CEL P0138 - Rear O2 Sensor

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2008 Mazda5 GT
Went to the store and suddenly notice the CEL. Luckily I have a Scangauge2 connected and pulled code P0138. A quick search show that it a faulty rear O2 sensor used mainly for emissions. My 2008 only has 31K miles so I am surprised to see a faulty O2 sensor already

Seems like the rear O2 sensor is very expensive (>$200). I would usually just replace it but at this price I am tempted to try and clean it first. Anyone have good results with cleaning and if so what solution do you recommend?
 
Went to the store and suddenly notice the CEL. Luckily I have a Scangauge2 connected and pulled code P0138. A quick search show that it a faulty rear O2 sensor used mainly for emissions. My 2008 only has 31K miles so I am surprised to see a faulty O2 sensor already

Seems like the rear O2 sensor is very expensive (>$200). I would usually just replace it but at this price I am tempted to try and clean it first. Anyone have good results with cleaning and if so what solution do you recommend?

I've never had a bad O2 sensor on a newer car so I'm not sure about the warranty, but isn't an O2 sensor covered under the 10-year emissions warranty?
 
I've never had a bad O2 sensor on a newer car so I'm not sure about the warranty, but isn't an O2 sensor covered under the 10-year emissions warranty?

No, O2 sensors fall under the 2y/24k mile warranty period.

http://www.epa.gov/obd/420f09048.htm

There's a couple a little cheaper than $200, but dang, that alot of $$ for a plain ole narrow-band sensor.
http://www.rockauto.com/catalog/raframecatalog.php?carcode=1441856&parttype=5132


Edit: Here's one much more reasonable - http://www.partsgeek.com/r667d4h-ma..._content=WC&utm_campaign=PartsGeek+Bing&gb=pp
 
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Quick update. On the date that CEL came on, I rest the light twice and it came back within a few mins. I was sure it had malfunctioned or something is not right with it. Parked it overnight and the light is gone and has not come back since (happy shrugg).

@ flcruising, nice find on the Walker O2 sensor! That is a much more reasonable price. I too look at rockatuo first and was surprised to see the even the lowest price... I've used Walker O2 sensors in other cars with very good success. PnP too and no need to reused the old connector.
 
Well, it's back the last couple of weeks and it does not go away for long. Way too early for an O2 sensor to fail but it looks like I'm the only one having problems. I did Google some Mazda3 owners having early O2 sensor failure.

Based on other Mazda forums, the O2 sensor is a wear and tear part coverer under bumper-to-bumper. Emissions only cover the CAT and Powertrain covers the mechanical parts...
 
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i would guess simple electrical issue before a failed sensor at 30k miles. Check the electrical connector to the sensor make sure no leads are black. Maybe spray with dielectric grease.
 
How easy is it to replace an O2 sensor? I'm currently getting an error for bank 1 sensor 1 (front sensor?). I'd like to replace it myself, but not sure where it is located. I assume all I need is a wrench and ramps.

M5 (2008) has been running fine, but I need it to pass DEQ for new tags. DEQ won't even test it with the check engine light on.

EDIT: Found one sensor that runs to the back/top. Another runs front/under to someplace I can't see. Which one is bank 1 sensor 1?
 
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Firstly, thank YOU for bring up this old post. I've never replaced the sensor b/c it cost WAY too much for a simple narrow band O2 sensor and I am not willing to pay that much to replace a downstream O2. Cheapest I could find at the time was $177 from partsgeek (post by flcrusing). I've also been searching for an O2 simulator but it doesn't seem like this is an option for the Mz3/5.

I rechecked the link from partsgerk and it happens to be on sale + 5% coupon = $55.85 free s/h -WOOT! Then again, this product has matured and I see more options available from different vendor on rockauto also (67.79+s/h) but I have used and trust Walker O2 in the past (PnP). Will update you on how hard it is to replace on the Mz5. I've heard and do believe it will be more challenging (stripping and rounding out threads and heads) on this car b/c material quality is poorer.

You *should use an O2 sensor socket (really just deep socket with cutout for wiring) but you could always just cut the wiring off the old sensor if you are trashing it and use a deep socket. I recommend using some penetrating oil to loosen the threads. I usually first tighten tough bolts just a tad (yes, tighten to help break it loose and get it moving) before I attempt to loosen it – a breaker bar (long steel pipe to give you more torque with the wrench) will help too. It is not hard if it is easily accessable. I do not know which sensor is bank 1. You can try searching Mz3 owners.
 
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Since our vehicle is a 4 cylinder, there is only one BANK of O2 sensors. Sensor 1 would be the front sensor on the exhaust manifold that can be accessed from above very easily. Do you have the specific trouble code (DTC)?

Silentnoise713, the easiest method I have found to replace an O2 sensor, is to cut the wires and slip the closed end of a regular 6 point combo wrench onto the hex head.

If you need more leverage with any wrench, just do this...

133835974_9ca588913c.jpg
 
Flcrusing is right. There should only be two O2 sensors, one in the exhaust manifold (pre-cat/upstream) and one in the exhaust piping, after the catalytic converter, under the car (post-cat/downstream). I got confused for min and for some reason though this car had two upstream O2s (but ya never know with tighter emission regulations and electronically controlled doohickeys everywhere).


