HOW DOES TORQUE CONVERTER LOCK INTO THE TRANNY ON A 03 ELANTRA

It sounds like the issue with your 2011 Hyundai Elantra might be related to the output shaft speed sensor, especially since you're getting a P0722 error code. This sensor is crucial for the transmission's ability to shift gears correctly. Since the transmission was working fine before the engine replacement, it's possible that a connection to the sensor may have been disrupted during the process. I would suggest double-checking the sensor's connection and wiring for any signs of damage or loose connections. Also, ensure that there's no physical damage to the sensor itself. If everything seems intact, it might be a good idea to have the sensor tested or replaced.

Front wheel drive tranny change is not easy or practical. Check the Torque converter lockup solenoid and the torque converter.

excessive converter slippage is seen by the vehicle's computer. This code, however, usually results in the computer disabling the converter clutch's application altogether in an attempt to protect the transmission from further damage. All of this is usually going to require replacement of the torque converter.p0741,

P0734=A/T 4th gear incorrect ratio. This has nothing to do with the speed sensor. If the speed sensor was faulty, it would also set OTHER gear incorrect ratio codes. My experience tells me the 4th gear incorrect ration indicates the torque converter clutch (TCC) is slipping and nothing more. Unless there was a speed sensor DTC (Diagnostic Trouble Code), the sensor is not the issue (yet). The fact P0734 was set (trust the computer), the TCC is slipping (out of range). Question: Maintenance history may indicate the wrong transmission fluid may have been introduced. Hyundai's Transmission fluid is strictly SPIII... there is no substitute. The incorrect fluid will cause 4th gear slippage. Now, having said that, it doesn't mean the 4th gear (lock-up) clutch isn't worn out. Have a qualified/certified Hyundai Service Technician properly diagnose this issue.

Hello and welcome to FixYa!
If you are running an automatic transmission then the problem will surely be on the torque converter. The torque converter controls the shifting of the car that when it fails the car may run sluggish or will just sit on a certain gear. If that's the case then you need a torque converter replacement.
Hope this helps and thank you for using FixYa! Have a good one!

The filter would be inside the pan. The best fluid change is at an oil change place that uses t-tech style machine. This style will change the fluid in the torque converter, cooler and lines. It takes a container of new fluid and runs that in while old fluid runs out...done until clean fluid is coming out.

A Service Department as a Hyundai Service Department typically uses a "Fluid Exchange" machine (approx. 7 quarts of Hyundai SPIII fluid) to do a flush. There is a drain plug at the bottom of the transmission (24 mm bolt) to drain it. However, this method is not entirely/completely effective. This drain plug only empties the pan, not the torque converter -- so you're only replacing about 3 to 4 quarts with this method (torque converter does not drain). So, once you've replaced the 3 to 4 quarts this way, there's still contaminated fluid in the torque converter which will circulate back into the rest of the transmission. RECOMMENDATION: Have a certified technician do a flush (fluid exchange).

Your transmission fluid is dangerously low (tranny is dry). The pops & jerks are the transmission
internal components trying not to seize onto each other due to lack of lubrication and/or overheating.
Low transmission fluid leads not only to loss of lubrication but lots of overheating (of the tranny).
The loss of Elantra performance (speed tops out at 55), is caused by power loss due to friction inside the transmission. All your power is lost overheating the transmission (toward destruction), instead of
being transmitted through the transmission to the wheels.

The 2004 Elantra takes Hyundai SP-III Automatic Transmission Fluid, with 8.4 quarts used in a
total refill - but just 5 quarts at a fluid change. For How-to transmission fluid change instructions:

http://en.allexperts.com/q/Hyundai-Repair-815/2004-Elantra-GT-transmission.htm

www.autozone.com claims that 2004 Hyundai Elantra can take these non-Hyundai ATF's:
Lucas Semi-Synthetic Multivehicle ATF
Pennzoil Muilt Vehicle ATF
Valvoline Maxlife Dex/Merc ATF
Castrol High Mileage ATF

the pump is deep inside the transmission just behind the torque converter, u must pull the trans out to replace it, at that point you might as well do the whole trans, on a scale of 1-10, 10 being most difficult, this is a 9.

DTC P0734 is "4th gear incorrect ratio" - that's your torque converter 'LOCK UP Clutch" It's worn out (or you're using the wrong Trans Fluid required by Hyundai - SPIII OR Both). The P1624 is simply the Trans Control Unit (computer) communicating to the Engine Control Module (ECM computer) MIL - on request signal to ECM = asking it to turn on "Check Engine" light. MIL = Malfunction Indicator Lamp. When the torque converter 'lock up" clutch slips or doesn't engage, the trans automatically returns to 3rd gear and stays there (it's called 'fail safe' mode) until you cycle the ignition key, then everything resets until the malfunction occurs again.
You'll need to have an AT (Automatic Transaxel) tester hooked up to verify this. Hyundai has a Service Bulletin specifically for "gear incorrect ratio' codes. If when you changed the trans fluid, was it done at a Hyundai Service Center? If not, someone may have installed the wrong fluid.