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Integrated diagnostic systems are present in most cars and trucks today. During the 1970s and early 1980s, manufacturers began using electronic means to monitor functions and diagnose engine problems. This was primarily to meet EPA emissions standards. Over the years diagnostic systems have become more sophisticated. The most recent standard is OBD-II, this standard was introduced in the mid 90s, it controls the engine almost completely and also monitors parts of the chassis, body, accessories and the car's control diagnostic network.



To combat the pollution problem in the Los Angeles basin, the state of California required emissions control systems on automobiles of the 1966 model year. The Federal Government extended these controls nationwide in 1968.

The US Congress passed the Clean Air Act in 1970 and established the Environmental Protection Agency (EPA). This began the publication of a series of emissions standards and requirements for vehicle maintenance. To meet these standards, vehicle manufacturers created electronic fuel supply and ignition systems where sensors measure engine performance and adjust the systems to reduce pollution. These sensors also began to be used to diagnose the vehicle.

At the beginning there were few standards and each manufacturer had its own systems and signals. In 1988, the Society of Automotive Engineers (SAE) established a standard connector and set of diagnostic test signals. EPA adapted most of the standards from SAE's diagnostic programs and recommendations. OBD-II is an expanded set of standards and practices developed by SAE and approved by the EPA and CARB (California Air Resources Board) for implementation on January 1, 1996.


The Environmental Protection Agency is responsible for reducing "mobile emissions" from cars and trucks and has the power to require manufacturers to build cars that meet increasingly stringent emissions standards. Furthermore, manufacturers must maintain automobile emission standards for the life of the vehicle. OBD-II provides a universal inspection and diagnostic method to ensure that the car is working within the manufacturer's specifications. While there is an argument as to the exact standards and methodology used, the fact is that there is a need to reduce the level of pollution, caused by vehicle emissions, in our cities, and we have to live with these requirements.


There are three basic OBD-II protocols in use, each with minor variations in the communication pattern between the on-board diagnostic equipment and the scanner. Although there have been some manufacturer changes between protocols in recent years, as a general rule, Chrysler vehicles, European and Asian vehicles use the ISO 9141 protocol. GM vehicles use the SAE J1850 VPW (variable pulse width modulation) protocol. and Ford vehicles use SAE J1850 PWM (Pulse Width Modulation) communication patterns.

You can also tell what protocol a car uses by examining the connector. If the connector has a pin in position #7 and no pin in position #2 or #10, then the car has the ISO 9141 protocol. If no pin is present in position #7, the car uses an SAE protocol. . If there are pins in positions #7, #2 and #10, the car can use the ISO protocol.

While there are three OBD-II electrical connection protocols, the command set is set according to the SAE J1979 standard.


Pre-OBD-II cars had multiple connectors in various locations under the dash and under the hood. All OBD-II cars have a connector that is located in the cabin easily accessible from the driver's seat, so that a scanner can be connected with a cable

The “Check Engine” light or MIL

The Check Engine light or MIL. It has three types of signals

Occasional flashes show a temporary malfunction.

Permanently on if the problem is more serious.

Constant flashing if the problem is very serious and can cause serious damage if the engine is not turned off immediately.

In all cases, a reading is taken from all the sensors and stored in the vehicle's central computer.

If the fault signal is caused by a serious problem, the MIL light will be on until the problem is resolved and the MIL light is reset.

Intermittent faults turn on the MIL momentarily and turn off before the problem is located. The reading from the sensors at the time of the failure, which is stored in the computer, can be of high value in diagnosing these problems. However, if the vehicle completes three driving cycles without the problem reappearing, the reading is cleared.


Fault codes are regulated by the SAE J1979 standard and is the standard used by vehicle manufacturers today. Fault codes consist of 5 characters, which are a letter followed by four numbers.

The first character, which is a letter, indicates the function of the vehicle according to the following.

P - Powertrain or engine and transmission (Powertrain)

B - Body

C - Chassis

U - Undefined

The second character indicates whether the code is generic, defined by SAE, or specific, defined by the vehicle manufacturer.

0 – Generic for all brands and defined by SAE.

1 – Specific defined by the vehicle manufacturer, the code is generally different for each manufacturer.

Codes 0001 to 0999 are completely defined by SAE. Codes from 1000 to 1999 are defined by the manufacturer and only follow the SAE standard in format.

The third character indicates the vehicle subsystem.

0 - The complete electronic system

1 and 2 - Combustion control

3 - Ignition system

4 - Auxiliary emission control

5 - Speed and idle control

6- ECU and inputs and outputs

7 - Transmission

The fourth and fifth characters indicate the fault.

The following image represents what we have described.



OBD2 error code readers began to be one of the first tools that car owners began to use in recent years to diagnose problems related to engine and transmission management due to the simplicity of their operation, generating for the which uses important failure data.

The popularity of these code readers has increased in recent years, having the same diagnostic connector, along with connection protocols already pre-established in all cars to connect with the engine computer, regardless of the brand or style. of these readers an almost pocket-sized basic tool due to its small size.

We could never compare a code reader like these with the famous scanners that most mechanics have, since these scanners are more powerful tools in exploring all the control units or modules that work in the car.

In this way, these code readers, for the most part, limit themselves to informing us why the service engine soon or check engine light is on (both mean the same thing), extracting the error code that has been generated and also having the possibility of delete it, however in almost all cases this information will only refer to the engine and transmission.

Some code readers, in addition to extracting and erasing the code, also allow you to see in real time the operation of some engine sensors (not all) in addition to having frozen data that gives us an idea of what condition the engine was in when the problem was generated. Error code.

There are also these Bluetooth tools so that a small interface connects to the diagnostic connector and with software either on a computer or even on a smartphone we will have access to all this information, making it so that even with our cell phone we can obtain a diagnosis that indicate why the yellow check engine light stays on, some of these applications allow you to create custom clocks for speed, rpm, engine temperature, average consumption and much more.


Extract the error code.

Provide frozen data

Reading data in real time from some sensors

Code erasing

Most are limited to engine and transmission.


ABS brakes



Traction control

Electric steering

Air conditioning


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