1994 jeep grand cherokee 5.2 o2 sensor

After doing the repair, you have to cancel the codes with a scan tool or disconnect the battery 10 minutes. Then see if it comes back. It should not unless there's another problem.

yes still losing power and same code

My info shows there are two O2 sensors so the code 21 could mean one or both need replaced. They are identical so buy another one and do both. Cancel code and recheck. It might just be bad connection, 1994 is getting pretty old with corrosion. Code 12 means the codes were recently deleted. Other things cause power loss, fuel filter, spark plugs, etcetera. Hope this helps.