1997 Stratus Codes

My "check engine" light goes on and off. I did a on board self diagnostic test by using the ignition key sequence. The codes that I get are:

12
31
21

When I check the codes, sometimes only 12 pops-up, and sometimes all three show up....please someone help. I need to repair this so I can register my car. thanks in advance

12 = problem with battery connection

31 = EVAP system fault

21 = Problem With 0xygen sensor signal citcuit. Sensor Voltage to computer not fluctuating

i would start with the o2 sensor

12 = problem with battery connection

31 = EVAP system fault

21 = Problem With 0xygen sensor signal citcuit. Sensor Voltage to computer not fluctuating

i would start with the o2 sensor




How can I repair this and where is the sensor at? thank you

There may be more than 1, but for sure you'll find one in the exhaust manifold