Transmision wont back up

recheck install -connections and sensors --have code read --replace with the same trans - if not ecu wont recognize it

no,
run at full speed, yes, can fail
the engine must crank to start, (a/t cars only) and if cant crank it cant start.
so if it cranks,(old post) then next is spark
then try test fuel if spark is good on unstated engine
its not a diesel is it?

With the engine off, can you put it in reverse? Can you put the clutch in and keep it in reverse, then start it, and it moves back? If you can, but can't get it into reverse again with the engine running, either the clutch isn't adjusted properly, or the clutch has oil on it and is dragging. Reverse has no syncro's, those are the things in the transmission that help it shift. All the forward gears have syncro's so they can kind of tolerate when the clutch isn't right, they will still go into gear. But reverse needs the clutch to work properly.

tell the full story, please.
you start car
and it runs.
and what do you do that causes the car to not respond.
are you driviing, what? we dont know what you are doing, or execting.
transmission slip
weak engine.
enigne stall.s
idle speed wrong (below 800)
idle surges by itself
car wont move in gears, one or all? and reverse?
what/?

if its automatic transmission check the transmision flued,level,seals might be corroded,and flued leaked out, causes lack of presure in the transmision hydrolick system,or the clutch plates inside the transmission are slipping cuase of hardening

possible that clutch has failed, to test do following
switch off engine and put gear into 4th, with brakes on try to start
engine while in gear. if the engine starts switch it off straight away

this confirms your clutch has failed, if engine stalls your clutch is good.but could mean that the clutch activator is not working. either way
recomment you see a qualified dealer or machanic about this issue.

I hope this was helpful

its a transmision problem. bearing in your automatic gear shifter needs to be replaced. this is not a DYI solution.

the shifter forks inside the trany is hung up take back out or slide back be sure in neutral when u get it to shift then put back in

Just a couple of things to check if you haven't compare the smell and apearance of the tranmission oil to so new, if it looks dark and smell burned you probably have a bad transmision, clutches or burned up. Does it whine or make any noise? a stopped up filter will do this. also is it full when warm on the dip tick runniing in park?

Known problem with gearbox control units. High failure rate. Gearbox lights flahing. May not select reverse gear. Take to dealer as may get some goodwill on this.