Had a new experience tonight and not sure what to make of it. The "Service Engine Soon" light comes on after 10-15 minutes on the highway, but not around town. It goes off when the engine is shut down and does not leave a code that can be read indicating what caused it. I haven't been able to take it in for a scan with the light on yet. Today, the light came on as usual, and several minutes later, engine speed suddenly increased for a second like the converter unlocked or the trans downshifted, but did not spin out of control like it was slipping. Then it resumed normal conditions, but the light went off. Several minutes later the light came back on, but no other symptoms were noted the rest of the drive. Any one have any ideas? I've heard that SES symptom is often related to the EGR, but the trans connection is new to me.