P0240 code after catalytic converter replacement

I recently had my catalytic converter replaced with a new oem converter. I had the codes cleared. But now the check engine light comes and goes. I've replaced the thermostat. Did a full tune up. Ran sea foam through it. But the light still comes on ( for about 300 miles) then goes off for about 500 miles. This is getting frustrating trying to figure it out. Im wondering if the o2's need replaced.