Limp mode?
 

2003 Santa Fe with 3.5. Car is currently in a limp mode. Engine runs smoothly, engine will rev in P or N. No problem starting. Car dives find until reaching 20 mph. When pressing on gas car will bog down but not quit or speed up. Guessing throttle control or maybe mass air sensor. A bit confused as engine runs normally when not in gear. Possible tranny sensor error? Any help appreciated . Only error code is an SRS B1112. Low battery.

First things first,
battery needs to be checked for state of charge,
Charging system needs to be confirmed working properly.
Before going any further, these basic things need to be verified.
Can you do this?

Battery changed. Error code gone. No change when driving car.

Are you sure it is in limp mode?
Could your vehicle have an intake or exhaust restriction.
Did you get a chance to check for that?
When was the last time the fuel filter was changed?

If vehicle is in limp mode then there should be a DTC (diagnostic trouble code).
Have you tried taking off the negative cable of battery for ten minutes?
Or, tried to clear the ECU??

Battery was disconnected for more than 10 min when it was changed. Do not know how to clear ECU but can look it up. I will double check intake and exhaust. Engine runs fine when in Neutral or Park. Also, I can manually shift to any gear when driving. Engine bogs down a 2000 rpm.

Taking battery cable off clears the ECU.
The engine won't go over 2k rpm while in gear??
But, is fine when in park or neutral??

Sorry, not very good with limp home mode

There have been times when there was insufficient voltage within the ignition system that it lacked the power to provide the spark required to handle a load when past a certain point..
There is always the possibility the fuel system cannot supply the demand for the addl fuel needed to accelerate beyond a certain point. That is why I asked the question about the fuel filter. If it is restricting fuel the engine cannot go beyond that certain point., Also fuel pump pressure should be checked and confirmed .

Thanks for the info. I will check on the fuel filter. Just FYI, when not in gear there is no problem Erving to 4000+.

Don't forget when in park/neutral there is no load on the engine.