2005 Honda accord lx 2.4l P0139 code
 

A couple of weeks ago my MIL came on with the code p0139. So I changed the downstream O2 sensor. About a week after changing the O2 sensor and resetting the code, the MIL light came on again. Took my car to my mechanic. He put my car on his reader came back with the same code. Reset the code again. Told him that the O2 sensor was new and had been changed for the second time within a couple months. So he told me that it was probably my catalytic converter. 2 days ago the MIL light came on again... so, I ordered a new catalytic converter. Today the light turned off. Wasn't reset or anyting. Is this a good sign?

P0139 is "slow response" in the 2ndary sensor. I don't know why it came back (presume it was actually reset by the scanner).

Yes. Every time the MIL came on, the codes were reset with the scanner. MIL came back on yesterday. I have a catalytic converter on the way so I hope it fixes the problem.