Sudden P0421 within the past couple of days; today P0302, rough idle and flashing CEL

LostinCanada

Member
:
2003 Mazda Protege5
Hey everyone, I just read through a wackload of threads re P0302 and P0421 codes but none of them sound like what just happened to me. As a side note, this car hasn't had a check engine light for over a year, since I did the quick and dirty on-the-car ring job and rebuilt the head. Plugs, EGR and many other things replaced new OEM. My car has been running awesome ever since.

Two days ago I got a CEL so I fired up the Torque app and found the P0421 - Powertrain: warm up catalyst efficiency below threshold bank 1 code. So I cleared the code and have been monitoring with the Torque app running since (just city driving - no highway). I got a pending fault warning a couple of times for the same code but the CEL never came on. This afternoon, I suddenly got the CEL - this time it was P0302 - Powertrain: cylinder 2 misfire detected. Shortly afterwards the idle dropped about 100 rpm and started behaving erratically at the stop lights. When I came off the light the CEL started flashing (not driving fast at all, but during the acceleration). I wasn't far from home so I took it easy but still got the flashing CEL. Clearly something is wrong and it seems to be sudden.

I don't know if it's related but when I got home I realized I left the sunroof partially open so I turned the key and closed it. That's when I heard this weird electronic squeal. I heard this noise once before about a month ago, when my battery drained from letting the car sit for a couple of months and the battery became too weak to hold a charge anymore so I replaced it.

Am I having an electrical issue of some kind? Hopefully I'm not looking at replacing the entire parts catalogue as others have done trying to figure out these codes!

Before someone says coils I can pull the old coils off my parts car and take a test drive tomorrow but I'm wondering about getting this P0421 first and then a sudden misfire. And then of course the random electronic squeal (key on/engine off).
 
Last edited:
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2003 Sunlight Silver Mazda Proteg5
Wow! I had those codes last year on my P5. One night after driving back from work car began to misfire and went into limp mode. Would not rev past 2K and had the CEL flash.

How are your plugs doing? Mine were full of carbon but most noticeable on cyl 2. A new set of iridium NKG ended up doing the trick.

The P0421 stems from the precat and the downstream 02. In my case the precat went bad from the times the car burned oil on start up and the misfire.

Just my $0.02 cents.


Sent from my iPhone using Tapatalk
 

LostinCanada

Member
:
2003 Mazda Protege5
Thanks @naguilar. Most of the accounts I've read seemed to have had misfire issues and then the P0421 - in my case I had the P0421 first and then the misfire/poor idle. I'll pull the plugs tomorrow but unless something has suddenly failed they should be fine - I replaced them last year (several times as I was burning so much oil... LOL). Sigh. I hope that doesn't mean my precat has caught up with last year's trauma. I definitely don't want to change the precat without finding the misfire reason first - unless of course the precat is the cause of the misfire. But if it is I would have expected random CEL problems over time, not sudden failure a year later.
 

pcb

The Diagram Dude
:
2002 MP5
A cylinder specific code is a lot better than a random misfire.
Your pre-cat is probably toast but just do the non-fouler and save your time effort and money.

Apparently when our car goes into limp mode it dumps all kinds of extra fuel into the engine in an effort to protect it... That could have fouled up your cylinder.

Here's the list for cylinder specific misfire.. (coils aren't listed but are for random misfire.... normally a bad coil would affect both connected cylinders.)

 

LostinCanada

Member
:
2003 Mazda Protege5
Thanks for your comments pcb - I think you're probably right. After doing a whack of research and some thinking I ended up pulling the precat off my parts car this afternoon. I'm thinking about using that as an interim measure while I give some consideration and more research into doing my first mod - intake, header, cat/exhaust. That's assuming the last owner(s) didn't blow the precat. I committed to buying the car sight unseen mostly for the interior leather and possibly the transaxle. I never checked it for codes and only drove it about 40 km on a temporary permit just to get it home so who knows what issues it may have hidden.

So given the order of codes, and the very likely possibility that last year's oil/coolant eating episodes could have started my precat on its fatal journey then theoretically I should be able to swap the pre-cat and possibly one or both O2 sensors (although I'm pretty sure I replaced the upstream one). Then check/replace the plug(s) and Bob's my uncle.

Hopefully.
 

pcb

The Diagram Dude
:
2002 MP5
If I ever throw the 421 code I'm just doing the non-fouler...
My pre-cat and all related nuts and bolts would just crumble and break off.

PS... Bob's my uncle too !!!

 

LostinCanada

Member
:
2003 Mazda Protege5
@pcb hahahaha.... Well, you'd be surprised what a little Tri_Flo can do for seized parts... My parts car is rusted and corroded - last registered in AB and I was pleasantly surprised that all the exhaust bolts came apart without issue. The same could not be said for the 10mm undershield bolts though... all the heads all broke off...

