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flagella · 07-07-2010, 05:08 PM

K so a couple months ago my parents Mazda Protege 5 got a check engine light. We went to a shop to get it diagnosed and the guy at the shop told me it's the problem with the cat. We bought new cat and replaced it but today we drove it and the engine check light came on again.

We went to the shop and the guy said there are two cats on Mazda and the one we replaced was the main cat. He said the original diagnosis showed the main cat failed, so we had to replace it, but now the diagnosis is showing that pre-cat is not functioning well also. He went on and said something about how the computer diagnosis only shows if one of the cats is having a problem, but does not show both. He said when both cats have problem, it's only after we replaced the main cat, it will show if there is a problem with pre-cat as well.

Can someone familiar with this chime in to comment on this? Does it make sense?

Thanks in advance.

Qmx323 · 07-07-2010, 06:14 PM

makes sense, one cat should be under the cabin, another one right after the header before it connects to the down pipe

lexluthor09 · 07-07-2010, 08:13 PM

It makes sense, however the guy could've tested the cats before just replacing anyone, its not exactly cheap to just throw cats at a car.

But since you've replaced the one, the next obvious choice is replacing the first one.

RabidRat · 07-08-2010, 05:02 PM

Uhh.. yeah there should be a primary and secondary O2 sensor, and you should be able to see which cat is fked using a diagnostic tool: I just confirmed this with a mechanic friend. However, neither of us has worked on specifically the P5 and can't know for sure how it's set up, so I guess we'll really have to wait for someone with direct experience to chime in with something definitive.

flagella · 07-08-2010, 06:23 PM

Thx for the replies. Just getting sick of dumping more money. Hopefully this one will fix it.

lexluthor09 · 07-08-2010, 07:10 PM

Even without a diagnostic tool, just temperature alone can tell you if the cat is functioning PRE and POST cat.

The first one is a small puny one designed so that it warms up fast and reduces emission even on cold start ups since its right next to the combustion chamber. The heated exhaust gas from there then warms up the main cat for faster optimal operation.

The problem is TOO much heat melts cats, and usually it'd be the smaller warm up cat that'd die, not the main one.

Anyways to OP, if you're still getting the code, then you probably will fix it since theres no other cats left.

BEEB · 07-09-2010, 11:15 AM

Before your gonna dump more money into the P5, I would get a second test with another shop. I know the owner Johnny @ Advance Automotive in Richmond/#6 road had a Protege 5 since the first day it came out (back in 2002) and he's been around for decades. So, give them a try and get them to hook up their snap-on and test it. 604-278-1803 Say Billy sent you and they should diagnosis it for you right away !

flagella · 07-09-2010, 11:23 AM

Alright guys! thanks =)

spydermanx · 07-09-2010, 05:43 PM

the 2 o2 sensors are before and after the first cat (pre cat) attached to the exhaust manifold. there's no sensor after the second cat. The sensors can only tell if the first cat is bad, not the second cat (main cat).