You can use this gizmo to check out all the codes available from the myriad of sensors on your C5 or other 1996 or later GM or Ford car or truck.  The product consists of the display software and an optically isolated connector/cable that goes between the OBD-II port on the car to the computer's RS-232-C port.  The program runs on MS Windows in DOS mode (yes, I said DOS).  There is also a mouse-driven Windows version.  The reason given by the AutoTap guys for why DOS is still available is because of it's low overhead.  I don't think that my 450 MHz portable computer will have much trouble running this simple code even with the Windows OS - so, I don't believe the overhead argument.

Cable and isolation unit

Anyway. as can be seen in the (DOS) screen below, the information is presented in a straightforward manner.  I have found that the program is pretty easy to use once you get used to the Alt/<key> functions and memorize how to navigate the program.

Data can also be logged to a disk file for later analysis.  The log files can be imported into a spreadsheet or other graphics program for creation of charts.  The ATView program can be downloaded from the autotap.com site - it offers more of a custom look into the data than a general purpose spreadsheet can.  A limiting factor with ATView is that only five items can be charted at once.  I think that ten would be more realistic.  ATView is a Windows based file viewer designed to simplify analysis and comparison of AutoTap Log Files. Simply load one or two AutoTap .log file into ATView and select which of the parameters you'd like to view in tabular and graphical format. You can playback log files to watch for key data points, or playback two files simultaneously to get an instant visual comparison of logging runs.

From the Windows version you can save the log file in a comma delimited flat file for import into a spreadsheet.  There you can perform all kinds of analysis, etc.  You can also play back the entire log file and watch it in "real time".  If you use the gauges, you can see the tachometer, speedometer and other indicators during critical portions of your run.  A fairly useless tool, but it looks cool.  Most results will come from the analysis of the data and not from looking at gauges.  Here's some screen shots:

So now, what is needed is a dissertation about how to interpret all the numbers you can get from the program.  What does each one mean in the context of improving performance?  What do we look at if we think the car is not performing well (missing, flat spot in acceleration curve,  etc.)? I know that I need to know about the following things:

• Short Term Fuel Trend (STFT) Bank 1 and 2 (%) - Instantaneous fuel trend indicator - a numeric indicator (-) is rich (+) is lean.  Any deviation from 0% indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 0%.
• Long Term Fuel Trend (LTFT) Bank 1 and 2 (%) - The long term fuel trim is a matrix of cells arranged by RPM and MAP. Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what long term fuel trim factor to use in the base injector pulse width equation.  A value of 0% indicates that fuel delivery requires no compensation in order to maintain the PCM commanded air/fuel ratio.  A negative value significantly below 0 percent indicates that the fuel system is rich and the PCM is reducing the fuel delivery (decreased injector pulse width).  A positive value significantly above 0 percent indicates that a lean condition exists and the PCM compensates by adding fuel (increased injector pulse width).
• Engine RPM - Pretty obvious.  Computed by the PCM from ignition reference pulses.
• Vehicle speed - no rocket surgery here either
• Oxygen sensor Bank 1 Sensor 1 (O2B1S1) - (A voltage reading) The oxygen (or lambda) sensor measures the amount of oxygen in the exhaust gas.  This is an indicator of the efficiency of fuel burn.
• Oxygen sensor Bank 2 Sensor 1 (O2B2S1) - They're both the same definition.
• Fuel trim cell - Cell number 0 - 23.  Indicates which trim cell the engine is operating in.  Engine computer function that keeps the air/fuel mixture as close to the ideal 14.7:1 stoichiometric ratio as possible
• Knock retard - 0° - 16° The amount of spark advance the PCM receives from the IC spark advance in response to the signal from the knock sensors.
• Coolant Temperature - Duh
• Throttle position sensor (TPS) - Electronic throttle control indicated position (0% = off, >99.5% = WOT)
• Timing -
• Intake air temperature (IAT) - As measured just behind the MAF.  Used to adjust fuel delivery and spark timing.
• MAF flow rate - Airflow rate to the engine in pounds of air per minute.