Where is the rear O2 sensor and how do I change it?
 

I got a CEL yesterday morning with a code of P0037 which I know is the rear O2 sensor. I believe it is telling me that it needs to be replaced. However, I had just filled up my car with gas the evening before I got the CEL. So, I checked the gas cap to be sure it was tightened. In fact, I removed it and replaced it to be sure it was screwed on properly, but I am still getting the CEL. It looks like I will probably end up replacing the rear O2 sensor. I have read a thread here that I can get one for around $80 on ebay so I will check that out. Is this something that I should be able to replace myself? If I have to drop the gas tank or something else major then I will end up going to the dealer (groan). Thanks for any help in advance.

If you have to ask then it is out of your league my friend.

Its in the CAT, but the code isn't because the sensor is bad.

Just reset your PCM and see what happens.

Sometimes a sharp change in the weather can set it off, especially on the 2005 models.

Thanks
 

MM, thanks for your reply. I read on another thread that I can just jack up the passenger side and unscrew the O2 sensor from the catalytic converter canister. I have a 2004 RX8 so I wouldn't be surprised if it has the same problem as the 2005. I have about 80K miles on my car so I am not surprised if it has finally given up the ghost so to speak. I am OK with jacking up the car, removing the O2 sensor and reconnecting the wire if that is all there is to it. However, I also thought that I had read about the need to reset the PCM anyway. Should I just reset it first, as you suggest, and see what happens? Since the car has so many miles and this O2 sensor has never been replaced I am prepared to just deal with it. Of course, if the CEL goes away after resetting the PCM then I guess I can save my money for a later date when the O2 sensor is actually fried.

Might as well.
Here is a thread describing how to perform the reset:
https://www.rx8club.com/showthread.p...ighlight=reset

Basically, what you want to perform is the NVRAM reset.
This is how you reset it:
1. Turn ignition switch to the "ON" position, but do not start engine.
2. Pump the brake pedal at least 20 times within 8 seconds after the ignition switch is turned ON.
3. Watch for the oil pressure gauge to sweep to the mid-position and back down
4. After that stop pumping the brakes and turn the ignition switch to the 'OFF' position.
5. Start the car.

Still no luck
 

Thanks for the information and the post. I tried the PCM reset that is being broadly communicated using the odometer reset, and you are correct in that it doesn't make the CEL go away. Now, it is still possible that the O2 sensor is dead so I may just follow the procedure you suggest and see if a real PCM reset works. If not, then I will just replace the O2 sensor.

In the event that the CEL still persists after a real PCM reset, that brings me back to my original question re: the rear O2 sensor of "how do I change it?". Is there a DIY on how to do this? Don't let the moniker of "ProfDean" fool you. Yes, I have been an adjunct college professor for nearly 20 years, but I have been a full-time engineer for the last 28 years.

Then you need to quit listening to your broad communicators because there is no such thing as an odometer reset. ;)

1. Disconnect O2 sensor's wire harness
2. Unscrew O2 sensor
3. Screw in new O2 sensor
4. Connect wire harness for new O2 sensor.

O2 sensors do go bad. I've had to change them on a number of vehicles over the years.

"Broad communicators"; isn't that sexist? ;)

Now we are getting some where!
 

Thanks for the post and I agree that the idea of an odometer reset is simply BS. Also, thanks for the simple step-by-step procedure. Based on the only picture I have seen posted of the rear O2 connector it looked like it was recessed and I wasn't sure if I could get an open end or box end wrench on it, or if I needed a special tool to get it out. So, do you think a 20mm wrench would fit? I'll have to see what my largest metric wrench is in my workshop (hopefully, it is large enough but if not I will go to Sears and get one).

Any recommendations on whether I should go with a Mazda OEM sensor or a Bosch, Denso, etc. brand? I am leery of sensors that say they are "universal".

I would just get an OEM sensor. When I changed mine I didn't want to risk breaking the old one so I purchased a socket designed for the job.

You can get to it with an open-ended wrench.
I would stick with the OEM sensor.
I forget what size wrench to use, but I used a standard size to get it off.

http://www.harborfreight.com/cpi/cta...emnumber=95045

Well worth the small money. "The right tool...."

7/8" = 22mm (close enough for government work ;)

Many thanks!
 

I ordered the socket. Now I have to order the OEM O2 rear sensor.

Did you ever change that O2 Sensor?

Did you have any other symptoms in your cars performance other than the CEL?

Hi... I recieved the CEL light last week and I found out yesterday that my O2 sensor is damaged or is not working. I think its probably because i bumped into a speed bump last time and I think I probably had broken the sesnor. My question is that do I really have to change the sensor? and is there any way that i just disconnect the sensor so that I can get rid of the CEL light?

1) You can't bump the O2 sensor and break it...you would have broken your cat off first.
2) Get your CEL read at an auto parts store and return here with the code.
3) No you can't just unplug it and make everything right with the world.

Thank you very much for your reply DeViLbOi

I've already got my CEL read and it says that my O2 sensor is not working....Does O2 sensor makes any difference in car's performance? and can I replace it by myself?

The front O2 sensor will make a big difference in performance...the rear will not. You can replace it yourself with an open ended wrench, or the special socket,...and depending on age you may require a torch.

Thank you very much for the quick reply. Can you tell me where the Sensors are located?

Note parts '6' and '9' in the enclosed for O2 sensor locations (one on the exhaust manifold and one on the Cat). 'SST' refers to the deep socket we're discussed.

Edit: added a closeup member's annotated rendition.

You can replace the sensor yourself, however, this is not what I would call "easy" as exhaust bolts are the worst in the world to remove.

There are two ways that I can think of to do this:

1st way (the way I've never done)
1. Disconnect o2 sensor wiring harness from above cat (this will be a very tight fit and i'm sure it can be done)
2. remove o2 sensor guard pictured in Huey's diagram. This has two bolts on each side the protect the sensor from stuff your run over.
3. Use a wrench and try to remove the sensor itself.

Again, all these bolts are going to be heat cycled on there and rusty because mazda has never heard of stainless steel fasteners. In theory, its real easy, but in practice, I'm not so sure.

The other way is to remove the cat, and with the car off the car, remove the sensor.
Remove the cat by disconnecting the front flange from the header (3 bolts, all of which will require a breaker bar or impact wrench when it will fit) and the rear flange (2 spring loaded bolts with donut gasket)
And disconnect the o2 sensor wiring harness from the body.

Exhaust bolts are a huge paid in the ass, so good luck. (JonG and I know about this first hand....)

If you're going to go through all the trouble, might as well get a mid pipe and at least gain some HP in the process. :rock:

I have my cat just chillin in the basement if you want a picture where the O2 sensor is along with the bolts and such.

^Or an RP SuperCat to maintain emissons passage, with increased flow.

A lift is a good idea as well. It can be done on jackstands, but way easier up in the air.

Soak the nuts in WD-40 for several hours and then use a breaker bar and/or impact wrench.

^understatement of the century

Steve...wanna crawl under my car and put my cat back on? :P