Since I turboed my 2000 civic w/ D16y7 I moved the O2 sensors down and under the engine. The primary is right at where the waste gate pipe meets the main turbo exhaust. The secondary O2 is about 20" down the pipe. No cat. I didn't have any problem with a code to begin with. Winter rolls around and snow and water hit the underside of the car and I got a CEL. I also have a S-AFC installed but I don't use it except as a tach and boost gauge. I do have one of the inputs connected to the O2 wire so I can watch the voltage on the primary O2 sensor. When I got a CEL the car would run like crap in closed loop operation. The voltage from the sensor was very low and not responding correctly. I figured that the snow and water was cooling the sensor and causing it to malfunction. Now days I sometimes get the CEL after a cold start and within a few minutes. I think the heater in it went bad.

I am going to do a major 100K miles tune up and will replace the O2 sensor. Will I need a new secondary sensor? I don't know if it even works or what it for. Something to do with the cat but I don't have one and it didn't seem to notice. Does it effect performance at all? I had an autoparts clerk tell me it was too "calibrate" the engine but I think he's full of ****.