I think I found out a potential cause for the strange (and likely unrelated) electrical squeal... My car has the Mazda alarm added after the fact and I think I might be missing part of the emergency override button and/or be having some other issues with the system. My led is on all the time now and doesn't flash anymore when the system is armed. More stuff to troubleshoot in my spare time... that and replacing the rear passenger door seal and leather door panel my puppy ate... :(
 

MazdaSpeeder

Member
:
Protege5 (5-spd)
FYI, when you do plugs, go with basic NGK V-powers. The iridium stuff isn't recommended for the FSDE, and is really just a waste of money.
 

pcb

The Diagram Dude
:
2002 MP5
FYI, when you do plugs, go with basic NGK V-powers. The iridium stuff isn't recommended for the FSDE, and is really just a waste of money.

^^^ What he said...
Ordinary copper plugs give the strongest most reliable spark.
Their disadvantage is that they wear and need to be gapped and replaced more often.
 

MazdaSpeeder

Member
:
Protege5 (5-spd)
The nice part is, those NGKs are SO CHEAP, you can replace them every 10k miles if you wanted to. I was not used to plugs costing more, and when I got another vehicle, I was surprised!
 

pcb

The Diagram Dude
:
2002 MP5
I added a whole bunch to my Rock Auto order to help offset the shipping cost.
As well as air filters, oil filters, light bulbs, fuses, pcv valves ...

 

LostinCanada

Member
:
2003 Mazda Protege5
Wow that is a good price! I usually buy mine at NAPA, or sometimes the dealer (I get 20%) but usually the NAPA price is better.
 
:
Protege5
Parts stores are pretty competitive on spark plugs.

Be thankful you don't have a Ranger or Dodge hemi, two plugs per cylinder, platinums at 5$ each, and on the Ranger they're a bastard to replace.
 

LostinCanada

Member
:
2003 Mazda Protege5
Hahahahah... yeah my zippy little Mazda replaces a Ford F-350 Super Duty crew cab/long box. It had the V-8 engine famous for the spark plugs breaking off in the head and most were horrible to reach also. My first plug job on the truck broke all but the two most front plugs (coolest part of the engine), took two days, cost $100 for a special tool to deal with the problem and then I had to modify a shop vac to suck the porcelain out of the head - despite following the Ford service bulletin to the letter. Apparently it's kind of a badge of honour to survive that project... I bought better plugs and eventually I got pretty good at extension combinations/contortions. Years later I later heard from a guy it was easiest to undo a few bolts and pull the cab off the vehicle to change the plugs! LOL. I don't remember the plug price but they were at least that expensive.

Bought my plugs at Lordco today because I missed NAPA by a few minutes (much cheaper) and NAPA is closed Sunday - I did not get a good deal, despite my negotiations to their lowest possible price. Oh well - serves me right for not having spares on hand. :(
 

LostinCanada

Member
:
2003 Mazda Protege5
Well, an interesting development. I thought the previous owner had busted the rear O2 wire and spliced a repair - until I compared the two vehicles I didn't realize that my parts car is stock, mine has been modified and the rear 02 sensor relocated behind the CAT. I'm wondering if the previous owner had a problem with the pre-CAT already, unless they did that mod for another reason? So I'm wondering what that means for my present situation. Since I've got everything apart I figured I would proceed with the pre-cat swap and relocate the O2 sensor behind the pre-cat (using the parts car's shorter O2 lead) to see what happens. That will replace both the pre-cat and the O2 sensor - albeit with old parts.

So is it possible that the cause of my P0421 is actually the rear O2 sensor? I verified that I did replace the front O2 sensor with last year's repairs so that one is probably fine.
 

pcb

The Diagram Dude
:
2002 MP5
So is it possible that the cause of my P0421 is actually the rear O2 sensor? ...

I would think so but the ECU is supposed to recognize a bad O2 sensor.




But it doesn't matter... You're gonna need to do the non-fouler anyway.... Just do that and plug it in..
 

pcb

The Diagram Dude
:
2002 MP5
O2 sensors wear out and slow down.

They need to reach a certain threshold before the ECU throws a code.
 

LostinCanada

Member
:
2003 Mazda Protege5
Well, now I'm annoyed.

The one on the right is my precat, the left is the parts car.

Here's mine:


Here's the parts car:


So I did the swap, disconnected and tie-wrapped the wiring for my rear O2 sensor out of the way. Plugged the parts-car rear O2 sensor in where it is supposed to go. Bolted everything back together. Changed the plugs.

Pretty much immediately got the misfire code. GRRRRRRR. Seems slightly worse than before. Now I'm wondering how much time I have before my "new" precat gets toasted if the system is going to dump more fuel in the mix. Plus now I can smell an exhaust leak. :(

Fuel injector next?
 

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