trips1

My mpg got worse. I am getting 14.5mpg in city driving which was previously about 15-16mpg. Note that i am overdue by about 1.5month/1200miles on my synthetic oil change. The oil looks fine so not sure if the reason for low mpg is old oil.

Anyways there is no engine light but based on one of the advice on low mpg post on this forum i decided to get the O2 sensor checked at an independent mechanic shop. He hooked up the diagnostic tool and his laptop and told me that my O2 sensors are bad. On his laptop he showed me that the voltage is constant 0.7V and ideally it should have been a wave with ups and downs.
Also the price i was quoted was total 600$ to replace all the 3 O2 sensors using Denso. 374*2 for the two front sensors and 275 for the rear sensor. This price includes parts and labor. For labor he quoted $85 for each sensor.

As such i have few questions:
- Can my O2 sensor be really bad without engine light being ON.
- Does all what he mentioned makes sense (about wave and not a constant 0.7V)? Should i go to another independent mechanic and ask for a free diagnostic? Do Toyota service center offer free diagnostic with oil change?
- After googling for Denso O2 sensor price his quote on parts definitely look expensive. What about labor? What should be a decent and reasonable price for the labor?
- Any suggestion on O2 sensor brand that i should look for (Bosch, Denso,etc). Any particular online parts shop or ebay seller with good rates for genuine parts?

Thanks and as usual i appreciate everyone on this forum for helpful tips.

trips1

After going through some more posts on this forum i got answers to some of the questions. The O2 sensor showing constant voltage definitely indicates weak sensor and it would be good to replace it.

Now i have question on A/F vs O2 sensor. I have 20001 2WD RX300. I am not clear on the difference between A/F vs O2 and what is there in my 2001 RX300.

Do i need the "denso 234-9009" from www.oxygensensor.com.
What is the meaning of "PRE-CAT FRONT" and "PRE-CAT REAR". As i understand the life of rear one's is much more than the front one's and i can replace the front one's first.
Unfortunately the independent mechanic i went to did not tell me what O2 sensor's are bad. He only mentioned that i can get the front one's done first if i want.

Thanks.

lexina

The 234-9009 is the front one. From what he said, more likely the front one is bad. A constant voltage means bad o2 sensor. I read here from another thread it was not hard to change yourself with a cutout socket tool. It is strange that you don't get engine light or code from a failing o2.

Pescao

Was he really doing the right job reading your O2? Lawyers, car sellers and mechanics will never go to heaven. Its very strange that your CEL it's not on. Maybe you have a blown CEL light too. Also, are you using 87 octane gas? I had a similar problem, change to 93 a Shell gas brand and know I'm getting about 18 MPG. $85 bucks per sensor is quite expensive, if you have some mechanics skills it won't be too hard to do it yourself.

trips1

Is there a way to check if the CEL light is working or not?
When i start my RX should it glow in the beginning along with all the other lights. Which one is the CEL light.

Also i am seeing another issue after getting my O2 sensor checked using the diagnostic tool. My key lock/unlock button does not light up the front and back small yellow lights. Can someone help me with what may have gone wrong. Probably i will post a separate thread for this as i could not find anything using forum search.

Sorry for ton of questions but being only slight knowledge of Auto's i am very wary of Auto Mechanic shops and always think they are doing something to rip me off.

Thanks again.

lexus114

The two Air Flow sensors (A/F) are located in each exhaust header. (front one,and rear one) the only o2 sensor is located after the cat.

lexina

Use the scan tool to check for Pending codes, if you do have codes, then cel light is defective. When you turn key to On position, all icons should be lid up, I think.