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which O2 sensor should be changed?

 
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Old 26 Oct 2017, 08:50 am
prowlerclone's Avatar
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Default which O2 sensor should be changed?

Hey guys,

you all just helped me with advice on a radiator swap, so I figured I could get some guidance on this one.

This has been an ongoing issue for a year or so now, but since it only happens periodically, I'd just clear the code and brush it off.
Finally looking to resolve it.

Anyway, periodically, usually when I'm in 5th gear at a rather low RPM, cruising at a STEADY speed (usually around 45 to 60mph) the check engine light will come on. I'll take my OBD-II scanner and it will usually be one of these two:

Fault 01/ Pending 01 : P 0420 / P 0420 P

Fault 01/ P0420

I'll just clear the code and be good to go.

The code corresponds to: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD BANK 1

so, I know it can be a multitude of things... clogged cat, spark plug issues, oxygen sensor(s), etc. etc..

I have a free gift card that will basically cover the cost of an oxygen sensor, so I want to use it to replace one of them.

So, in YOUR experiences, with this code, should I replace the Upstream sensor, or the Downstream sensor? and I found some confusion on which one is the upstream and which one is the downstream.

Which one is closest to the FRONT of the car and which one is closest to the REAR of the car?

thank you so much guys. appreciate all the help.
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Old 26 Oct 2017, 09:13 am
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Default Re: which O2 sensor should be changed?

P0420 Catalyst System Efficiency Below Threshold
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Old 26 Oct 2017, 09:56 am
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Default Re: which O2 sensor should be changed?

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Old 26 Oct 2017, 10:35 am
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Default Re: which O2 sensor should be changed?

Check wiring first. Unplug and replug the oxygen sensors. Look for vacuum leaks and assure there's not a rough idle or missing cylinder.

If no joy, check for a clogged cat converter or exhaust system. Connect a vacuum gauge to the vacuum on the intake manifold. Get the reading at idle and then raise and hold engine speed at 3,000 RPM. The needle will drop when you first give her some gas, but should then rise and level off. If the vacuum reading starts to drop, pressure may be backing up in the exhaust system. The may indicate a blockage somewhere in the exhaust system.

If no joy, take an IR gun and check the temperature of the exhaust pipe before and after the cat converter. With engine are full operating temperature and at around 2500 RPM, there should be a difference in temp of around 100 degrees F.

If no joy, remove and clean the oxygen sensors with a brass wire brush. Reinstall and see if that works.

If no joy, replace one or both O2 sensors. And yes, use Mopar or OEM Denso sensors. This is one of those cases where some aftermarket sensor don't work in some cars. And this one is a real thing and not just an old-wives tale. It's not worth the bother to risk the introduction of another problem. You can get the Mopar sensors online as cheap or cheaper than the aftermarket kinds in the stores.

DISCLAIMER: It should be noted that while I am an ASE Certified Master Automotive Tech and an excellent lover, I primarily work as a Safety Engineer among other things and I haven't worked as a professional mechanic since I was young strapping lad with long sexy flowing hair before college. And even then, I mainly worked on big trucks and heavy equipment. Nowadays, I enjoy working on these funny looking little cars for fun and relaxation. And I help others when I can.
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Old 26 Oct 2017, 10:54 am
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Default Re: which O2 sensor should be changed?

i would suspect if it is a sensor i would GUESS both sensors are in need of replacement.

here is how a new hego vs and old hego look on my datalogger.

the grey line is a new sensor
the yellow line is and old lazy sensor
the blue line is mph divided by 100. i chose this clip as it was a low throttle 5th gear 60-65mph cruise, similar to what you describe when this happens.
__
Capture.jpg
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if you have a p0420 that says the readings from each sensor are too similar, my guess would be both look like the lazy yellow line and that is why they are throwing the p0420 so changing either one may fix your problem,or make it throw a different code. how is your gas mileage?
if i HAD to pick ONE to replace, it would be the upstream sensor. it's less for emissions and more for making the car run correctly, meaning you may get a bit better MPG even if you still have the code pop up occasionally. i prefer NTK brand as i believe they are the highest quality. they they make lab spec wideband sensors and looking at them side by side you notice the quality. ...little things like gold plated pins, connector quality, etc.

the only time i ever had a car throw a p0420 was in a honda with a gutted catalytic converter. it only occured moping along on the highway, like you describe. in that case a $2 spark plug non fouler fixed it. might be another option to consider.

not a mechanic though, so just my 2
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Last edited by rob302; 26 Oct 2017 at 11:09 am.
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Old 26 Oct 2017, 11:57 am
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Default Re: which O2 sensor should be changed?

Re: which O2 sensor should be changed?
At 50k miles, defiantly at 100k, as a preventative maintenance or if a engine code points to an issue in which case replacing both at the time is easier while you are there.

ANY aftermarket HO2 will work well if you follow the proper way to install them. There is no need to spend a lot of money for name brands or OEM H)2 sensors.
There has NEVER been any real evidence pointing to failure of any specific HO2 sensors used in the common Chrysler 2.4 gasoline engine in any platform it is used in, but a lot of installation and diagnostic procedure not done the best way.
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Old 26 Oct 2017, 12:35 pm
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Default Re: which O2 sensor should be changed?

The key to successfully replacing any HO2 or O2 sensor.

Spray area of sensor with a air dry cleaner
Remove sensor from exhaust component.
Either use a tap and chase the thread or a small wire brush to clean the threads.
Apply a very small amount of anti-seize to the threads.
Using only a Oxygen sensor installation socket install and torque to value as per the instructions from the sensor used or as per a Factory Service Manual.
Apply dielectric grease to the connector housing and re-connect with the chassis harness
Secure the harness away from direct contact of exhaust components.
Using a Diagnostic tool go into the diagnostic connector and check and clear any codes making note of any existing codes.
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