Chrysler Repair: 2001 Chrysler T & C: harsh downshifting, 4 digit numbers, diagnostic capability

Question
What would be causing it to be downshifting hard?

Answer
Hi Lisa,
The first thing to do is check that the transmission fluid level is in the correct range by removing the dipstick on the transmission while the engine is idling and warmed up. There is an indicated area on the stick for level when the fluid is "hot". Also notice if the fluid looks foamy which means that it may be low, but bubbled up to an apparent normal level from that problem. If fluid is needed, only add ATF +4 type and don't overfill it.
Another possibility is that the idle speed of the engine is too high. The tach should read below 1,000 rpm in neutral when warmed up.
Mechanical possibilities can range from improper pressure level of the fluid (can be tested with a gauge) to serious internal issues requiring rebuild.
It may be that the harsh downshifting is occuring when you are not moving at a low enough speed for that to be required (you would note the tachometer is running higher than expected after the downshift). If the trans refuses to return to a higher gear, that would be called "limp-in" mode which is a self-protective strategy which it adopts to prevent unnecessary damage. The trans will simply stay in second gear until you turn it off, but then drop back into the mode soon again. The 'check engine' light will also be 'on'.
This is where taking advantage of the self-diagnostic capability of the transmission computer is wise. The best approach would be to get a fault code readout of the memory of the transmission computer. You may be able to get the codes using the ignition switch/key: "on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time, Then watch the odometer window to see if the mileage reading changes to show 4-digit numbers preceded by a P. If not, and if there is an Autozone parts store near you then they often will do the readout for free. If not, an independent repair shop (not a dealer, not a 'chain') will often do the readout for $40 or less. Ask them for the 4-digit fault code numbers that are revealed, ask what they mean, what they recommend, and for what cost. Then write back and we can see how that compares to what the shop manuals say about those codes.
Roland