O2 Sensor (P0138) CEL Code?

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dfskinner

Member
:
2006 Mazda 5
Anyone out there had any issues with this sensor or code?

CEL came on yesterday for a little while and then went out........ came back on this morning so I dropped by Auto Zone for a free reading.... P0138

Bank 2 Sensor 1
P0138
Rear HO2S circuit high input ON 2 HO2S C, O, R
(See 0102A58 DTC P0138 [L8, LF])

****************************************************************
DTC P0138 Rear HO2S circuit high input

DETECTION CONDITION
The PCM monitors the input voltage from the rear HO2S. If the input voltage from the rear HO2S sensor is above 1.2 V for 0.8 s, the PCM determines that circuit input is high.

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.

POSSIBLECAUSE
Rear HO2S malfunction
Short to power supply in the wiring harness between rear HO2S terminal A and PCM terminal 2AH
Rear HO2S or PCM terminal is shorted
PCM malfunction
 
Every car will have at least one go at some point..you can get a direct replacement with the plug or connect the wires yourself and save some $$. Which reminds me. I need to find out how many the 5 has. My old Maxima had 3..
 
Pretty interesting, today the light is out? I'm going to try and clean it first......... Auto Zone; Front sensor is about $65.00; Rear sensor is $245.00....... Kind of expensive for the second one, which is the one that's giving me some trouble...... Advanced Auto; $114 pre cat and $114 post cat for NGK's. I'm going to check with the dealer and just see what they list...... never hurts to get a part number.

I'll let you guys know how it goes.....
2006Mazda5O2Sensors.jpg

Here's a copy of what's in the 2006 Repair Manual..... I believe the Mazda 5 only has the two O2 Sensor's;
 
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I think you're going to have to replace the unit. They DO go bad, and when it's cold, it can happen more often. Just drive until you get $$$ or find a good deal. It will not affect the driveability or reliability of the vehicle. You MAY see some reduced gas mileage.
 
sounbwoy/skyhawk..... thanks..... cleaned the O2 Sensor and connectors, ensured everthing had good connections and cleared the code........ the price at AutoZone was for NGK's........ hopefully this will take care of it, if not I'll be replacing the O2 Sensor.....

I've not seen any difference in gas mileage or power, but I’ll keep an eye out......
 
"I've not seen any difference in gas mileage or power, but I’ll keep an eye out......"

You probably won't. The secondary O2 sensor is there mainly as an emission control. It main purpose is to monitor catalytic converter efficiency. If your primary is good, you should continue to be ok. You will notice that at many auto parts stores they only have the primary available as an aftermarket direct fit. If they sell the secondary at all, it may well be OEM (also why it is so expensive). I've known customers that have gone for years without a secondary with no ill effects (except of course, the CEL staring them in the face all the time).
 
I have the p0138 code, bank 1 sensor 2. Would this code be related to the front Oxygen sensor? upstream or downstream? It looks like a fairly easy DIY job. Can someone give me some guidance? Thanks

Which should I be buying? are there any difference in Life expectancy from a Generic/NTK, Bosch or Denso, ACDELCO?

Is there benefit to getting a better brand o2 sensor, Difference is quite a bit.
 
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mileage and age of car

I just had the P0138 code pop up earlier this month. I replaced the downstream oxygen sensor, i.e. the heated oxygen sensor, which is NTK24447 from Amazon. It was around $50 for this OEM part.

From the top of the engine compartment, I disconnected the electrical connector, the plastic tie down (which I simply broke off, the new sensor came with a new one), and unscrewed the nut holding the mid-pt of the wire (10mm ratchet required here). From underneath the car I managed to use a 22mm open ended wrench to disconnect the sensor from the exhaust. I did spray every threaded connection with PB blaster the day before, but the sensor came out surprisingly easily.

After driving the car with the new sensor (which comes with antiseize on the threads already) all day for about 30 miles, the CEL went out by itself.

BTW, my Mazda 5 has ~65k miles and is a 2009 for reference.
 
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Is there any real data on how long these o2 sensors are supposed to last? I don't really go for the "replace at a given mileage" bit. That being said, I can't really understand why some fail at 65K and others will go the life of the car. My old Subaru puked up a primary sensor at about 85K and the replacement is still fine at 263K
 
The oil you use can cause them to fail faster. My guess is they're made by Bosch, so of course crappy.
 
All of my cars still have the original downstream O2, 1 of them even above 255KM. They in theory shouldn't wear out if the system is designed right (your cat will get poisoned first) and made right. The upstream wears out much faster and got lazy at around 100-150K miles in my case. No CEL, but smog check said they are lazy with both high NOx and HC, CO at 15mph.
 
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