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Using a Vehicle Diagnostic Code Reader

Diagnose car problems without going to a mechanic with an auto code reader. Simply plug it into the car's computer system, then interpret the trouble code readout.

By the DIY experts of The Family Handyman Magazine

Use the code reader as a starting point

Nothing can knock your day off track faster than a “Check Engine” light popping up on your dash. You wonder if you should pull over and shut off your vehicle or drive right to a shop. A code reader/scanner can help you make the drive/no drive decision and even help you fix the problem. It works by plugging into the car's computer system and displaying a “trouble code.”

A code reader/scanner is worth buying if you're a fairly competent amateur mechanic who understands how an engine works. But it's not a silver bullet that will always tell you exactly what's wrong. It'll give you a head start, but you'll still have to do some detective work before you start pulling and replacing parts (more on this later).

The least expensive units are simple code readers that burp up an alphanumeric trouble code but no information about what it means. You'll have to look up the code in a reference book or search the Internet. Midpriced units actually display the problem on the screen, like “P0115 Engine Coolant Temperature Circuit Malfunction.” One model even accesses the Internet, so you can upload the trouble code to a Web site that has information on the most likely cause of the problem.

But if you're a true grease monkey, go for a more expensive scanner. A scanner gives you real-time “live” information so you see the same data your car's computer is seeing. That saves you the hassle of diving under the hood (with a wiring diagram in hand), piercing wires and taking sensor readings.

Code reading and scanning sound simple, right? Well, there's more to it than that. A code that indicates your oxygen sensor is “lean” can mean the sensor is dead, or it can mean that the air/fuel mixture really is lean and you've got either a vacuum leak or a fuel problem. How do you know? Here are three ways to get to the root of a problem without replacing good parts.

  1. Go to the code reader/scan tool manufacturer's Web site to see if it has information on your trouble code.
  2. Take advantage of Internet forums. Just search for your car's model and add “forum” to the search term. Register for the site (usually free) and post your question, including your vehicle's year, mileage, code number and what you've done so far. You'll be surprised by the number and quality of responses you get.
  3. Subscribe to an online shop manual. It will have not only all the carmaker's technical service bulletins listed but also the complete diagnostic procedure for your particular code. It will walk you through the testing procedure, telling you which wires to check and what voltages you should see. The services also include component locators to help you find the part in your vehicle, and wiring diagrams showing the connector position for each wire.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

You'll need a code reader or scanner, along with a computer with internet access for interpreting the trouble codes.

Comments from DIY Community Members

Share what's on your mind and see what other DIYers are thinking about.

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January 22, 9:45 AM [GMT -5]

I got one and it works on most cars.. but beware I tried it on a Honda Civic and you needed a special purchase to pull the codes.. It worked on a toyota matrix and vw passat.. I guess just make sure it'll work on your car without buying anything else. If you find the tool at your store there may be multiple types I'd rcommend getting the best one --- the cheaper ones lack features that help in investigating the codes. Clearing the codes will only clear them.. it won't show as "passed" if you're going to a state emission test - so you'll need to actually fix it.

January 10, 9:08 PM [GMT -5]

Although I frequently buy tools and hardware items from Harbor Freight, I had a bad experience with a Harbor Freight OBD Diagnostic Reader. It read, "No Diagnostic Codes Present." But the Check Engine Light was on.

A visit to an AutoZone Store resulted in a FREE OBD Code Check reading. The resulting code was caused by a damaged gas tank cap. After replacing the gas tank cap, the Check Engine Light eventually went out; but it took nearly a week to disappear.

Again, I'm not bashing Harbor Freight, just saying I would NOT recommend their OBD Code Reading Tools.

December 26, 2:19 AM [GMT -5]

I recommend the Eqqus 3100 Inova model. You can pick one up at WalMart for about $80. I have one, and I love it. I prefer it over Actron for the novice user. (They have a nice digital electrical meter up there too.)

It's easy to use (just hook it up to the OBD-II port in your car and view the readout) and comes with software and a very informative owner's manual that contains educational information about the "On-Board Diagnostics" capabilities of modern cars and trucks, as well as a list of dozens of trouble codes for all sorts of problems. The unit is updatable via the internet also.

It's a nice way to find out what is wrong and keep your mechanic honest (not that he wouldn't be...). Sometimes, the Check Engine light is on merely because the gas cap is loose, so always check that first. Good luck and have fun!

September 24, 5:39 PM [GMT -5]

i have not used one yet but would like to know where i can buy a vehicle diagnostic code reader

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