HI there

a few moments ago I got the code, so I was wondering which sensor is it; the car has 3 sensors down to the exhaust pipe

Help please!

Instead of typing a long boring post just look at this..

https://www.obd-codes.com/p0421

How many miles on the upstream 02 sensors. And spark plugs?

Spark plugs about 15K; oxygen sensors from the factory; however, the 2nd has been replaced, 100K miles ago easily

I tend to replace upstream sensors around the 90k mark. I think they get slow, before throwing a code. Might be worth it to try on yours. But the cat may be degrading.

OK I thank you guys;

I am inclined, it is the one was replaced last time; But I wanted to ask this too, where is it coming from or even better, what is the underlying cause for failing O2 sensor or a catalic converter for example.

I am asking this because recently I have implemented some changes to my car such as using different gas station (Quick Trip) and started using synthetic oil such as Super Tech, currently at 2nd oil change while from the beginning has been used dino only; could this change been related to the code somehow please

O2 sensors just go bad with mileage. Its an electrical sensor jammed in the exhaust stream. I'm amazed that they last as long as they do. Catalytic converters can be damaged by poor maintenace. Misfires send fuel down and the cat burns it, but its not good for the cat. Lean mixtures also make the EGT go up and that can damage a cat as well. The o2 sensors can kill a cat if you drive around with a bad one.

Oil should not have that much effect, unless you are burning a lot of oil, if so high zinc and phosporus oils are not good for the cats of an oil burner.

As long as you are running the correct fuel( don't try e-85 in a non flex fuel etc.), i don't thing brand matters.

I thank you for you valuable input!

my car uses/burns some oil between oil changes, at 5K interval i'd say about 1qt or less; I plan to use Mobil 1 HM Synthetic for next time, so meanwhile is there any traditional way to test the O2 for its function, or is it possible for the sensor to go to off and on randomly, and should I use fuel cleaner to benefit the sensor for example

needless to say, I registered my cars last month, so the repair could wait; but at the same time, I like my M6 so Ill be continuously proactive about it

I'm guessing this is a Ultra Low Emissions Vehicle? Your Catalyst isn't "lighting" in a timely manner!

In my experience, it absolutely does. I went with the cheapest O2 sensor off of RockAuto to get my wife's old Cavalier to pass smog and it did pass, but her MPG was terrible. Suspecting the sensor, I swapped it out with an OEM ACDelco one, and gained 5 MPG.

I was referring to brand of fuel, not o2 sensor. 02 sensor brand makes a huge difference. Denso or oem only for my engines.

latest follow up on 02 sensor;

this morning on my way out, check engine light went away and it been like that since; I kind of knew it its going to happen like this;

so my question to BITOG community, what has caused it, bad gas or new prolonged(5k) oil change interval/low synthetic quality oil (ST) vs Mobil1 for example
I'm inclined to bad gas


one more thing, I went to all auto parts establishment and no one could point out which out of three O2 sensors that my Mazda has is the faulty one; this is why is your imperative and do yourself a favor take care of your car on your own!

P0421 IS NOT a HO2S sensor fault code. A auto parts house is not responsible for diagnosing your vehicle, Nor is it their responsibility to provide Repair or Component Location information.

A competent Technician can test the Catalyst Oxygen Storage Capacity with a 2 Channel Lap Scope & some Propane, This will also test the HO2S sensors as the Lap Scope will be directly reading the Signal outputs.


If this is a 3.0L V6? You have 4 HO2S sensors. Bank #1 is the bank nearest the firewall, Bank #2 faces the core support.

7 is Bank 1 Sensor #1
5 is Bank 1 Sensor #2.....This sensor monitors Bank 1 Catalyst efficiency.

8 is Bank 2 Sensor #1
6 is Bank 2 Sensor #2

I never meant in imply that the codes meant replace your o2 sensors. I was saying that at high mileage i replace mine before they throw codes to protect the cats. They get slow before they fail. Your cat is taking too long to work, as the upstream and down stream senors compare data.

The cat is probably degrading as i said above, but it would not hurt to change the upstream sensors if you are going to keep the vehicle. I would make sure the ignition system is fresh, and if there is high mileage on the 02 sensors, i'd change ALL the upstream sensors before changing the cat, to protect the new cat.

I'd replace the upstream sensors and run it on the high way for a good while to converters good and hot, see if that helps. If not, get someone to diagnose it before spending any more money. But it could be the catalytic converter starting to give up.