Next to the crimson red of a highway patrolman's spotlight, few lights can raise the ire of even the most patient motorist as the amber glow of a check engine light.

In the old days, check engine and warning lights signified immediate or impending disaster. As onboard engine management and diagnostics systems became more sophisticated, the lights generally have taken on the meaning of "something to address soon."

I was both fortunate, and unfortunate, to have a check engine light on my New Beetle go off the day before my scheduled maintenance appointment. Due to the fact that I had a Volkswagen Master Guard extended warranty, I thought my car would be well covered. That thought was soon proven to be illusory. This, then, is the tale of the MAF sensor and me. Upon plugging my car into the VAG tool, it was determined the check engine light was caused by an intermittent MAF sensor failure.

For those who don't know, the MAF (mass air flow) sensor measures the mass of air entering the engine to control the air/fuel ratio. The sensor is incorporated into the intake system after the air filter. Bosch uses a hot-wire MAF sensor, which is a platinum wire heated to 212*F (some say 180*F) above ambient temperature. As air is sucked into the engine past the wire, it has a cooling effect that's offset by an increase in voltage to the sensor to maintain the 212*F. The engine management system then determines how long to keep the fuel injectors open. Upon shutdown, Bosch MAF sensors briefly heat the wire to 1,000*C to burn off any contaminants.

For those reading Internet message boards and newsgroups, there has been a buzz for some time about the failure rate of MAF sensors on newer cars such as my 2000 New Beetle. To add insult to injury, not only was the MAF sensor not covered by the powertrain warranty, or even part of the extended warranty, it wasn't even covered by the same warranty rules that govern emissions systems. If that weren't bad enough, the dealer wanted $135 in labor for a 10-min. job, plus $52 for the part. Many dealerships claim that aftermarket intakes, exhausts or replacement filters are the cause of failure. My car, however, was stock.

It could have been much worse. I talked to several dealerships and found the MAF sensor that now costs $52 had once cost almost $600, but since then has gone down to $350, to $180 and now $52. There is some speculation this price adjustment is due to the outrage caused by the part failure. My research further concluded that due to the number of failures and short supply, I would have to wait several days to get one in. One parts manager said, "I order two, three, four, five per day." Plan B: Although I've never replaced a MAF sensor, I set out to do it right and save myself a pile of dough. I know what it is, and where it is, but I needed the diagnostic tools, the part and a Bentley manual, the definitive source of VW/Audi repair information.

european car readers might remember Dan Barnes' "Tool-of-the-Month" column in the November 2001 issue, featuring a device called the VAG-COM. This is a computer interface cable and proprietary software, used to read the operating parameters and fault codes in VW/Audi systems. It can also be used to log data over time-- very important for tuning and development.

I gave VAG-COM a call and got hooked up with its latest cable. The software can be downloaded from the company's Internet site in trial mode and can be activated by purchasing a license ($199 for cable and license).

Uwe Ross, the developer of VAG-COM, was inspired to design the tool after the untimely death of the developer of VWTool, an older, less advanced system.

The second call I placed was to Robert Bentley Publishing, the source for official VW/Audi repair manuals. In recent years, Bentley has expanded its product line to include manuals in CD-ROM format for quick browsing and convenient hyperlinks between related subjects. (Not incidentally, Ross-Tech also sells Bentley CD-ROMs to its VAG-COM customers at a lower cost.)

We at european car have long been Bentley fans, and the company was recognized in our recent european car 100. Because I wanted to work in the shop as well as at my desk, I got both the manual and the CD-ROM. The manual can be read anywhere, but since I was going to be using my laptop with VAG-COM, it made sense to load it up with the Bentley manual as well. The installation of both the VAG-COM and Bentley manual can be accomplished with a few clicks. You'll need an Internet connection to register the software or it can be done by phone.

With laptop in hand, I set out to complete the task. The first thing is to locate your vehicle's OBD (onboard diagnostics) plug. Mine is just above the clutch pedal. Interestingly, the VAG-COM connector has an LED that shows red if there is a wiring fault. Mine showed green.

In talking to Uwe Ross, it seems the use of some aftermarket sound systems can cause +12V to get sent through to the diagnostics plug, which will damage the laptop computer or VAG tool used by the dealer. Some dealers won't even work on cars with aftermarket sound systems lest they damage the expensive VAG diagnostics tools. For this reason, Uwe developed "VAG Saver" as a separate plug-in, which dealerships buy to test the plug before hooking up their diagnostics. Green LED = good. Red LED = bad.

With cables hooked up, I started the car, started the laptop and waited for the program to load. The opening screen is where the user clicks to enter different areas of the program. I selected "Select Module" followed by "Engine," which took me to the engine module. From there I was able to read information about the onboard electronics, including the ECU part number.

The next step is to read the fault codes (click on the button labeled "Fault Codes.") This greeted me with "1 DTCs Found: 17536 - Fuel Trim: Bank 1 (Mult): System too lean 35-00 - -". VAG-COM operates a message board on Yahoo ( where people discuss codes and troubleshooting, and it was suggested that I compare the values in data blocks 32 and 002 against the printed standards. Looks like a bad MAF sensor, indeed. The Bentley manual shows extensive diagnosis procedures for the MAF sensor wiring system if you should get stumped.

The next step of Plan B was to install the new MAF sensor. The dealer wanted $135 to do the job, but I knew it could be done much faster--so I'm going to raise an issue here not often addressed. A repair shop does not determine how much time is required for a job. That information is provided via Mitchell's or whatever proprietary standards the dealership uses.

My problem is that many of these procedures list a time that would be required only by a one-armed monkey. This means a mechanic can theoretically bill more hours in a working day than there are hours. Case in point: My 4Runner was once in for an exhaust manifold replacement. I was in and out in far less time than I was billed, and my arguments to the service manager fell on deaf ears. Solution: Seek knowledge, buy tools and do it yourself.

Although I had never done this job on this particular type of car, I found that the whole process only took about 10 min. With the right tools and knowledge, you can save lots of time and lots of money. Your mileage may vary.

Ross-Tech LLC
888 Sumneytown Pike
Lansdale, PA 19446
(215) 361-8942
Fax: (215) 893-3816

Bentley Publishers
1734 Massachusetts Ave.
Cambridge, MA 02138
(800) 423-4595
(617) 547-4170
Fax: (617) 876-9235

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