I'm Back! (P0421 CEL Code)

[7milesout]

I've got dain brammage or dimensia-lite (or a combo of both), as I didn't remember any of that crap I had written previously.

[Navysteveo87]

*** Updated regarding my O2 sensor spacer trick ***

The CEL is back on and I knew it was gonna come back on shortly after driving it.

Reason I knew this is because the mpg gauge on the dashboard would fluctuate from 25-40 mpg. When I drove it easy it wouldn't even go over 40 mpg (before the CEL it would display 48 mpg when it was driven very easy)

When I drive highway speeds (65-80 mph) I'm currently getting 28-35 mpg....before it was 38-43 mpg. Now that the weather is getting nicer and summer gas starts soon (May 1) I should be getting a few more mpg.

Question. With the CEL on does this put the car in a "limp" mode state? Even if it's running really good and only needs an O2 sensor or something minor regarding emissions? Thanks

[Fummins]

The front O2 sensor is used to monitor and adjust air/fuel ratio, the rear sensor is just there to make sure the Catalytic convertor is working properly. If the front sensor is hooped it could run like pooh, if the rear sensor(or cat) is hooped it shouldn't affect how it runs unless the cat is actually plugged.

[Navysteveo87]

Good to know. I didn't mess with the front O2 sensor...just the rear sensor after the first converter. Like Johnny said I think that first cat is clogged due to the misfire I ran for 2 weeks and dumped all that raw fuel into the converter. Looks like I wont be passing emissions check anytime soon.

Do you know how the ecm knows the converter is clogged? What sensor monitor's that to throw code P0421? And what is the code that would display for a bad downstream O2 sensor?

[Johnny Mullet]

If I had your issue and since there are no emissions in my area I would remove and gut the cat.

[Navysteveo87]

I was thinking about doing that but will it still throw a CEL code even with the spacer installed?

[7milesout]

Navysteve. What Ed Zachary is your spacer? I bought a minicat for my son's car and it has worked flawlessly. I don't know if a spacer and mini cat are the same.

I guesstimate that the rear O2 sensor is measuring ... oxygen. Or the lack thereof. Seems a catalytic converter takes oxygen and carbon monoxide and converts it to carbon dioxide and H2O. The process depletes the oxygen so I would imagine that rear sensor is always supposed to signal "rich / no oxygen." If the cat fails, oxygen would begin to show up in the exhaust. At some threshold toward lean (oxygen content), the ECU must interpret that as a cat failure. All speculation on my part.

[Navysteveo87]

Ed Zachery? Ok sure but I purchased this from Walmart...

https://www.walmart.com/ip/2Pcs-O2-S...ole/1093527612

I thought the spacer would of worked because the cat is doing "some" cleaning versus not having one installed at all. The extender pulls the entire O2 sensor out of the path of the fumes and only senses a little bit of the exhaust

[7milesout]

I'll find the one I bought from walmart. It look like the same thing, except it's called a minicat. Supposedly it has a bit of catalyst material tucked into it and the exhaust that makes it through that to hit the sensor has been "converted." All I can say is, it is working on my son's RX8 ... and his has NO cat at all.

[Navysteveo87]

7miles. Can you send me the link to what O2 spacer your son bought? Thanks