Whats the difference?

I am getting a code P1135 and the guy at AutoZone said it was the downstream sensor. I think it is the upstream one. I am planning on replacing both, want to make sure I understand and order the correct DENSO part.

Thanks,!

 

P1135 is a Bank 1 Sensor 1 heater circuit malfunction. Sensor 1 is the pre-cat A/F sensor not the O2 sensor after the cat. You should be able to check this with your volt/ohm meter. Check for heater resistance. If the heater is burned out, you will get an open circuit. If you get a low resistance (a few ohms) it is ok. Then check to see if its getting power.