Needed a new O2 sensor for my Mustang GT, Bank 1 sensor 2, so I replaced it and a few days later I throw a P0420 cat eff. code. Which is weird because I have MIL eliminators so I at first swapped out the MIL eliminator for another one that was brand new. Code comes back so I assumed AZ sent me an upstream sensor rather than a downstream sensor. They swapped the sensor for me but told me the upstream and downstream are the same which I find odd. Just curious if there is a problem with Bosh O2 sensors for our cars, I never really had a problem with Bosh sensors ever before. I can see the sensor switching fine on my scanner so I know it's working. I just put the new sensor and will see if the code comes back.

 

So the code came back again tonight. New O2 sensor and new MIL eliminator didn't help so it makes me believe the PCM is bad.

 

The upstream and downstream sensors are functionally identical, as are all platinum/zirconia narrowband O2 sensors. It is only the pigtail length and connector that are different.

Bosch sensors are often maligned as part of many forum's conventional wisdom, mostly because they are commonly sold at FLAPS and "we all know they sell junk"--makes you wonder how they have managed to stay in business, offer lifetime warranties, and grow for 30 years doesn't it?

However in 25+ years of using them I have never had an issue with any Bosch sensor; and since Bosch invented them I doubt theirs would be inferior to anyone else's. My '03 GT has been running fine for 3 years and 65k miles with Bosch sensors up front.

As to your issue I have known MIL eliminators to not work properly when new rear sensors and paired with old front sensors. This is because of the manner in which the PCM monitors the cat efficiency. It expects the rear sensors to "switch" more slowly than the front because catalytic converters store oxygen; a new sensor may and often does switch faster than an older sensor.

Here's more about how MIL eliminators work, and more about the PCM cat efficiency monitor...

 

Thanks for the info. I went and replaced sensor 1 on bank 1 so both bank 1 sensors are new so hopefully the P0420 code will not come back. Now I need to work on the EVAP code I have.

 

Never had a problem with bosch

 

After replacing the O2 sensor in the front the code bever came back.

 

It might, I make no guarantees--however I have seen this problem before...

 

So the code came back again but it took a little more driving this time to set the code. I cleaned all O2 sensor connectors and swapped yet another MIL eliminator on that bank and so far the only code that returned was the pesky 1450 that I am taking care of this weekend if I can free up some time. If it comes back again I guess I will have no choice but to put cats on the car.

 

P1450 has nothing to do with O2 sensors or catalytic converters--so why would you you feel you need to install cats?