Oxygen Sensor Locations
 

It's driving me nuts I'm getting P0135 O2 sensor heater malfunction, "02 Heater ckt Bank 1 Sensor 1"

I found the o2 sensor on the bank closest the firewall and removed it. I then tested the heater circuit. I had 7 ohms across the element in the o2 sensor and 12V coming from the car (with ignition on).

I was perplexed but replaced the sensor anyway with a known good sensor I had laying around.

Next I figured maybe I was nuts so I checked the one downstream the cat. Same story.

Is there a 3rd o2 Sensor on this motor? I cannot for the life of me find anything on the front of the engine closest the radiator. But think maybe I'm looking at the wrong one?

It's a 1998 Buick Century with 3.1 L V6.

Thanks!!!!!!!