1995 Chrysler Cirrus shutting off, limp mode

You have code 12, 21, and 55
12 and 55 is the beginning and ending of test OK to have
code 21 my book saying its an O2 SENSOR staying at center

let me know

TEST TC-32A - UPSTREAM O2S STAYS AT CENTER

Start engine. Allow engine to reach normal operating temperature. Using scan tool, set engine speed to 1500 RPM. Using scan tool, read upstream Oxygen Sensor (O2S) state. If upstream O2S state is not switching, go to step 5). If upstream O2S state is switching, go to next step.
While observing scan tool display, wiggle upstream O2S connector and harness. Upstream O2S is located on front exhaust pipe. If at any time scan tool displays UPSTREAM O2S LOCKED AT CENTER, repair connector or harness that caused upstream O2S to lock at center. If scan tool does not display UPSTREAM O2S LOCKED AT CENTER, go to next step.
At this time, the condition required to set DTC is not present. UPSTREAM O2S LOCKED AT CENTER DTC sets if upstream O2S output voltage stays at 0.5 volt for 1.5 minutes with engine temperature more than 170 °F (76.6 °C) and engine running for 2 minutes. Possible causes are: upstream O2S circuit open, failed upstream O2S, failed PCM, failed connector terminals or connector wires. Go to next step.
Inspect upstream O2S wiring and connectors. Repair as required. If wiring and connectors are okay, test is complete.
If upstream O2S state is not switching in step 1), turn engine off. Turn ignition on. Disconnect upstream O2S connector. Using scan tool in ohmmeter mode, check resistance of upstream O2S connector terminal No. 1 (Black/Dark Green wire on FJ22 and F24S bodies; Black/Light Blue wire on JA and PL bodies). If resistance is more than 5 ohms, repair open to ground in Black/Dark Green wire or Black/Light Blue wire. If resistance is less than 5 ohms, go to next step.
Connect a jumper wire between upstream O2S connector terminal No. 2 (White/Black wire on FJ22 and F24S bodies; Black/Dark Green wire on JA and PL bodies) and positive battery terminal. Using scan tool, read upstream O2S voltage. If voltage is more than one volt, replace upstream O2S. If voltage is less than one volt, go to next step.
Turn ignition off. Disconnect Powertrain Control Module (PCM) connector. Inspect PCM connector. Repair any terminals that are damaged, pushed out or miswired. If PCM connector is okay, go to next step.
Using an external ohmmeter, check resistance of Upstream O2S signal circuit (White/Black wire on FJ22 and F24S bodies; Black/Dark Green wire on JA and PL bodies) between upstream O2S connector terminal No. 2 and PCM connector terminal No. 8 on F24S, FJ22 and PL bodies or No. 30 on JA body. If resistance is more than 5 ohms, repair open White/Black wire or Black/Dark Green wire. If resistance is less than 5 ohms, replace PCM.

Code 12 will set if battery feed to PCM has been disconnected within last 50-100 starts. PCM monitors circuit when ignition is on.

You have code 12 I gave you the meaning of it
you have code 55 I gave you the meaning of it
and the same goes to code 21
Code 12 and code 55 not a concern
should work on checking code 21
what is it that you don't understand?
What else you need me to explain to you!

Let me know

Limp mode?
Is it just the idle
or its in limp mode and not shifting?
You can disconnect the battery and clean idle (throttle body )
reconnect the battery, start engine (it may stall on you its OK )
take it for a drive (few miles )
let me know