I have a 2001 Toyota Camry 2

I have a 2001 toyota camry 2.2L LE of which I have repaired d exhaust for leakage and it started having lots of petrol consumption problem.I took it to my mechanic and they diagnose with OBD II and found it has P1135 error. He then serviced the injector and change the heater circuit sensor at the exhaust manifold. After driving for a day same problem condition came back.I was adviced to buy the catalytic reactor beneath the car, which I did. No curative response. Then I was ask to change the entire car exhaust sytem: from the manifold to the the tail end. After fixing the entire exhaust.I drove d car for only four days and started noticing d car abnormal petrol consumption is back. I took the car back and the mechanic checked and found that it is still having P1135 erropr code: "Air-fuel heater circuit sensor response malfunction". Where could this problem arise from?