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post #1 of 4 (permalink) Old 12-31-2018, 04:17 PM Thread Starter
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Precat failure need advice

Hi all,

I read a lot on the forums but don't post much. I do the basics on my cars, brakes, oil changes, general maintenance, but don't have the knowledge or skills to go too deep. So her's my story. I've owned a 2004 Mazda 6 Sport Wagon since 2004. With 180,000+ on the clock, it has been one of the best cars I've ever owned and other than some coil pack issues and pre-cat replacement (due to cracking not failure) I've not any major issue. It is getting rusty (I live in northwest PA) and I have been so happy that I set my sights on buying a lower mileage cleaner 6 wagon. I thought I found it in Chicago. I bought a 2004 with 95,000 miles. The body and on a brief city test drive it seemed OK. About 200 miles from home I threw a CEL Also, on the highway it seemed to lack power and the mileage stunk (<20 mpg) I have a cheap scanner, which read the code as P0421. I was hoping it was an O2 sensor, but I knew it was not. A local muffler shop changed out both pre-cats with ones I bought on line. The told me the bank 1 cat was completely empty and it had puked the guts into the rear cat. They told me the rear cat is just a back up but legally could not hollow it out. I friend of mine helped pull the cat and clean (hollow) it out. The car runs well, no codes, much better power. All this to ask if I should hollow out the second cat on bank 2 to match the flow on bank 1? The muffle shop said its clean.

Now that I actually posted I will ask another question. Should I change plugs and coil packs as a PM, even with no apparent issues?

Thanks and Happy New Year!
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post #2 of 4 (permalink) Old 01-01-2019, 02:16 AM
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If it were my car, and it was running good, I'd just leave it as is. Especially if you put it back together already...if you had it apart still then I might've done it.

If the coil packs haven't been replaced, I'd probably replace them. I found the Mustang coils from Accel to be the best deal, I found a set of 8 for under $120 shipped. Catch is you have do some slight modding to complete the install.

Happy New Year!

- 2007 Mazda 6s Sport Wagon Sport VE, A6, bought Nov 30th '18

- 2017 Chevrolet SS Nightfall Gray 6mt & LS3
- 1957 Chevrolet Sports Sedan
- 1992,1994 Infiniti G20 2.0L, 5mt
- 2000 Infiniti G20 2.0L, auto

Previous Mazdas: 89 & 2 x 92 626s (GD)

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post #3 of 4 (permalink) Old 01-01-2019, 08:31 AM
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@Calvin I know the exhaust is routed different on an 04 than my 07 but if the other cat is fairly easy to get to and hollow out I would think it would probably be a good idea to do it. As for the coils I would probably replace those for exactly the reason you said, “preventative maintenance “ here’s why I say this, I replaced the plugs in my car at 149k when I bought it at 155k I started having a bad misfire when I pulled the plugs in the process of confirming my diagnosis I found that 2 of the bank 2 plugs looked new even though I had not felt any issues they clearly had not been working very well if at all for 5000 miles and I never felt a thing I bought my coils on eBay and they have been good now for over 10k and counting I think they cost like $30 for all 6


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Last edited by bluedevil6; 01-01-2019 at 10:48 AM.
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post #4 of 4 (permalink) Old 01-01-2019, 02:17 PM Thread Starter
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You confirmed what I was thinking. I am not looking forward to either, but I think I can manage. Thanks again
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