Perhaps it is a matter of preference but I always prefer socket wrench > combo (closed end) wrench. I do use the connect-a-wrench trick if the space is too tight to use a socket but I likes me a good 2’-3’ leverage when and where possible :D
 
Since our vehicle is a 4 cylinder, there is only one BANK of O2 sensors. Sensor 1 would be the front sensor on the exhaust manifold that can be accessed from above very easily. Do you have the specific trouble code (DTC)?

DTCs are P0134 and P2243

Are these the positions of the two sensors?
SlogA.jpg
 
Sensor 1 would be the one you are after.

Here it is straight from the service manual for DTC P0134.
DTC P0134Front HO2S no activity detected
DETECTION CONDITION
  • The PCM monitors the input voltage from the front HO2S when the following conditions are met. If the input voltage from the sensor never exceeds 0.55 V for 83.2 s, the PCM determines that sensor circuit is not activated.
MONITORING CONDITIONS
— HO2S, HO2S heater and TWC Repair Verification Drive Mode
— The following conditions are met
• Engine speed is above 1,500 rpm.
• Engine coolant temperature is above 70 C {158 F}.​
Diagnostic support note
• This is a continuous monitor (HO2S).
• The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
• PENDING CODE is available if the PCM detects the above malfunction condition during the first drive cycle.
• FREEZE FRAME DATA is available.
• DTC is stored in the PCM memory.​
POSSIBLE CAUSE• Front HO2S deterioration
• Front HO2S heater malfunction
• Exhaust system leakage
• Open circuit or short to ground in the wiring harness between front HO2S terminal A and PCM terminal 2AG
• Insufficient compression
• Engine malfunction

No record in the manual for P2243, but I would guess it is probably a result of P0134 (dead sensor).
BTW, sorry to thread-jack Silentnoise713, if you want to move this to another topic for searchability purposes, then by-all-means.
 
Am I correct on the position of Sensor 1 in the photo above?

p2243 - O2 Sensor Reference Voltage Circuit/Open Bank 1 Sensor 1

So both codes seem to indicate it is a dead sensor.
 
Two comments:

1. Check the sensor with a DMM first. If you dig around, maybe call the dealer or look in Haynes/Chilton, you can get the resistance across the leads and confirm whether it's good or bad.
2. Clean it with throttle body cleaner that says it's safe for O2 sensors. It needs to dry 100% before re-installation.
 
Never thought about that Robotaz as I'm accustomed to narrow-band sensors that don't work so well with a DMM since they produce an oscillating voltage. The front sensor on our vehicles is a wide-band type which requires a reference voltage (12v). If you can figure out which is the signal wire, you can theoretically test it with a DMM as it will vary (0-5V) based on A/F ratio. The sensor must be powered to work, so I would isolate the signal wire and disconnect it from the harness so you don't risk damage to the PCM.

The resistance test for a narrow-band sensor could only tell you whether the heating element was bad or not; I don't know if the heating element works the same for the wide-band O2.

Let us know how it goes.
 
Does Sensor 1 have pins in the connector and Sensor 2 have the blades, or is it the other way around?

I also noticed Denso and Walker are the same price for the downstream sensor. Best to go with the Denso OE?

Also, I'll ask a third time, did I correctly indicate the positions of the sensors in my photo above?
 
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I know Sensor 1 is the one I'm after. How about an answer to my questions so I know I'm ordering the right part?

Does Sensor 1 have pins in the connector and Sensor 2 have the blades, or is it the other way around?

I also noticed Denso and Walker are the same price for the downstream sensor. Best to go with the Denso OE?

Also, I'll ask a third time, did I correctly indicate the positions of the sensors in my photo above?
 
Am I missing something here? This is from last week...

Since our vehicle is a 4 cylinder, there is only one BANK of O2 sensors. Sensor 1 would be the front sensor on the exhaust manifold

Then you replied with an image and the sensors labeled 1 and 2. I told you that sensor 1 (front) was the one you wanted to replace.

Downstream would be rear (sensor 2), therefore upstream would be front (sensor 1).

I have no idea why the connection type matters. Again, the upstream sensor is a wide-band sensor (5-wire), whereas the downstream is a narrow-band (4-wire). They aren't interchangeable. You are welcome though to open your hood and confirm as the connectors can be easily accessed from the passenger side. If something isn't clear, by-all-means, ask for clarity. I'm just not sure any other ways I could explain, otherwise you will have to wait for another contributor.
 
"Sensor 1 is the one you are after" as an answer to the question "is the photo correct?" led to my confusion. I'll stop asking and assume that my photo is correct on both locations.

The reason I asked about the connector type is that there are several O2 sensors listed for the vehicle on partgeek, some listed "front" and "rear" and some listed as "upstream" and "downstream." One listed as a "front" and one listed as "upstream" differ by $43, yet both are made by Walker. One had pin connectors and the other looked like pins, but it was hard to tell for sure from the photo. So I only asked to be absolutely sure I was ordering the right one AND to be absolutely sure I had the correct location of Sensor 1.

Sorry about the misunderstanding.
 
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