Dspr

Hi, this is my first post here, and I could really use some help! My daughter recently purchased a 2005 Lexus ES330. Recently the check engine light came on, and the code indicated “02 Sensor Signal Biased / Stuck Rich Bank 2 Sensor 1”. I purchased a new Denso Sensor, but I am confused as to which location is correct. I have attached a picture of the two sensors underneath toward the front. Is one of these the correct one? I really appreciate any guidance you may be able to offer. Thanks in advance!

Arsenii

Hello,

Starting to wonder if that's about time to make sticky thread for A/F sensors on those cars..

Here is a Parts Catalogue showing all the sensors.

4th-gen ES300 came with 4 sensors - 2 Upstream (or Sensor 1) sensors, installed Before the Cat, and 2 Downstream (or Sensor 2) sensors, installed After the Cat. This engine is a little backwards, with Bank 1 being towards the Firewall, and Bank 2 closer to the Radiator.

Both sensors that you pictured are Downstream sensors that measure the efficiency of the catalyst. Bank 2 Sensor 1 is located on the Front of the engine, Before the catalyst, it should be visible from under the hood, and lucky you, it should be the easiest one to replace.

Hope this helps and best of luck!

Dspr

Thank you!

Saintsfan1

I also had this code come up on my '01 es300. The mechanic suggested it wasn't that big of a deal, if I didn't mind the Check engine light. I don't. Any advice otherwise??

Arsenii

As written before, there are Two kinds of O2, or A/F sensors, Upstream and Downstream, the difference is where they are located in the system, Upstream is Before the Catalyst, Downstream is After respectively. Number of the sensor tells which one is it, Bank 1 Sensor 1 being the Upstream sensor for Bank 1, Bank 1 Sensor 2 being the Downstream sensor for Bank 1, or just a Downstream sensor for 3rd-gen ES, as there is only one of them in the car for all 3 Catalysts.

While Downstream sensor doesn't play a major role in engine operation, it is there to monitor the efficiency of the catalyst, Upstream sensor is a vital component for the Closed Loop operation, where the ECU can constantly vary the amount of fuel referencing from the MAF sensor, and then monitoring how fully it combusted using A/F sensor. A setup like that greatly increases gas mileage, as well as reducing the pollution of the outside air, not to mention the spared wear and tear on the catalysts that won't have to burn all the raw fuel that was wasted otherwise.

In short, Lack of an Upstream sensor forces the ECU to operate in an Open Loop mode, referencing from the predefined charts, which is not even close to being as accurate, making the engine a lot less efficient. Driving like that, you will spend a lot more in fuel at a distance than the price of that sensor, so there is literally no reason to leave it alone, it should be replaced as soon as possible. If your mechanic suggests leaving it alone, I wouldn't trust him for any engine diagnostics.

Hope this helps and best of luck!