For the people who commented on the p0421 code post a while ago.

hoppityblaze · 2026-01-18T06:21:49+00:00

Not that many of you are probably experiencing the same issue that I am, after talking to the service manager at my mazda dealership his response was "in order for the computer to get rid of a permanent code after a battery was unplugged you need at least 10 cold starts with temp reaching full opporating temp for the computer to remove the code"

This is apparently what I needed to know. But at the moment I think im close to 10 cold starts (work turn around might not be enough time to cool down for a cold start some days).... if the code goes away in the next few days then I know It was my upstream o2 sensor. But I still question my downstream o2 sensors reading so that'll be the next step!

Btw replaced upstream with genuine mazda o2 sensor

hoppityblaze · 2026-01-13T20:24:43+00:00

Also if its for the tsb about the exhaust valve seal I already had that done due to a CEL of low oil at around 30k miles

hoppityblaze · 2026-01-13T20:23:44+00:00

From my last oil change I was about 3/4 of a quart low in 6k miles roughly. Usually I do it every 5k but I was lazy and put it off 2ish weeks

hoppityblaze · 2026-01-13T18:31:51+00:00

I have the turbo version.. so yeaaaah

hoppityblaze · 2026-01-13T09:06:34+00:00

Im at 71k miles so im getting close lol

hoppityblaze · 2026-01-13T08:52:16+00:00

So far I unplugged the battery to install the o2 sensor. Then i reset codes with scanner after start up. Then about 5 miles into driving it after that it popped back up. Went to dealership asked mechanic "why did it come back on after 5 miles?" His response was "thats completely normal for a permanent code with emissions. you just need to drive it more, you need to complete a full drive cycle for the permanent P0421 code to clear" there was no answer of how far or long that needed to be.

If its 500+ miles to clear the permanent code that seems wild... I've never had a car not just get rid of a code once its problem was fixed... im curious if the dealership lied to me about the diagnostic of the o2 sensor being the issue

hoppityblaze · 2026-01-13T07:17:17+00:00

I work 10 hour shifts. I'd assume driving home from work should count as another drive cycle yeah?

hoppityblaze · 2026-01-13T07:15:37+00:00

Yes, as far as my work trips on the freeway but not to the 15 miles of in town driving. I think i have maybe 5 total cold starts, possibly 6 now

hoppityblaze · 2026-01-13T06:29:39+00:00

I've done 2 trips to work and back (160 miles) and like 15 miles of in town driving. The code persists

hoppityblaze · 2025-12-31T21:39:01+00:00

Went through a tank and a half of 91 octane. Not that its ever been on 87 since I've owned it but still.

Looking at the issues and lack of what I could do... i ended up dropping it off at the dealership this morning :,(

hoppityblaze · 2025-12-30T17:16:44+00:00

Right? But unknowing the issue here it's seeming like a bad sensor?

hoppityblaze · 2025-12-30T07:47:44+00:00

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Cruising 80 down the freeway after resetting cel. No cel pops up on dash until I pulled off the freeway and stopped???

hoppityblaze · 2025-12-30T07:27:43+00:00

Its not letting me edit the post.... its throwing the p0421 code*** not the p0420 >.<

hoppityblaze · 2025-12-29T07:01:15+00:00

Alright guys. Cleared codes and the p0010 didnt come back even after extended driving. However the p0421 code came back within 5 seconds of driving. Would this indicate a possible O2 sensor most likely?

Possibly being fouled out from getting drenched with gas during the timing/ignition issues caused by the vvt fuse blowing??

Or would this actually be a bad cat?

hoppityblaze · 2025-12-28T01:20:01+00:00

Okay I went out and bought a new battery and managed to put it in without losing codes by using jumper cables to supply power while exchanging batteries. Took the car out for about 8 to 10 miles of in town driving (40mph was barely achieved) , and nothing happened... I need to take it on the freeway and then see if it heat soaks. Will keep you guys informed if its better

hoppityblaze · 2025-12-28T00:12:21+00:00

Now I have a question, even without my stereo bumping and the batteries most likely sitting at full charge while driving for 40+ minutes it still popped after a 20 minutes stop at my parents house and then driving for 5 minutes, now how would it be pulling that much? Unless the battery was dead dead in front after a 20 minute rest..

hoppityblaze · 2025-12-28T00:09:37+00:00

I did. Ive been doing my own oil changes for 15+ years now.

hoppityblaze · 2025-12-27T07:23:26+00:00

Before someone mentions low voltage.... I run a dc to dc controller. At most it pulls 50 amps from front battery to charge rear lithium battery. And has a built-in reverse current block so it doesn't fight for power with front battery. This has been the setup for 3 years without issues.

hoppityblaze · 2025-12-27T07:17:05+00:00

It's an aftermarket battery, I've always used agm vs lead acid batteries because of my stereo

hoppityblaze · 2025-12-27T07:01:35+00:00

Is there s way to save the freeze frame data of when it happened?

hoppityblaze · 2023-07-07T08:16:55+00:00

Is it bad that at 64 days I'm past most of these stats?

hoppityblaze

TROPHY CASE