Check Engine Light nightmare

I have a 2002 2WD 4.7L PreRunner, with error code P1135 Air/Fuel Sensor Heater malfunction (Bank 1 Sensor 1). I bought and replaced the upstream O2 sensor (suggested by autozone diagnostic) and 15 miles later the check engine light is back on. How tight does the O2 sensor need to be in the flange? If air is leaking by will that cause an error code?
My plan is to replace the downstream O2 sensor next.

What else can I throw my money at before going in for Cat convertor?

Doug

 

I just got back from Autozone, I now have a P0420 code "Catalyst system efficiency below threshold - bank 1
Probable cause:
Air leak in exhaust before rear HO2S
AF sensor error
Fuel system fault
Faulty catalytic converter

Ok, any good ideas as to where I should go next?

Doug

 

The truck has 145,000 original miles. It does a have a 3" lift but has never been wheel'n. Very straight truck. My choses are to buy and replace the catalytic converter. Question, it looks like there may by two cats on this rig. The first one seems to be part of the 1st stage below the exhaust manifold coupling. The next one is between that one and the muffler.

Is swapping these parts a project that a average mechanic can accomplish?

Shopping for full exhaust system online, any ideas?

Thanks,
Doug

 

Swap the left and right side sensors from side to side to see if the trouble follows. I believe you have a bad sensor after the cat with that 420 code.