My car is a Volvo XC90,2011,3.2, 176,000 miles, everything works very well and the engine performs well, except Check Engine Light is on, the code is 420 related the Cats and I have replaced both cats (brand new, aftermarket part, 5 years guarantee)and two sensors on bank1 side, things get better but CEL is still on every now and then, comes on and disappears automatically, probably once a week, always the front O2 sensor on bank1 side problem. The technician told me, it's related to oil burning of the engine. To try to figure it out, I went to official Volvo car care center, the advisor there told me the real problem is the aftermarket Cats I used, which is defective, no great importance to the engine burning, by the way, the engine consumes 1quart every 2000 miles, which is normal according to Volvo car care center said. Also, Volvo told me, I have to redo it, by using original Volvo parts, which will be a huge cost. My questions are 1.Does engine burning oil damage the sensor and Cat Converter? 2. Aftermarket parts match my car? 3. Do I have to Chang my engine or fix the oil burning, other options?

 

a P0420 is a catalytic efficiency error which is a calculation based on the delta between the front O2 sensor and the rear O2 sensor. If as you noted you replaced the cat, then its not likely to the parts bur rather the sensitivity of the computer (thus the slam on after market parts not quite being to factory standards). This can be due to the cat or the brand of O2 sensor. This is more common to turbo cars with ECU upgrades - which is why its common to insert a "cel-boss" spacer between the rear O2 sensor and the cat or even have the ECU disable the input from the rear O2 sensors.