hi,,I have a 96 tacoma 2.7 L and code p 420 that says cat operating below efficent threshold and so I replaced the rear oxygen sensor which threw a code up as well and then replaced the cat ....the check engine light went off for a month or so and came back on....any ideas why.....no other codes...no code for the oxygen sensors......any input would be helpful....

has anyone experienced this problem....the code went out in cold weather and came back on in hot weather