OBDII Codes PO156 & PO161
 

Hi,
My 1999 ML320 displayed Check Engine Light, it went on and off a couple times. At the present time, the light has been off for more than a week. OBDII reader scanned code PO156 (bank2, sensor2) and pending code PO161 (bank 2, sensor2). Is the O2 sensor defective? Or might it be a wiring? The car has only 49k miles.

Thank you.

You need a left rear O2 sensor!

I had a dealer replace the left rear sensor (was throwing PO156 and 161 codes). About a year and few month later, CEL came on and now I have PO161 again(O2 sensor heater circuit malfunction) and pending code PO173 (fuel trim malfunction) stored. And it is the same bank and sensor again. Should I replace it again or is it an indication of something else going wrong?

Anyone? I suspect it might be Mass Airflow Sensor, but I wanted to confirm with you folks first. Thanks.