding out exactly what the problem is lies in retrieving and then interpreting the error code stored in the ECU.

Diagnostic Trouble Codes

The OBD-II code consists of five characters. It begins with a letter and is followed by four numbers.

how to scan obd ii codes using an obd ii code reader

  • P stands for powertrain (engine & transmission), B stands for the body of the vehicle (which includes A/C and airbags), C stands for chassis and includes the ABS system, U stands for the user network, including the wiring bus.
  • The first number will be a 0 or 1. A 0 means the problem is generic, while a 1 indicates that the problem is manufacturer-specific.
  • The second number will be between 0-9 (and may also have an A, B, or C if you have a hybrid vehicle). Here is a further breakdown of this placeholder digit:
  • 1 and 2 refer to the fuel & air metering system, with 2 specifically referring to the injector circuit.
  • 3 refers to the ignition system or misfire.
  • 4 means the problem is with the auxiliary emissions controls.
  • 5 refers to the vehicle speed control and vehicle idle control system.
  • 6 refers to the computer output circuit.
  • A 7, 8, or 9 means the problem is with the transmission.
  • The last two numbers describe the specific fault in the location specified in the previous number. It is most helpful to refer to a database of fault codes in order to discover the specific problem or use an online code interpreter.

Choosing an OBD-II Scanner

There are many OBD-II scanners available, in many auto parts stores and online, and they range in price from about $13 to upwards of $100. The more expensive models perform more complicated and advanced functions, while the least expensive types are able to perform the bare minimum of tasks - usually retrieving the code and resetting the ECU.

While some low cost models might have all the functions you need (retrieving and erasing codes), a small percentage of them may be incompatible with your vehicle. Typically, they are designed to work for the most common car models and protocols. Protocols are essentially different types of ECU's, or computers. All protocols perform the same functions, but different car manufacturers use different protocols. To find out what protocol your vehicle uses and see if would be compatible with a low-cost device, refer to this document.

Higher-end OBD-II scanners will be more expensive but also perform more advanced functions. Some may have ABS or airbag capability in order to read or reset the ABS or airbag systems. They also typically have on-screen definitions to explain what the error code means. Despite your unit having an on-screen definition, it is always a good idea to investigate the code yourself to see how it relates to your specific make and model of vehicle. Identical problem codes on two different vehicles may mean different things. Other advanced functions may include:

  • live data capability being able to see engine performance in real time as you are driving is helpful for diagnosing problems.
  • memory ability to store codes in the scanner's memory for future reference.
  • printing capability
  • updates be able to install updates to keep up with changes in vehicle protocols.

OBD-II scanners are sometimes available for rent at your local auto parts store if you would rather not buy one. Other times, an auto parts store will read the code for you and possibly print out the codes and results that it retrieved for free.

OBD-II Scanner Alternative

There are Bluetooth adapters available that can retrieve problem codes using your smartphone. The OBDII Bluetooth adapter plugs into your vehicle's OBD-II port. Then you pair the adapter with your smartphone. After that, you will need to download a free app that reads and interprets problem codes. You can use the OBD-II Bluetooth adapter as an alternative to purchasing an OBD-II scanner. One type of this cable is called a  GoPoint GL1 cable, compatible with a free smartphone app.

Here is a list of highly rated OBD2 Bluetooth adapters:


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