Modern cars contain many sensors, actuators and computers. These produce, read and process data all about the car's current state, including both faults as well as general operating parameters.  

An OBD port facilitates a secondary computer to be plugged into the cars onboard ECU, allowing the data to be downloaded and even displayed in real-time.  

The physical OBD port is a standardised connection between all makes and models of car, the software on the other hand, can be less universal, as we’ll talk about below. 

What is an OBD port used for? 

Like most connections between computers, data can be transferred in both directions, this allows more than just the reading of stored error codes: 

Reading data 

There are upwards of 10 different sensors and data streams on a modern car, including basic things such as engine temperature, and engine RPM. But in order for an ECU to do its job, it needs to know everything about the car, from exhaust gas composition to throttle pedal position.  

All of these parameters are available to a mechanic, or even the vehicle owner, with the right diagnostic tools. For example, even a basic OBD reader can tell exactly what temperature the engine coolant is at, or what the engine timing is. 

Diagnosing problems can be made much easier by using the data to eliminate possibilities, such as ruling out a faulty throttle pedal sensor - a common issue on some cars - by examining the percentages being outputted to the ECU, through an OBD tool. 

Reading stored codes 

When a fault develops, the ECU can usually detect the abnormality via the various aforementioned sensors. In most cases, something like a misfire caused by faulty spark plugs will simply cause the engine warning light to illuminate. 

After the problem has occurred, even if it is an intermittent fault - the ECU will store the relevant information in its memory. This is then displayed as a code when viewed, allowing a mechanic to diagnose the problem. 

Modifying your car 

Because the ECU defines so much about the car and engine, many parameters can be modified to suit the driver or upgrade the car. 

With the relevant software, changes can be made such as increasing the engine’s performance - also known as a remap - and tweaking the way the lights operate to a more convenient way. 

There are some consumer-grade options for doing this such as Carly.  

What is the difference between OBD1 and OBD2? 

The beauty of the OBD system is that the ports are standardised, allowing any car to be connected using the same cable, in a similar way that USB is the standard for mobile phones and laptops.  

However, because the first iteration of OBD (OBD1) was introduced in 1981, the technology in cars was fairly limited. This meant certain features we now take for granted had not been made commonplace.

It was also only added to certain cars sold in certain locations, mostly from American manufacturers to the Californian market.  

OBD2 was introduced in 1996 and included many more features. This new standard was developed to be future-proof, unlike OBD1, and is still used today.

Below is an OBD2 port.

What year cars have OBD2? 

OBD2 was introduced in 1996, but not all manufacturers adopted it immediately, many makers only introduced it due to legislation, the guide below will help identify if your car is likely to have OBD or not.

• Cars made for the US market - All cars from 1996
• Petrol cars made for the EU market - All cars from 2000
• Diesel cars made for the EU market - All cars from 2004

Remember, your car may have an OBD2 plug, even if it was made before these dates, check below on where to find your OBD port to be certain. 

Where is my OBD2 port? 

Whilst all manufacturers differ on the best place to put the port, there are some usual places to check first, namely under the dashboard. The port will almost never be in the engine bay. Some cars hide the port using a piece of plastic trim, it may have ‘OBD’ written on it. 

The best way to check your specific car is to read the user manual, failing that a quick search online will almost always help, don't forget to include the year of your car. 

How do I test my OBD port? 

If you have tried to plug in a device to your OBD port, and it’s not working correctly, it may be difficult to pinpoint the cause.  

First, check the OBD connector itself for damaged pins or breaks to the cable. If you have access to another OBD scan tool or device, try that. Alternatively, if you have another car with an OBD port, try connecting your device to that.  

If these tests prove fruitless, it may require a trip to a garage, their professional equipment will be able to tell the reason for the port not responding as it should, and advise best how to fix it.  

Can you drive with an OBD tool plugged in? 

Yes! In fact, there are a few devices that are designed to be used this way, additionally, gauges and control devices for example, allow you to read extra information about your car whilst on the move.  

It is best to only use devices that enable wireless communication, such as those with Bluetooth that connects to a smartphone, to avoid cables running in the driver's footwell.  

Make sure you have a passenger to monitor these devices though, you should not use any mobile device whilst in control of a vehicle. 

What OBD scanner should I buy? 

If you need to read some error messages stored in your ECU, want to know more about your engine, or are just curious, there is a wide range of products to choose from, to suit every budget.  

Basic tools 

For around £20 you can buy a basic scanner tool, these typically plug in via a cable and can display basic error messages and many system parameters such as engine temp, RPM, throttle position, ignition timing and more 

They can't typically change any parameters though, and are difficult to use to gather data whilst on the move.  

Advanced Scanners 

If you’re willing to drop a bit more cash, for around £50-£100 you can get tools that will do all of the above and more.  

Devices such as Carly, can enable changes to your car's lighting and mirrors, such as enabling auto-dipping mirrors when reversing. They can also display ECU error messages. 

As well as displaying the error messages, these levels of tools often have built-in libraries of information that can help diagnose problems. In some cases, they even guide you through step-by-step troubleshooting using physical and digital tests on the car.  

Many connect to your phone or laptop via Bluetooth, meaning you can easily use them whilst on the move to get more information. 

Professional Scanners 

Aimed at garages and mechanics, these tools often run into the hundreds of pounds, and offer connection to manufacturer-specific software such as those used by BMW and VW, to enable further customization.  

Due to the complexity of these systems, they are often used in conjunction with a laptop computer and sometimes need a certain amount of training to use effectively.  

For more information on OBD tools, Toms Guide did a great run-down here. 

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