Skip the auto shop - Our mechanics make house calls

What Is the On-Board Diagnostics (OBD) System?

obd system

Your car contains a vast number of different systems, all of which must work in harmony to ensure proper operation. There must be a way to monitor your ignition and emissions systems and the On-Board Diagnostics (OBD) is a computer that keeps tabs on what’s going on with your vehicle.

What the OBD system does

Simply stated, the OBD system is an on-board computer that communicates with other systems, including the ECU, TCU and more. It monitors the performance of your ignition system, engine performance, transmission operation, emissions system operation and more. Based on feedback from sensors around the vehicle, the OBD system determines if everything is working properly, or if something is beginning to go wrong. It’s advanced enough to warn drivers before a serious problem occurs, often at the first symptom of a failing component.

When the OBD system identifies an issue, it turns on a warning light in the dash (generally, the Check Engine Light), and then stores a trouble code (called a DTC, or diagnostic trouble code). A mechanic can connect a scan tool to the OBD II connector under the dash and read this code. This provides the information needed to begin the diagnostic process. Note that reading a code does not necessarily mean that the mechanic knows immediately what has gone wrong, but that the mechanic has a place to begin looking.

As a note, the OBD system also determines whether your car will pass emissions testing. If the Check Engine Light is on, your car won’t pass the test. There’s also a chance that it won’t pass even if the Check Engine Light is not on.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.


Post a question and get free advice from our certified mechanics.


More related articles

How to Buy Fuel Treatments
Adding a fuel additive to your gas tank when you fill up is one way to clean vital engine parts of deposits, improve the performance of your engine,...
P2159 OBD-II Trouble Code: Vehicle Speed Sensor B Range/Performance
Diagnostic Trouble Code (DTC): P2159 P2159 code definition Vehicle Speed Sensor B Range/Performance Related Trouble Codes: P2158: Vehicle Speed Sensor B P2160: Vehicle Speed Sensor B Circuit Low P2161:...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...

Related questions

Q: How do I know that my OBD system is working right?

The OBD system on your vehicle is a self-monitoring system. The Powertrain Control Module monitors and controls all of the systems and functions that regulate fuel, ignition, emissions, and engine cooling as well as having some minor control over air...

Q: air conditioner not blowing air from dash vents, but blowing air from side vents

A few different items can cause the air to not blow out of the dash vents. The most common are a blend door or a control switch. The best method for the AC diagnosis feature can be found in the...

Q: Are Code Readers and Scan Tools Different?

There are differences and similarities between code readers and scan tools. With the advent of the vehicle on board diagnostic system (OBD) there have been many different makers of scan tools and code readers. The code readers were the first...