P0421 code

[Mitz]

@littlemoule

I've just re-read [again] the FSM...

TECHNICAL DESCRIPTION

The signal from the heated oxygen sensor (rear) differs from the linear air-fuel ratio sensor, because the catalytic converter purifies exhaust gas. When the catalytic converter has deteriorated, the signal from the linear air-fuel ratio sensor becomes similar to the heated oxygen sensor (rear).

The ECM compares the output of the linear air-fuel ratio sensor and heated oxygen sensor (rear) signals.

DESCRIPTIONS OF MONITOR METHODS
Linear air-fuel ratio sensor and heated oxygen sensor (rear) rich/lean switching frequencies are nearly equal.



There.. it's very clear.. the 2 sensor outputs must be read and compared. I cannot see any entry that this has been ever done

The O2 sensor might be getting the signal so it does not throw an O2 sensor code but the process of switching from open loop into closed loop condition is getting longer than normal during the WARM UP process.. lazy sensor?



The FSM lists the O2 sensor among others as one of the possible causes of DTC P0421, if it's not, it should not be included there in the first place.

I can only wonder why exclude it [automatically], on the assumption that it does not trigger any O2 sensor code. You may be able to find out what's wrong using the process of elimination.

Well, anyway, it's your own interpretation and decision.. just wishing you best wishes and good luck!

[littlemoule]

FSM also lists Catalytic converter deteriorated at the top of the list. Why overemphasize O2 sensor is beyond me. In anycase, not much luck is needed here, I will get to the bottom of it - I am not saying O2 sensor is not the possible cause but 9 out of 10 times its the catalytic converter itself and the car ran on misfire for 1 month. I put O2 sensor as the lowest possible cause. The rest of the exhaust system has been inspected and it is fine. Dead set to sue Krown are we?

[7milesout]

Why overemphasize O2 sensor is beyond me.

Because the O2 sensor is much easier to replace, because it is much less expensive to replace, because it is much quicker to replace, because you can do that yourself, and because if you do replace it and it wasn't the issue, it would have been needed at some point in the future anyway. So the money you spend on replacing it is not exactly wasted even if that wasn't the issue. There's not one doubt in my mind I'd replace that 02 sensor before I replaced the cat. Heck, I'd replace both sensors before I replaced the cat.

Dead set to sue Krown are we?

Both O2 sensors look pretty darn clean to me. So it appears to me that contamination is not the issue.

[littlemoule]

Is it just me or you just contradicted your own paragraph above about replacing the O2 sensors FIRST with this statement:

Both O2 sensors look pretty darn clean to me. So it appears to me that contamination is not the issue.

The only thing that comes to my mind reading that post is basically this: WT*

[klroger]

I think I would have taken 8 minuets & hooked up a scanner & drove down the road & monitored the O2 signal & the Cat Monitor signal. In that time, you could have been sure where the failure is without buying a "one size fits all but doesn't really fit anything" part & paying to change it. I guess a "direct Fit" part from a wrecking yard you can change in your drive way doesn't interest you??? Sorry, just me taking the easy way out...

[Mitz]

I think I would have taken 8 minuets & hooked up a scanner & drove down the road & monitored the O2 signal & the Cat Monitor signal. In that time, you could have been sure where the failure is without buying a "one size fits all but doesn't really fit anything" part & paying to change it. I guess a "direct Fit" part from a wrecking yard you can change in your drive way doesn't interest you??? Sorry, just me taking the easy way out...

@7milesout & @klroger .. that's correct.. I fully agree with your insights

here's a good reading

Common mistakes when diagnosing the P0421 code

"A common mistake that a mechanic could make when diagnosing a P0421 code would be skipping over the full diagnostic and just replacing the catalytic converter. Although this is the most likely cause of a P0421 code, it is not the only cause, and every other possibility should be ruled out before replacing any parts. This holds especially true when considering that the catalytic converters are usually the most expensive part of the whole exhaust system."

https://www.yourmechanic.com/article...y-andrew-quinn

the guy thinks he's absolutely correct (sorry for the word, no pun intended).. although it's included in the troubleshooting list, it was deemed unnecessary to check the O2 sensor as it did not give any corresponding DTC Code on the oxygen sensor itself... maybe Fummins can share his thoughts on this matter too.

Nevertheless, a Magnaflow universal catalytic converter that does not fit was purchased.

[littlemoule]

I think I would have taken 8 minuets & hooked up a scanner & drove down the road & monitored the O2 signal & the Cat Monitor signal. In that time, you could have been sure where the failure is without buying a "one size fits all but doesn't really fit anything" part & paying to change it. I guess a "direct Fit" part from a wrecking yard you can change in your drive way doesn't interest you??? Sorry, just me taking the easy way out...

Guide me through this code reading thing, I don't have a code scanner right now but if it's inexpensive and can pinpoint the issue I'll buy the scanner.

Nevertheless, a Magnaflow universal catalytic converter that does not fit was purchased.

The mangaflow catalytic converter is not installed yet and is still in it's sealed bag and within return period. In my next visit to the shop I was gonna get some assessment if it can be installed. But it seems like you guys are certain it wont fit.
Everyone I spoke with doesn't want to take the O2 sensor off to clean it because apparently it can be a liability issue and might break. :/

[littlemoule]

By the way, in the picture, howcome the catalytic converter and mounts look so rusty? Or is that surface rust? lol

"one size fits all but doesn't really fit anything"

lol true.

And common my friends, I even took the time to post the picture for you guys, Will get more and better pictures soon. I'm not mean. Even if I say f u, I am not being mean. Just an FYI.

Jokes aside, I do feel like we are making progress with logical and critical thinking. I'm gonna figure this sucker out I promise.

[7milesout]

Is it just me or you just contradicted your own paragraph above about replacing the O2 sensors FIRST with this statement:



The only thing that comes to my mind reading that post is basically this: WT*

O2 sensors fail every day, most of the time I'm betting they look perfectly fine from the outside.

[littlemoule]

Alright so the shop attached some diagnostic stuff to read the upper and lower values from the computer. They confirmed it's not the O2 sensor, otherwise the values would be way off - 1.5 hours to replace the first catalytic converter, the second cat is not monitored by the computer. No immediate aftermarket parts available but the mileage isn't that bad but a new cat is needed.