Milt’s Tune-Up Advice

May 1, 2009 | By Milt Webb

Some Insight On OBD I & II Code Reading Equipment.

MILT, I HAVE a ’91 Galant VR4 and a ’97 Buick LeSabre and until you came along in Auto Restorer, I had no idea you checked the sensors with a multi-meter.

With that in mind, what diagnostic tool/code reader would you suggest to buy for computer cars?

I see the ones that look like they are miniature oscilloscopes and can read and adjust the fuel and timing, etc. but do not know if they are worth the money. Are they mainly for the racing tuners?

Is a straight code reader enough anymore? I don’t mind spending the money if the tool is helpful enough. Thanks.

Paul Jarrett Littleton, Colorado

I am a nine-year subscriber to Auto Restorer and read it cover-to-cover every month. I was thinking about the “Milt’s Tune-Up Advice” column and thought that Milt may be able to help me out.

I am looking to purchase an On-Board Diagnostics II code reader tool. A couple of Internet searches revealed many different manufacturers and models, in a variety of price ranges. I will only need this tool once in a while, so the top-of-the line models are not necessary for me.

I would need the tool to read and troubleshoot codes that would appear on my personal vehicles. I learned during the Internet searches that some tools can be used while test-driving used vehicles to determine their condition. And, some of the tools can store the data while you drive a vehicle so you can read any findings after you have stopped driving. I have also heard where some of the tools have software that is upgradeable—all of these are desirable features.

Would you be able to recommend a few affordable tools ($100-150 range) and their features? Are there any additional features that I didn’t mention that would be useful? This way I (and perhaps other AR readers) would be able to make an educated decision when it comes time to purchase one of these tools. Thanks.

Larry Williams Keenesburg, Colorado

Milt’s Reply

In this response I will summarize data stream equipment components and computer control system diagnostic trouble code strategies.


Pre-1994 vehicles were equipped with OBD I (On-Board Diagnostics, phase I). And while most ’94-95 vehicles were equipped with OBD I as well, the OBD II systems were introduced in ’94, and OBD II was supplied on most ’96 and later vehicles.

OBD II vehicles have standardized 17- pin diagnostic connectors and a “generic” data stream.

OBD I vehicles have their own brands of diagnostic connectors. All OBD I GM vehicles have a 12-pin connector, Ford EECIV vehicles have 5-pin connectors, and Chrysler uses a 4-pin connector. OBD I import vehicles have different connectors than domestic OBD I vehicles.

Most all of the domestic OBD I vehicles have adequate data streams for tuneup and sensor testing. The OBD II generic data stream is standardized for all domestic and import vehicles. These data streams are also adequate for tuneup, sensor and actuator signals.

They Identify the Circuit Only

It’s important to realize, however, that the trouble codes in both OBD I and OBD II systems only inform you of which circuit(s) have a problem. You must still use digital volt ohmmeters, graphing analyzers and/or lab scopes to analyze each circuit, and sensor calibrations. Calibration data are available in performance tune-up manuals such as Motor ( Most all tune-up manuals include titles for OBD I and OBD II diagnostic trouble codes.

A Range of Readers Is Available

There are numerous “code readers” on the market. The lowest-priced readers display code numbers only. Higher Priced readers include code numbers with the diagnostic trouble code title.

Meanwhile, “scanners” include code readers, titles and data streams. I personally use a Snap-On scanner (model MT2500) and purchase calibration modules that are five years old (2004 in 2009). (Snap-On scanners like mine are available for about $500 on the Internet. Calibration modules will run some $200- $300, depending on the year.)

The scanner data stream “update rate” is limited by the scanner and/or the vehicle data stream strategies. For very fast update rates, I use a Snap-On Vantage (MT2400) to analyze sensor and actuator waveforms. For ignition patterns I use a Snap-On KV module (EETM306A) “piggy backed” to the Vantage analyzer.

It’s your choice! A $50 digital voltmeter all the way up to “miniature oscilloscopes” and computer scanners ($ $ $).

I recommend that you shop auto parts houses and catalogs, search the Internet and compare equipment technical specifications. These specifications are briefly outlined for all available equipment in their packaging or catalogs. Compare what each will do for you, and then compare costs. Evaluate your supplier or catalog source.

The vehicle’s factory supplied computer systems and calibration components (chips) are adequate for regular driving on the highway. They provide good drivability, performance, best gas mileage available to a specific vehicle and low emissions, provided all the specified maintenance has been performed to specifications and the adjustable components are at specification.

It takes lots of time to learn (in depth) what functions the above equipment can perform. I recommend you start with servicing and tuning your own vehicle(s) and then measure sensor operations using a digital voltmeter.

Purchase the factory service manual(s) for your vehicle(s) or after market tuneup manuals. Study the “how-to” on your own vehicle, then perform the same services on different vehicle makes.

You will discover most makes are generically similar in operation. However, tune-up, sensor and actuator specifications do vary considerably.

I would like to stress again that I believe each user has to decide the value of which tools he will need. In addition, the user must understand all specified tune-up maintenance is to be performed prior to “reading the codes.”

Editor’s note: For more on Milt’s book, “Tuning Up Autos And Trucks, A Guidebook of Solutions for Testing, Evaluating, andAnalyzing Computer Controlled Vehicles,” visit or call ($60).