by chriscarter » Sat Sep 24, 2011 12:03 pm
Should be able to leave it connected and watch for codes to pop up, but I can't be sure without seeing your reader and trying it real-time. Give it a try and see what happens. Again........try to think this through. What input systems are active at 40 mph and how do they influence driveability? Is there a direct correlation between the auto trans shifting and the problem experienced? What else is "on" at the time....A/C, cruise, etc. Do any other warning lights come on? Have you done anything differently in terms of maintenance or repair since this started?
For diagnostic purposes, try leaving the trans in D2 and seeing if the problem still occurs. Change the inputs to see if it affects the symptoms....maybe accelerate hard through the speed range where it happens just to see if it still happens....maybe accelerate very slowly for the same reason. You could also leave the trans in low and normally accelerate to the engine RPM range (not the road speed range) where the problem happens. Do a differential diagnosis in an effort to eliminate certain things and to concentrate on others. It's not just about error codes and CEL indications.