Introduction
One year after replacing the rear oxygen sensor of my 2010 Mazda 5, the yellow engine warning symbol came on again. I read out the error code, with my 'OBD 2' device, and it was again reporting fault code P0138 ("bank 1 sensor 2") problem https://cartreatments.com/p0138/
As the device was fairly new, I susptected it was something else. Clearing the fault code while the engine was warm, seemed to work, so I suspected the built-in heat element was not working. However the fuse #37 was OK (see https://www.autogenius.info/mazda-5-2007...)
I then removed the sensor and cleaned it using ultrasound (and gazonline), but that did not solve the problem.
In disbelief I went on and checkes all (!) the relays, but they alle seemed OK
Finally I decided to order a new sensor from a webshop (somehow all webshops I found seem to share the same Berlin address...). This is the part I ordered: https://www.motordoctor.dk/products/8126...
Before mounting the new sensor, I did a non-scientific check of the pins (see the steps below) and the results are listed in this table:
| Pins | New sensor | Old defect sensor |
|---|---|---|
| 1-2 | 5.06 Ohm | 6.2 Ohm |
| 1-3 | 48 pF | 6.5 Ohm |
| 1-4 | 49 pF | 52 kOhm |
| 2-3 | 46 pF | 0.9 Ohm |
| 2-4 | 49 pF | 53 kOhm |
| 3-4 | 46 pF | 53 kOhm |
As you can see from the table above, there clearly is a difference in the behaviour of the new and the old device.
I installed the new device and followed the instructions:
- clear the error codes
- disconnect the car battery for som minutes
- take the car for a 10 minute drive
After this, the problem was gone.
Tools
Parts
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Unplug the sensor and decide on a numbering scheme. Also clean the pins to ensure this is not a connection problem
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Check all combinations of the pins of problematic sensor. See comparison chart above (with the new replacement sensor).
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If you do not have such a tester, a normal multimeter (resistance and capacitance) should be sufficient
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If you would like a transistor tester, you can search Aliexpress for one. It is an amasing little device. https://www.aliexpress.com/item/10050015...
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Leasson learned: probe the oxygen sensor pins if you experience problems:
- Pin 1-2 is the pre-heating element, and should show around 5-6 Ohm (meaning that the pre-heating must be using around 60W)
- All other combinations of pins should show a capacitance around 50 pF
There is a clear difference in the meassurements when comparing the defect part and the new replacement part.
Here are some good resources on Mazda and oxygen sensors, but remember: meassure the pins and look for signs of a defect device before you start taking tings apart
https://www.youtube.com/watch?v=jFFR4307...
https://www.youtube.com/watch?v=XC5HWd7P...
