Hi can anyone tell me how many 02 sensor on the above car , I check today and only saw 3 , I thought there should be 4, seeing that is has 2 catylic convertor.

Code po138 came up replace the sensor that is mounted in engine , but the same code is still showing up .

Thanks

 

There are 4 sensors. One upstream and one downstream of each catalytic converter. There is a cat converter for each bank of cylinders. Bank 1 is the rear bank, Bank 2 is the front bank.
The upstream sensor on the rear bank is accessible from underneath but you'll be working blind. When pulling an O2 sensor, be aware that the threads may be corroded a bit where the sensor protrudes into the exhaust pipe. You don't want to drag that corrosion through the threads if you can help it. If the sensor starts to get stuck as you are threading it out, run it back in and work it in and out a little to knock off the chunks of rust.