1997 Plymouth Voyager Check engine light

12 = Direct battery input to PCM was disconnected withing the last 50 key-on cycles.*

21 = Upstream oxygen sensor voltage shorted to ground (tested after key off and at start to run).**
or
Left oxygen sensor input voltage maintained above the normal operating range.**
or
Upstream oxygen sensor response slower than minimum required switching frequency or valve does not go above.65 volts.**
or
Upstream oxygen sensor heating element circuit malfunction.**
or
Downstream oxygen sensor voltage shorted to ground (tested after key off and at start to run).
Or
Downstream oxygen sensor input voltage maintained above normal operating range.
Or
Neither rich nor lean condition detected from the downstream oxygen sensor.
Or
Downstream oxygen sensor heating element circuit malfunction.

31 = Insufficient or excessive vapour flow detected during evaporative emission system operation.
Or
An open or shorted condition detected in the duty cycle purge solenoid circuit.

55 = Completion of fault code display on Check Engine Lamp (end of codes).*