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Reading Audi A4 ECU Fault Codes
 
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Pelican Technical Article:

Reading Audi A4 ECU Fault Codes

Peter Bodensteiner

Time:

1 hour1 hr

Tab:

$250

Talent:

**

Tools:

VAG-Com Diagnostic System (VCDS) code reader, Windows-compatible computer

Applicable Models:

Audi A4 (1997-01)
Audi A4 Quattro (1997-01)

Hot Tip:

Auto code readers provide direct information about your car and what it's doing

Performance Gain:

Everything from clearing annoying trouble lights to diagnosing a poorly running engine

Complementary Modification:

Restore or replace central gauge display, bleed brakes

Electronic controls are a major component of any modern car. Certainly there are still a number of projects that an enthusiast can tackle at home that don't require any interaction with these electronic control systems. But as time goes on, the tools available to the hobbyist mechanic for reading and interacting with these systems keep getting better and cheaper. At this point, there's not much excuse for a well-equipped home shop to not have an engine code reader of some sort.

For Audis (and all Volkswagen Audi Group or VAG cars), the standard for many years has been the Ross-Tech VAG-Com Diagnostic System (VCDS), which is fairly well standardized for the German company's wide-ranging lineup. It can read trouble codes that are stored in the car's ECU; it can test various sensors and other electronic components in your car, and in some cases it can actually operate mechanical components and systems in an isolated way. For example, the VCDS can operate an anti-lock braking system pump to purge old fluid and air that it may have collected, simply by clicking on the right series of commands.

The VCDS is a very powerful tool, and as such its use should be accompanied with patience and care. Ideally you should purchase one of these tools before you really need it, so that you can connect it to your car and explore the software before anything goes wrong. That way you can familiarize yourself with the various functions available to you and see what kind of information a properly running vehicle delivers. Ross-Tech also offers a support forum on their website, www.ross-tech.com.

In practice however, you may experience VCDS for the first time with a car that is already showing warning lights or is having other problems. Again, tread lightly and work methodically; don't start resetting things and erasing codes before you are familiar with the menus and controls and without recording and saving any codes that the VCDS reads.

The VCDS, as delivered, really only has one part--this cable/plug--which connects to the car's diagnostic port on one end, and to a Windows-compatible computer on the other.
Figure 1

The VCDS, as delivered, really only has one part--this cable/plug--which connects to the car's diagnostic port on one end, and to a Windows-compatible computer on the other. Note the large label on the vehicle end of the cable; this indicates which cable you have purchased and also dictates what version of the VCDS software you need to download from the Ross-Tech website, in this case, HEX + CAN.

Aside from the cable, you'll find this quick-start guide in the product box.
Figure 2

Aside from the cable, you'll find this quick-start guide in the product box. It's very straightforward to follow. You'll also need a compatible computer, of course. I picked up this old but still capable Windows XP machine for $200 on Craigslist, and that was a couple years ago. It does the job of running the VCDS software just fine.

Here is the diagnostic port that you will find under the dashboard of the A4.
Figure 3

Here is the diagnostic port that you will find under the dashboard of the A4. It's to the left of the steering wheel, just above and to the right of the hood release handle, which is also pictured.

This is just another view of the diagnostic port, showing the angle at which the VCDS cable connects to it.
Figure 4

This is just another view of the diagnostic port, showing the angle at which the VCDS cable connects to it. Again, you can see the hood release handle in the background.

The dashboard shows a few warning lights upon startup--for most these would prompt a visit to a repair shop.
Figure 5

The dashboard shows a few warning lights upon startup--for most these would prompt a visit to a repair shop. With the VCDS however, you can discover the causes of those warning lights, figure out what needs to be fixed if anything and then turn off the warning lights. At the very least, you can read the codes yourself and be better informed when you do take the car to your mechanic.

This is the main menu for the VCDS software as seen on a Windows XP desktop.
Figure 6

This is the main menu for the VCDS software as seen on a Windows XP desktop. From here you can access all VCDS functions--the primary ones a home mechanic would use are: Select Control Module, which allows you to operate and test various individual devices within the car; Auto-Scan, which performs a full scan of the ECU and other electronic components to search for fault codes; and Service Reminder Interval Reset, because presumably you are doing your own maintenance and not relying on a mechanic to reset these. Rather than supplying a manual in the box with your diagnostic port reader, Ross-Tech provides guidance through support documentation, a wiki, and user forums on their website.

Before performing an Auto-Scan, you must select your chassis code from this drop-down menu.
Figure 7

Before performing an Auto-Scan, you must select your chassis code from this drop-down menu. In this case I selected 8D, for the 1995 through 2001 B5 chassis Audi A4--only one of a multitude of Volkswagens and Audis that the VCDS can scan. The Auto-Scan function checks through all of the car's electronic controllers, one by one, and returns any fault codes that it sees. It takes several minutes to do the complete Auto-Scan. Once the scan is performed, you can save the output messages to a text file.

Alternatively, you can address only individual control modules.
Figure 8

Alternatively, you can address only individual control modules. From the main screen, click the "Select Control Module" button. That will take you to this screen, and to the "Common" tab, which groups the most common control modules in one place. The 2000 A4 in this project is a relatively simple car with few controllers; newer models have more controllers.

I've selected the 03-ABS Brakes controller, which brings up the screen pictured here.
Figure 9

I've selected the 03-ABS Brakes controller, which brings up the screen pictured here. As the software communicates with the controller, the various part numbers and code fields are populated with data. In this case we are going away from the "safe" buttons in order to do an output test of the ABS pump. Click on the button marked Output Tests - 03 under "advanced functions."

This is the ABS pump under the hood of the car.
Figure 10

This is the ABS pump under the hood of the car. You can easily see the hard brake lines running to and from the pump, as well as an electrical connector next to the windshield washer fluid reservoir. I suspect that a soft brake pedal may be caused by air in the pump, and performing the test of the pump will purge any air bubbles that may be lurking in there. Following up with a standard brake bleed should move that air out of the system.

This smaller window gives you the ability to run any available output tests.
Figure 11

This smaller window gives you the ability to run any available output tests. In this case the only thing that can be done is to activate the ABS hydraulic pump. Click Start/Next and the pump should run for about 30 seconds to one minute. You can clearly hear the pump operating during this process. Then click Done when the test is complete.

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