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..