Chrysler Repair: check engine light on: codes 12 and 21, oxygen sensors, chrysler intrepid

Question
I have a 1996 Chrysler intrepid with a check engine light on, car still running ok cycled  key 3 times and got code 12-21-55 would like to know what the codes relate to...

Answer
Hi Gary,
The 12 code means that the battery was disconnected from the circuit for the engine controller (or the car as a whole) sometime in the past 50-100 starts, and this is usually of no significance. The 21 code means that one or both of the oxygen sensors, mounted on the exhaust manifolds, one on each, is not producing the proper voltage reading. It usually means replacement. You could check the voltage on the signal wire of each sensor (measure on the harness side of the disconnect plug) compared to ground after the engine has warmed up and is idling. On the left side measure the black/dark green and on the right side measure the tan/white wire's voltage compared to ground. Either a 0.1 or a 1.0 volt reading is suspicious, something in between would be normal. Use a fine pin inserted through the insulation to make contact with the wire to read the voltage.
The 55 code means "end of readout".
Roland