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

pcb

The Diagram Dude
:
2002 MP5
Apparently you can wreck a pre-cat in just a minute or two. But as soon as the misfire happens the ECU does what it can to stop the misfire.... It retards the timing and other things during limp mode.

As far as I know, the cel stops flashing and goes solid as soon as the ECU has the misfiring under control.

The injector seems like the next thing to look at.

It's possible that your injector is stuck closed or clogged and no gas is getting into the cylinder, you would still get the misfire but no fuel to get on the pre-cat.
 

LostinCanada

Member
:
2003 Mazda Protege5
Ok, well I have 4(well, 2 if I obey the colour coding) spares on the parts car... I'll try the injector. Hope I didn't toast the precat - I drove the car locally after the repairs and got the flashing CEL often. I drove like a grandmother instead of my usual assertive driving style but there was no stopping the misfires, only hopefully minimizing.

I forgot to mention in my earlier post that #2 plug was blacker than the rest with soot on the threads - though not crazy fouled w/black like I've seen before. I have noticed a whiff of fuel smell now and again since this problem began - originally I thought it was nothing but maybe not...
 
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pcb

The Diagram Dude
:
2002 MP5
Sounds like it's getting fuel.

I remember one guy had an injector stuck open... It was spraying all the time.

Worse case scenario, you just gut your pre-cat (in case it melts and gets plugged) and do the non-fouler.
 

LostinCanada

Member
:
2003 Mazda Protege5
Well... an update. Pulled a green injector off the parts car (grrrrrr what kind of stupid jerk invented those plastic electrical connectors!!!), replaced on mine. Cleared codes.... f'ing one random missfire then repeated pending P3020 on the first few rev-ups. Decided, what the heck - pulled the coil off the parts car, replaced on mine. Cleared codes... 2 revs, no code so far! The only thing I don't like if this is the solution is the locking tab on both of buddy's coil packs are worn off so could potentially vibrate loose unless I jury-rig a tie-wrap.

LOL if the coil is the fix guess I should have tried that first given the parts car... turns out you can have one cylinder misfiring from a bad coil pack - maybe because the coil is just starting to go bad and #2 has the shortest connection (directly underneath)?

Anyway, taking the dog for a test drive now and we'll see what transpires... hopefully a happy ending!
 

pcb

The Diagram Dude
:
2002 MP5
Fricken coils... They misbehave differently every time they fail.

I always thought it was Jerry-rig....
When something breaks and has no conventional fix, you call Jerry to rig something up...
 

LostinCanada

Member
:
2003 Mazda Protege5
Hahahaha... actually I googled it because I wasn't sure - apparently both are "right".

At any rate I am pleased to report that after driving the heck out of it, my car is back to normal! Two things I can't stand are oil leaks and CEL's! I guess the good news is that I now have a "new" pre-cat and my O2 system is hooked up properly. I didn't notice an exhaust smell today - hopefully that was just a random whiff but I'll keep monitoring.

I have to say having a parts car is sure handy for troubleshooting - and to keep my young German Shredder in steady supply of new upholstery :(

I guess the system probably did throw a valid pending/P0421 code although I'm still confused considering the distance between the O2 sensors in my original setup. And why I didn't get it sooner - I blew enough coolant and oil through the engine with last year's shenanigans I can't believe the pre-cat didn't die then.

Thanks for the suggestions - they were very helpful in troubleshooting! :)
 

pcb

The Diagram Dude
:
2002 MP5
I'll try to keep in mind to start with coils if I ever throw any kind of misfire.

I've keep my working old ones in the trunk just in case.