[Salmanilla]

I have an 08 RTX with 165k miles on her. Lately, I have had intermittent P0420 CEL warnings... After reading the ODB2, it is the "Bank 1" O2... Here is the question... The Catalytic Converter has 2 sensors. An upper and a lower.... Which one do I replace?

Is there a "more likely to be.." one?

Sal

 

[beatle]

The output of the upper actually affects the air/fuel ratio corrections made by the ECU. The lower only indicates whether the cat is operating properly.

I think it's more likely to indicate the second one as being the problem. At least that's what Mazda told me when I had a P0420 code and I wanted my cat replaced under warranty. I also smelled rotten eggs which indicated the cat was failing. I haven't looked, but I think the lower sensor would be easier to replace as well.

 

[chiph]

If you're going to do this work yourself, buy the Denso sensor, not Bosch.

Chip H.

 

[eurban]

I would buy the OEM supplier NTK rather than Denso or Bosch.

 

[OwnerCS]

I would buy the OEM supplier NTK rather than Denso or Bosch.

+1

I replaced all 4 with new NTK/NGK that I purchased from www.RockAuto.com.

I did notice a mileage improvement around town. I think my old sensors had gotten lazy.

 

[John's Ridge]

On this same topic. My '08 with 128K miles started having the check engine light come on (and off) severial times over the last couple of months. My local dealer says it is the lower CAT. I do not smell the rotten egg smell. How do I know it is not simply the sensor? As always I appreciate the help.:act018:

 

[Showkey]

Rotten egg smell can often be related to sulfur in the fuel and not a key indicator of a failing CAT.

 

[Dnick]

On this same topic. My '08 with 128K miles started having the check engine light come on (and off) severial times over the last couple of months. My local dealer says it is the lower CAT. I do not smell the rotten egg smell. How do I know it is not simply the sensor? As always I appreciate the help.:act018:

Only by changing the O2 sensor.

 

[John's Ridge]

Only by changing the O2 sensor.

That would be cheaper than the CAT. I guess the error code can't break down the difference between the sensor and the CAT...Thanks!:act024:

 

[Dnick]

That would be cheaper than the CAT. I guess the error code can't break down the difference between the sensor and the CAT...Thanks!:act024:

Correct-o-mundo!