Need help in interpreting the "Check Engine" codes for a Kia Rio
May 31, 2018 8:50 PM   Subscribe

I have a 2008 Kia Rio. It failed inspection because of the "Check Engine" light. My Google-fu is failing me in diagnosing the exact problem.

Here are the error codes associated with the Check Engine condition:

P0139 O2 sensor 12 circuit slow response
P0140 O2 sensor 12 circuit no activity
P2096 Post catalyst fuel trim system too lean bank 1

I gather that an oxygen sensor might be bad, but which one? And is that the only problem, or is something else going on, too? Some sources say that there is an upstream O2 sensor and a downstream one. Other sources just refer to an oxygen sensor (singular) without any implication that there might be two of them in the car. The more I read about it, the more confused I get.

One of my neighbors is good with cars but has poor English skills. He told me that if I buy the part (or parts), he will do the repair work for me. But he can't research the codes himself, so I need to figure out which part(s) to buy.
posted by akk2014 to Technology (6 answers total)
If you can't figure anything else out, check the Haynes manual (or better yet find a copy of the service manual online) and see if there are indeed two O2 sensors and if they are interchangeable, as they are on some cars. If so, swapping them, clearing the codes, and reading them again once the CEL comes back on should change the reported faults, which should give you an answer as to whether it's a bad sensor or something else is actually wrong. (I'm assuming you/your neighbor have already checked to make sure it isn't just a loose connection)

Also, depending on which state you are in, the emissions equipment may still be under warranty. I believe California requires 12 years.

One other thing that might help if you are a person who is generally good with logical troubleshooting is to get a cheap Bluetooth OBD2/Canbus dongle from Amazon and the Torque app for your phone so you can see the sensor values in real time.
posted by wierdo at 10:08 PM on May 31, 2018 [1 favorite]

Best answer: The 2008 Kia Rio uses an inline 4 cylinder.
There is only one "bank" of cylinders in this car.
There are two 02 sensors. Sensor one is before, or upstream, of the catalytic converter. Sensor 2 is after the cat or downstream.

P0139 O2 sensor 12 circuit slow response - This means that the downstream 02 sensor is showing bad voltage when fuel trim is changed.
P0140 O2 sensor 12 circuit no activity - The downstream 02 sensor is not responding at all to fuel cut on deceleration.
P2096 Post catalyst fuel trim system too lean bank 1 - The 02 sensor downstream of the cat is reporting lean trim. This is because it is not responding as shown by the above codes.

I'd look to make sure that the 02 sensor connectors are plugged in. If it is I'd be tempted to replace the downstream sensor without further diagnosis. Autozone has them for $101 You may be able to cross shop or find a used one for cheaper.

Keep in mind that there are a lot of things that can cause this failure. However, they would cause a noticeable drop in power and probably do not apply to you.
posted by pdoege at 10:11 PM on May 31, 2018 [7 favorites]

If you use a code scanner to clear codes, bear in mind that the OBD II system has "readiness monitors" which log the results of various self-tests the vehicle does on its emissions and engine control systems. The emission inspection procedure I'm familiar with requires that (depending on the model year) either most or all of the readiness monitors are set in order to pass the test. (This is to prevent people from clearing their codes and then immediately going to be tested before the check engine light comes on again.) So, if you make repairs and clear the codes, be sure to drive the car around for a bit (including highway driving) before taking it for a retest.
posted by Larry David Syndrome at 6:14 AM on June 1, 2018 [1 favorite]

Response by poster: I ordered this downstream oxygen sensor from Amazon, for $71.04:

Bosch 15376 downstream sensor

Amazon tells me it's compatible with my car model / year.

I'll have my neighbor check the existing downstream sensor. If it's plugged in, then I'll have him replace it with the new one. Hopefully, this will fix the problem.

Thanks for all the advice.
posted by akk2014 at 7:03 AM on June 1, 2018

What Larry David Syndrome said. Any decent code reader will show you pending codes that have not yet triggered the CEL, so if you get a clean read you should be fine for inspection.
posted by wierdo at 7:03 AM on June 1, 2018

Some public libraries have factory service manuals and Haynes manuals available. Don't buy them unless you must.
posted by dlwr300 at 9:37 AM on June 1, 2018

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