OBD-II is the diagnostics standard built into the stock ECU of 1995 and up DSMs and EVOs. It's most commonly used for several things:
The ECU+ is the only engine management system for the DSMs and EVOs that includes a complete OBD-II scan tool. With other engine mangement systems, you'd need to buy (check out that list price!) a standalone device to do what the ECU+ does. With the ECU+, you just click the OBD menu item. And unlike simple code scanners, the ECU+'s OBD-II scan tool module is a complete implementation of the OBD-II protocol, and can read and display pretty much all of the information available from the stock ECU, as well as display and clear check engine codes.
The ECU+ includes a real-time display of a variety of values "known" to your car's stock ECU. Included in this is:
Depending upon your vehicle's year and make of ECU, other values may also be available. The ECU+ supports all of the real-time data available in OBD-II "mode 1," and will adapt on the fly to your vehicle to display only the values that your car supports.
Note that the above includes the vehicle "fuel trims," which are used when tuning low and medium throttle conditions. With most other engine management systems, you have no way of knowing whether you're tuned properly for larger injectors. The fuel trims will tell you that with the ECU+.
In addition to the normal mode 1 values above, the ECU+ can also display the vehicle's internal test results. OBD-II specifies that the stock ECU must run a series of "continuous" tests that monitor various emission-related parameters. The results of these tests, as well as the results of the non-continuous tests, are also displayed in a separate tab of the ECU+'s OBD-II display. With this, you'll be able to see whether your car will pass emissions before you go to the emissions test station.
The ECU+ can, of course, display and clear any "check engine" diagnostic codes that your stock ECU has thrown. The ECU+ will display the codes along with an explanation of the trouble, and unlike most cheap "code readers," the ECU+ displays the freeze frame data along with both the stored and pending trouble codes. What's the difference? Well, when the stock ECU first senses a problem, it sets a "pending" trouble code. The pending code is later promoted to be a "stored" code if the problem reoccurs. Stored codes generally turn on the check-engine light as well. By showing you the pending codes, the ECU+ can warn you about intermittent problems that haven't quite become serious enough to turn on the light. These intermittent problems can impact performance.
Freeze frame data is another ECU+ feature that only a few scan tools support. The freeze frame data is a snapshot of the vehicle at the instant that a trouble code is detected. This is a big help in diagnosing trouble codes, as it can often show you the condition that caused the code, even if the problem has gone away. An example might be diagnosing an intermittent code with the throttle position sensor, where things look ok when revving the car in your driveway. By examining the freeze frame data, you might see a condition where the car was under high load, but showed 0% TPS. This might indicate that you have a loose wire to the TPS sensor that is being jarred loose when driving.
Even though the OBD-II protocol is only implemented in 1995 and up stock ECUs, earlier ECUs implemented a different protocol, called MUT-II. The MUT-II protocol is also supported with the ECU+ (along with the hardware "auto on" feature) so that the 1G DSMers can enjoy some of the ECU+'s diagnostic goodness.
With MUT-II mode, the ECU+ can display a similar set of real time data, including:
The MUT-II protocol doesn't support freeze frame data and only includes a limited number of trouble codes, but the ECU+ does still support displaying and clearing these trouble codes.
The MUT-II protocol is also, it turns out, supported in 1995 and up DSM and EVO ECUs, so the ECU+ can also display MUT-II real time data for those cars.