I have an ‘07 Mazda 6 that recently started giving me only the P0421 code. What would be the best plan of attack to figure out what’s wrong? The car runs fine, slight power loss but nothing major. I get a rattling noise under acceleration, but only when the car is in drive. I know faulty cats tend to make that noise, but wouldn’t it do it all the time and not just in drive? I would appreciate any help or to be pointed in the right direction.

 

Check for exhaust leaks. you will need to get under the car and may need to have it up on the rack and cold start it to more easily detect if there is one. The other way is with a "snifer" but I know you don't have that.
Stay clear of the BS on the internet about the CAT. Nothing you posted points as of yet to a CAT problem.

When you claim power loss how did you determine this? Most individuals can not feel a power drop on less then 5hp and most cant feel an increase in less then 12hp... lol
And if you dropped 5hp then you do have a mechanic issue and that is another whole ball gaming of diagnostic procedures.

As for noises from the exhaust the CAT seems to always get the target on forums, but the reality is the year and miles of service as well the lack of some service will cause exhaust system to eventually start have strange noises, sung to the tune and words "squeaks rattles and roles" LOL


https://www.bing.com/videos/rivervie...stvo&FORM=VIRE

.

 

Welcome to the forum

 

I greatly appreciate your input! I will have to check for exhaust leaks. I suppose power loss isn’t substantial, it is a four cylinder and I’m sure it needs a tune up. If for whatever reason it isn’t an exhaust leak, what else should I check?

 

When you first start the engine cold i the morning does it seem loader for a few second (up to 30 seconds) and then quiet down?

How many miles on your Mazda

What was the last miles and service done?
Have you recently changed where you normally get gas from
When you scanned the ECU what was all the information in the other areas like I/M monitors and permanent DTC file found. If not that needs to be done.

 

It’s almost at 150,000 and the noise is very random. It isn’t louder upon a cold start, but it comes and goes. Sometimes I can step on it and it’ll be quiet, other times it sounds like a rock in a tin can. Also I only have a very basic scan tool, so I’m unable to do anything other than pull the code and clear them.

 

Ok that really is pointing to a small exhaust leak. When the gasket is cold it can contract and not seal 100% then the opposite when hot it may seal as long as exhaust velocity is steady and not up and down the PRM rang like getting stuck in traffic in the city or highway and then the DTC+check engine light will appear. Because the correction has not been done if you were to clear the DTC it would only turn off the dash check engine light but the DTC would remain in the permanent ECU file and send the information to the ECU after 1-4 complete engine cycles which would activate the check engine light again. Not correcting it could also cause by way of anomalous ECU adjusting operation cause other DTC to appear in the check for DTC file and cause you to start a wild goose chase and pounding the key searching what to do on the internet. lol
The short good news is the exhaust gasket we are talking about is rarely the exhaust manifold to head but rather the small two bolt/fastener exhaust gasket either pre cat or after the cat and mid muffler/resonator in the exhaust system were front and mid join. The only down side is the fund getting under the car to change it if you are a DIY with rams. The gasket itself is less the 12 bucks .
The only best way to know if it is the gasket is visually looking to see is there is carbon residue around the gasket or with an exhaust hydro carbon gas sniffer which most shops don't have those anymore.

Now I never said that it was the gaskit abosulutley only that it is pointing in that direction based on your responses.