The following is an account of testing the OBDScan to verify the function of both the Mode 3, Read Trouble Codes, and the Mode 4, Clear and Reset Trouble Codes and Malfunction Indicator Light (MIL), also known as the 'Check Engine' light. Since all of my vehicles are currently in good working order, I decided to force a malfunction on the 1996 Chrysler Sebring. The Manifold Absolute Pressure (MAP) sensor is located on the intake plenum and easy to reach, so I disconnected the electrical connector from the MAP sensor and started the car. The idle was very rough and the Check Engine light came on right away. My intent was to force the ECU to set a trouble code so that I could use OBDScan to both read the trouble code and then clear the trouble code. After about 5 minutes of operation with no MAP sensor, I stopped the engine and switched the ignition back on then connected a laptop computer and OBDScan. To my surprise there was no trouble code set. I know that it sometimes takes several driving cycles to set a trouble code, so I left the MAP disconnected for several driving trips around town, which by the way was not easy since the engine ran very poorly with no MAP sensor. After returning home I reconnected OBDCsan and the Laptop and promptly found that there was still no trouble code set! As long as the engine was running the 'Check Engine' remained on but with the engine not running and the ignition 'on', there was no check engine light and no trouble codes were reported. Not wanting to drive around barely able to move, I reconnected OBDScan and, with the engine running and the MIL on, read the diagnostic data then read the trouble codes. Sure enough, the Sebring responded with a trouble code of P0108. I assume Chrysler has implemented the reading of a trouble code by a scan tool which is tentative, by that I mean a condition that the ECU detects as a problem but it has not existed long enough to be written to EEPROM as a stored trouble code. The Chiltons repair manual for the '96 Sebring defines code P0108 as "MAP voltage too high", which fits nicely with the problem I created. Next I reinstalled the MAP sensor connector and started the car. The 'Check Engine' light remained illuminated, so I tried driving the car around the block and the "Check Engine" light remained illuminated. I next tried to clear the trouble code, using OBDScan, with the engine running, but the vehicle ECM would not permit it. I stopped the engine and switched the ignition back to the on position and then successfully cleared the trouble code. Now the Sebring is back to normal, with the MIL off, as it should be. This process is what you could use to troubleshoot and repair an actual problem if one exists.

Dan Harrison 5/25/00