'03 6s-mt V6 174k
followed normal schedule until..
-new clutch and rear main seal at 110k
-replaced throttle body at 155k
then..
two months ago at 172k started getting various cyl misfire
codes. to start, replaced plugs-it had been almost 100k.
ran better for a week and the misfire codes were back.
replaced the ignition coils and inspected wires for wear.
they were ok, in the clear..
another two weeks and i'm getting the cyl misfire
codes again. following other threads, it's looking
like a cat issue. am i on the right track changing one or
both?
it's been a great commuter car driven mostly highway miles,
just wondering if this is the beginning of the end for my 6.

 

Hey! Welcome to the forums!! I am no expert on the cat plugging issues but I am sure one of the other guys that are will be on shortly, from what I can remember reading I think your on the right track as well, also its just a new cat converter, nothing to trade the car in for

Dont forget to fill out your signature! Enjoy the forums!

 

Unless your getting cat codes or can hear the cat rattle when revving I would jump
Into replacing the yes there are two and there not cheap! There also a pain in the butt to do! U might have a fuel pressure issue. The v6 is also known for carbon buildup in the lower intake causing fuel soak and making the cylinders misfire usually followed by lean codes.

 

I have personal experience with this issue.
Carbon are issues from EGR valve recirculating back in to manifold AND Change your PCV valve as Mazda never does because it is not on there service schedule!
If you don't know how long ago you have replaced ignition coils, or haven't changed them yet, change them now.
Click on and watch: . I have heard the stock ign coils are OK but that is it. I would not trust your old coils.
MSD has a new ign coil out---it is Part # 8248 --- it looks exactly the same as ours, it is 90 degree and has a mount --- it's for the new Ford 5.0 ltr Coyote Engine. Just make sure the B+ sides of the coil plug line up.
Check your fuel pressure.
Pre-Catalytic converters: Yes they will cause a misfire. It happened to me. After changed spark plugs, ign coil, fuel injector and installing new fuel pump and gas tank, i was still getting a misfire. After changing pre-cat, no more misfire. All my work was covered under warranty! Invoices shown below.
A good place for replacement cats is Magnaflow. Click on and read this thread: https://www.mazdaforum.com/forum/maz...3-0-ltr-30510/ . There is a link to search your specific vehicle. Let us know if you get one and post the pic of it. According to the pic, the design looks better.
174,000 is a lot of miles. Mazda recommends changing cats after 100,000 ish. I haven't heard of any going past 140,000 with original pre-cats. Plugs every 75,000, ign coils 100,000 ish but i would change them earlier as they have been known to fail, and when they do your pre-cats will become clogged, so it is better to change coils sooner than later!

FOR A GREAT VIDEO ON DIAGNOSING MISFIRES CLICK HERE:

 

GpInUt:

I understand you changed the coils but not the wires as they appeared o.k. They may look o.k. but still leak voltage. I would replace the wires so all major ignition parts are new.
Checking wires takes more than a visual inspection. In a dark place open the hood and idle the engine, spray some water mist over the engine. You might see sparks flying. Measuring resistance can also reveal bad wires.

 

thanks for the welcome and quick responses

didn't do the plugs or coils myself, but did ask about
the state of the ignition wiring--i got an ok..
had the same guys give the car a thorough inspection
ie compression (really good!) do an injector clean and
decarbonize. also checked fuel rails & pump pressure.
the paper trail is all with this shop, also including doing
the diagnostic test before changing the coils.
given that i'm still flashing the same codes, i'm inclined
to make them deduct the additional diagnostic charge
when i bring it back for hopefully the last time

if it does point to the pre-cat etc. will go
with the magnaflow over oem.

cheers,
greg