I have a 97 TJ with a check engine light on. the codes are

12 Direct battery input to PCM was disconnected within the last 50 Key-on cycles.

21 Upstream oxygen sensor response slower than minimum required switching frequency. Upstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor input voltage maintained above the normal operating range. Oxygen sensor voltage too low, tested after cold start. (Upstream or Downstream) Left oxygen sensor input voltage maintained above the normal operating temperature.

On the code 21, do both O2 sensors need to be replaced or what? I really don't understand which O2 sensor is failing. I got a quote on the O2 sensors, $55.03 before cat and $67.07 after.

it is a good idea ti change both and keep the good one as a spare. if you hook your jeep up to a monitor you can watch the voltage drop, spike, flatline at 0 or whatever, for the O2 that isnt working right.