OBD-II is a widespread standard for self-diagnozing and reporting of car faults implemented on most cars produced after 1996.
It started as a way for automotive producers to measure and control car emissions, but today OBD-II is the standard communication interface for diagnozing various vehicle subsystems.
Your car is packed with sensors and small individual computers that integrate and monitor information from the sensors. When something goes wrong and needs your attention, you will see a blinking LED somehwere on your dash or some error or warning on screen. Or even worse, there will be no warning but you will notice a malfunction in some subsystem of your car (lights, parking mechanism, etc). Your auto technician will connect through the OBD-II car connector (see Figure 1 below) with some physical device (hand-held device, a laptop etc) and will pool your car’s computers for status information and error codes. All this is made possible by the OBD-II standard.
Differences between OBD and OBD-II
OBD (actually, the correct name is OBD-I) and OBD-II are two versions of the same thing: a communication interface. OBD-I is older and OBD-II is the currently used standard. Until the industry settled for one standard, there were many versions and iterations. There were ADLD, M-OBD, OBD-I, OBD 1.5 and finally OBD-II. There is also EOBD (European on board diagnostics), which is the European version of EOBD, JOBD (the Japanese version) and ADR 79/01 & 79/02 (the Australian OBD standard).
Unlike the OBD-I connector, which was sometimes found under the hood of the vehicle, you will find the OBD connector somewhere where the driver can reach it from her seat, usually below the dash or behind the ashtray.
The OBD-II connector has 16 pins, while the OBD has 20 pins. The physical OBD-II connector looks like the image below. The black is the female and the blue is called the male connector.
The OBD-II connector is specified by something called SAE J1962, a documentation where the hardware interface is described in detail.
OBD-II type A and OBD-II type B
There are two types of OBD-II connectors: type A for vehicles where the supply voltage is 12V and type B for vehicles at 24V. In Figure 1, you see a groove between the two rows of pins going from one end to the other of the connector. This is a type A female connector. The type B female connector has a groove which is interrupted in the middle (see Figure 2 below). This modification prevents type B male from being accidentally inserted into a type A female (a 12V male into a 24V female), because it would lead to a hardware fault.
When you purchase an OBD-II scanner, make sure the connector type matches the one on your car.
The OBD-II protocols
There are 5 singalling protocols that work on the OBD-II interface. These 5 protocols adhere to a common overall main structure, but also have various customizations (minor differences).
- ISO 15765 CAN – starting with production year 2008, all vehicles tend to use this protocol
- SAE J1850 PWM – mainly used by Ford Motor Company
- ISO 9141-2 – maily used by Chrysler, all European and most Asian vehicles.
- SAE J1850 VPW – General Motors
- ISO 14230 KWP2000
You will normally use a tool that scans your car through the OBD-II connector. The particular protocol used by your car is important and here is why. The scanner tools that implement all five protocols cost thousands (dollars / euros). The less expensive tools, which are the ones individual car owners and small shops will use, are usually implementing one particular protocol. So, when purchasing a scanner, make sure it suits the protocol of your car.
If you need to figure out which OBD-II protocol is used on your car, you have to examine the OBD-II connector. In an OBD-II connector, not all pins are populated (look at the blue connector in Figure 1 and you will understand what I mean). Here are the differences in the 5 protocols:
- SAE J1850 VPW –The connector should have metallic contacts in pins 2, 4, 5, and 16, but not 10.
- ISO 9141-2 / KWP2000 –The connector should have metallic contacts in pins 4, 5, 7, 15, and 16.
- SAE J1850 PWM –The connector should have metallic contacts in pins 2, 4, 5, 10, and 16.
- ISO 15765 CAN –The connector should have metallic contacts in pins 4, 5, 6, 14 and 16.
Does my car have OBD-II ?
Almost certainly, it does.
If your cars was manufactured after 1996, your car must implement the OBD-II standard. Also:
- you should be able to see the OBD-II conenctor from Figure 1 somewhere within reach of the driver.
- there should be a sticker with the text ”OBD-II compliant” under the hood
Take a look at Figure 1 above to see what kind of connector you should be looking for. Pay attention to the number of pins (see section Differences between OBD and OBD-II above)
OBD Fault codes
Through the OBD-II interface, your car will present you with something calles diagnostic trouble codes (DTCs). These codes are stored when a sensor in the car reports a reading that is outside the normally accepted range. These Codes should be interpreted in conjunction with the vehicle’s service manual.
Many people look into OBD-II scanning tools when they deal with a ”Check engine” light on the dashboard or some driveability problem. The information provided by OBD cand help save time and money as it can pinpoint to a specific problem cause rather than guessing and replacing components.
The Diagnostic Trouble Codes consist of 5 characters:
- The first letter indicates the family of DTC.
P: Powertrain, (i.e. engine and gearbox)
U: User network
- The first digit indicates if the code is generic or not (green digit):
0: Generic fault
1: Manufacturer fault
- The last three digits / characters indicate the actual fault code
There are 50000+ generic and manufacturer OBD-II Diagnostic Trouble Codes. When you encounter a code, simply Google it and you will reach one of the many online databases which stores them all, together with the interpretation and advice for how to proceed.
For example, let us say your OBD-II scanner is showing you DTC P0301. Googling what DTC P0301 means we find not just the meaning of this error code, but also common symptoms, causes and possible solutions.
There are roughly four main types of OBD-II scanners:
- Hand-held scan tools
These tools can be:
simple fault codes readers (possibly with the ability to reset the error codes) aimed at private car owners (Figure 3)
complex professional and expesnive tools with more advanced functions like setting specific electronic control unit (ECU) parameters, graphing engine parameters etc.
- Mobile devices
Mobile apps (that you can install on your phone or tablet) connect to your car through USB, bluetooth, WiFi or cellular adapters plugged into the OBD-II connector. See Figure 4 for 2 examples.
- Laptop based scanners
They consist of a cable that converts the signal from OBD-II to USB and plugs into your laptop (Figure 5) and a software that interprets the incoming data.
A laptop based scanner is definitely better than a hald-held scanner because:
- you have a larger display where much more data can be presented
- there is a wide array of software to choose from, for various functions
- a laptop has a large a amount of memory to store data
Of course, various laptop tools will have different abilities and limitations, as is the case with all other tools (mobile or hand-held)
You can see a compiled list of OBD software.
- Data loggers
These tools record data for later analysis. Usually, the regular car owner does not need to look into this kind of devices.
Is it okay to drive with an OBD-II scanner plugged in ?
It depends. The short answer is that driving with OBD-II plugged in should take place for short intervals of time, only when and as loong as it takes for you to investigate the particular issue.
The problem is twofold:
- Some OBD-II scanners draw more power from your car than others. This will not be a big problem while driving, but it can drain the battery when the car is idle.
- Many OBD devices switch the car into diagnostic mode which affects how your car prioritizes incoming safety data.
If you are looking into purchasing a device that you plan to leave plugged in on your car, you should read about how it behaves on the above two points.
There were instanced when people reported leaving hand-held scanners connected on OBD-II while driving the car and having the ECU malfunctioning within a few kilometers. In these cases, they had to pull over, disconnect the scanner and everything went back to normal – ECU worked properly again, with no DTCs shown.
Other people reported having a scanner from a repair shop being left on their car for a while to test emissions while driving and did not encoutenr any issues.
In conclusion, some scanner will cause problems while driving and other will not. When purchasing a new device, you should actively look for this information (on the device specifications, ask the seller or the manufacturer or look up online).