What has been suggested here is that a fault code is being registered by the sensors, when operating normally and detecting an incorrect mixture.
I note that the car had been stolen. Did the thieves try to bridge the ignition circuit (hot wire it), or otherwise mess something up in the loom? Are the O2 sensors in fact faulty? They are easily checked together with the circuits involved.
With the engine idling there should be between 0.1 and 1.0 volt, pin four to ground at the associated connector.
With the connector disconnected:-
The resistances on the sensor side, at pin 4 should measure, 1 Meg ohm, (1000,000 ohms) minimum, to a ground close to the sensor body.
Ignition switched on, there should be 10 volts or more, on the loom side at pin 2. Pin 1, should record a direct connection to ground.