No passing gear

Okay, first check the linkage going from the shifter to the tranny. Block the wheels and have someone watch the shift linkage on the trans while shifting. Maybe it's just loose, broken, or stripped. If linkage is good, it's either your shifter solenoids or torque converter

No load on engine. When a car is in any gear except park and neutral it has the load of the transmission. Which causes the rpm to drop. Therefore the rpm at idle will be a little higher or it would stall

This sounds dangerous, this should be inspected by a dealer.

lost engine power, or transmission slips????
which is it?
the engine rpm tells you if the engine is weak or transmission slips.
2.0 liter , DOHC I4, J20 engine.
which transmission, my guess A/T box. (sounds like?) sure..
test one
1: in drive. pointed in safe direction, say a field or ?
2: parked, hand brake and foot brake held hard and fast.
3: race engine to wide open throttle for 1 to 3 seconds.
and lift right foot. after noting RPM. quckly.
4: rpm below 2200 rpm is a weak engine , if the engine spools
wildly up to 3500 rpm or more, the transmission is bad. (slips)
this is called the stall test. (in the FSM)
2,300 - 2,600 r/min. for H25/J20 engine
stall test passes. so does the transmission reach 4th gear.
1,2,3,4? yes/ .no.
yes, the its not the transmission.
if engine power lacks in any gear, that is the engine. weak.

this post is about the engine power, (my answer)
will you change 100 more parts guessing?
why do that?, any shop can diagnose it for a fraction of your guessing.

if the trans dont slip

1. over heating? the engine?
2. never done a tuneup this decade?> really?
3. low fuel pressure?
4. low engine compression.?
5. Bad MAF sensors or just dirty?
6. check engine light on running?, why ignore that, scan it.
7. weak spark , see #2 and 6
8. leaking or clogged injectors. or stuck on, for 2 reasons.
9. Intake vacuum air leaks>?

both the engine and transmissions are complex
as is the vast number of tests to do. based on previous tests and logic.
do you want to test or gue$$?

RPM at 800 , in drive
can you race motor in neutral, to 2500 rpm
?
1: does rpm rise above 1500 in drive and no movement, (slip and bad tranny)
2: or holds about 1500 but can move. (brakes frozen , with ice)
try to push car in neutral on flat ground, can you>
no , the brakes are iced.
yes, then engine power is bad.
with A/T trans, you get to first find out.
1: is engine weak.
2: is tranny slipping (races to very high rpm and no go)
3: or brakes ices, (post location so we can see climate, duh?

The engine does not need to be raced when cold

The vehicles computer will adjust the idle according
to how the manufacture programmed it

After about one mine or so, the idle on most vehicles
comes down to approx 750RPM, generically speaking

When you put the car in gear,assuming it is an automatic,
the rpm will drop a 100 at most & may be adjusted by the
computer, just as it does when you put the A/C on. That is
to compensate for the change in engine load

You should be waiting for the rpm to come down to 650 to
800 & then a couple more minutes for some temp to come
up, before driving off. In cold weather wait at least 5 min or more

I think your vehicle is not working correctly, or you not explaining
it in a way I understand what answer you want

When was the last time the transmission fluid and filter changed? I would have that serviced first. If it is an electronic transmission, the line pressure might also have to be reset.

Transmission will never hold the engine back if it is going through the gears properly.If it gets into high gear, eventually the engine will reach "redline" if you stay into it long enough. ("redline" for any engine is the point at which it cannot run any faster without harming itself.) DO NOT confuse that with proper shift points which should happen just before you reach max torque which happens way before you reach "redline" at which point you are way past the torque peak of the engine, and making no usable torque or horsepower. An example of this is a race engine with a max rpm limit of 10,000rpm.("redline") Depending upon the torque curve (determined by compression, stroke and type of cam grind) you would likely be shifting gears at about 7500 to 8500RPM not at 10,000.
Therefore, your shift points are like climbing a ladder. you get just so high and though there are more steps, you will fall off if you go there. Each time you shift, you are putting the rpms back into the usable power band, till you reach the next step, then start over again. If your trans shifts early the engine will lug and labor. shifting too late and it will quickly reach "redline" and self destruct or go "flat". In a conversion, you need to make sure that the electronics that control shift points are working within the usable torque range.
OR install a manual valve body and shifter and control it like a stick shift car, using your brain as the computer and a tach to tell you where you are.

The gears in your transmission are not functioning properly, Check tranny fluid levels, or take it in for a tranny inspection

I changed the transmission control module, located on the firewall on the passenger side under hood. Now the transmission works fine. Bought it at pull a part junk yard for 20.00 and thought what the heck I will try this. It worked.. amen