Dodge Stratus ES 2.5 6 cyl

Hi! I had posted a couple months ago about my check enging light. I had codes of 12 - 31 - 21. I was informed that I should replace the O2 sensor, and it should cleaar out the problem for 21. The 31 code I believe was a gas cap code....stating that the gas cap I had did not tighten securely. I was told by a mechanic.

Well I had both O2 sensors replaced $443.00 and the 21 code is no longer their. I replaced my old gas cap with a new one and my check enging light still pops-up with the code "31". Any help would be appreciated. My smog was due in FREAKING JULY. I had been working on this code "31" for 6 months.