Computer problem
1995 Ford Mustang 6 cyl Two Wheel Drive Automatic
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I've been having a lot of problems with the transmission shifting in and out of gear at highway speeds (but only when the engine has heated up). Sometimes it will downshift from third to second (right around 52 or 53 mph), and sometimes it shifts from third into neutral and I either have to manually shift it down into second or let the car coast down to about 20 mph before it will let me accelerate.

Because of this, we tried hooking up a scanner to see if any sensors were bad; the scanner begins the test, but when it gets to the transmission-related scans, it prematurely stops the scan, stating that there was an error because the car was not in park or neutral - but it most certainly IS in park. We have replaced the range sensor, checked all of the grounds, and cleaned and replaced the filter and fluid (just to say we did so). Any ideas?