The check engine light is one of the most stressful lights on the dashboard because it doesn’t tell you much on its own. It might mean a loose gas cap, or it might mean a problem that needs attention soon. A car diagnostic tool, usually called an OBD2 scanner, lets you read the reason the light came on, so you can decide whether it’s a quick fix or a job for a mechanic. Here’s how these tools work and how to use one.
What is OBD2?
OBD stands for on-board diagnostics. Cars have computers that constantly monitor the engine, emissions system and many other parts. When something falls outside its normal range, the computer stores a diagnostic trouble code (DTC) and may turn on the check engine light.
OBD2 (also written OBD-II) is the standard version of this system. In the United States, it has been required on cars and light trucks since the 1996 model year, and similar standards exist in many other countries. Because it’s standardized, the same scanner can read basic codes from most makes and models.
Where is the OBD2 port?
The OBD2 port is a 16-pin connector, usually under the dashboard on the driver’s side, near the steering column or above the pedals. On some cars it’s behind a small cover. Your owner’s manual will show the exact spot if you can’t find it.
Types of car diagnostic tools
Basic code readers
The simplest tools plug into the port and display the trouble code on a small screen. Most also let you clear the code and turn off the check engine light. They’re inexpensive and fine for occasional use.
Bluetooth and Wi-Fi OBD2 adapters
These small plugs sit in the port and send data to an app on your phone. Depending on the app, they can show codes, live sensor readings and fuel economy data. They’re compact and easy to keep in the glove box, but what they can do depends heavily on the app you pair them with.
Handheld scan tools
More advanced handheld scanners show live data, such as engine temperature, oxygen sensor readings and fuel trims, and may include code definitions on screen. Some can also read codes from other systems like the ABS brakes or airbags, which basic readers often can’t.
Professional diagnostic tools
Repair shops use advanced scan tools that can run tests, program modules and access manufacturer-specific data. These are expensive and more than most drivers need.
How to read your check engine codes
- Park the car and turn the engine off.
- Plug the scanner into the OBD2 port.
- Turn the ignition to the on position. Some tools work with the engine running; follow the scanner’s instructions.
- Read the codes. Write down every code shown, not just the first one, along with any freeze-frame data the scanner offers. Freeze-frame data is a snapshot of conditions when the fault was recorded.
- Look up the meaning in the scanner’s manual or app, or in a reliable repair reference for your make and model.
Understanding the code format
OBD2 codes have five characters, such as P0420. The first letter tells you the system:
- P: powertrain (engine and transmission)
- B: body (things like airbags and power seats)
- C: chassis (things like ABS brakes)
- U: network and communication between modules
The second character shows whether the code is generic (0) or specific to the manufacturer (often 1). The remaining digits identify the specific fault area.
What a code can and can’t tell you
A trouble code points to where the computer noticed a problem, not always to the part that failed. A code for an oxygen sensor reading, for example, can be caused by the sensor itself, a vacuum leak, an exhaust leak or a fuel delivery problem. Treat the code as a starting point for diagnosis. Replacing parts based only on a code can get expensive if the real cause is elsewhere.
Some common situations:
- A loose or worn gas cap can trigger evaporative emissions codes. Tighten it and give it a few drive cycles.
- Misfire codes point to one or more cylinders not firing properly. Our guide to common causes of engine misfires explains the usual suspects.
- Low battery voltage can cause odd codes and warning lights, especially in cold weather. A weak battery is worth ruling out first. See our guide to using a car battery charger safely.
When to stop and see a mechanic
- A flashing check engine light usually means a severe misfire that can damage the catalytic converter. Reduce speed and get the car checked as soon as possible.
- Warning lights for oil pressure, temperature or brakes need attention right away, whatever the scanner says.
- Codes that come back after clearing mean the fault is still there.
Clearing a code doesn’t fix anything; it just turns off the light. Clearing codes right before an emissions inspection can also cause a fail, because the car’s readiness monitors reset. Our guide to emissions tests explains why.
Useful extras for DIY diagnostics
Diagnostic work often starts in a dim footwell, and it often turns out the battery is to blame. Two items that help:
- A rechargeable LED work lamp to find the OBD2 port and look around the engine bay.
- A jump starter kit like the Car Emergency Start Pump, which combines an emergency starting power source with a tire inflation pump, for the days the battery gives out.
Many auto parts stores in the United States will also read your codes for free, which is a good option if you rarely need a scanner.
For more on routine care that prevents many warning lights in the first place, see our tune-up basics and the home garage tools guide.
FAQ
Will an OBD2 scanner work on my car?
If it’s a car or light truck sold in the United States from the 1996 model year on, it has an OBD2 port and a standard scanner can read its basic engine codes. Coverage of other systems varies by scanner.
Can I drive with the check engine light on?
If it’s steady and the car drives normally, you can usually drive carefully to get it checked soon. If it’s flashing, or the car runs badly, reduce speed and have it checked right away.
Does clearing the code fix the problem?
No. It only turns off the light. If the fault is still there, the code will return.
We run the NicheStuff store; some links in this post go to products we sell.
Top image: freestocks.org, CC0 1.0, via Flickr.