My 2000 Honda Civic is throwing a P0135 code. I have determined that the O2 sensor needs to be replaced. How do I know if I need to replace the upstream or downstream sensor? Thank you!!

 

The code P0135 implicates the primary sensor, the one closest to the engine. Check the resistance of the heater before replacing; with engine cold unplug sensor and measure resistance between the two wires that are the same color. Should be something like 40 ohms I think. Open circuit means the sensor is bad.

 

First make sure 7.5A dash fuse 15 is not blown.

 

Awesome, thank you everyone.