Pre-Cat Failure

[Naomi]

Check engine light came on with error 420. Catalyst system warmup under threshold. Checked both o2 sensors, returning 10kohm of current at 12vdc, they check out fine.

I traced another sensor to what looks to be a pre-cat, or a warmup cat, which explains the error code. What I'm wondering is has anyone else had to replace this. They're warrantied for 80,000 miles by federal law, however the labor is not (i'd do it myself wether it were free or not from the dealer.)

I'm also wondering if there's an alternative since this rice burner doesn't need two cats.

http://www.all-catalytic-converters.com/mazda-protege-converter.html

Since its a proprietary fit to the exhaust manifold (the bolt pattern and gasket are a proprietary fit) it would be complicated but not impossible to make a straight pipe from the manifold outlet to the primary catalytic converter inlet. It would likely be cheaper too, even if it cost a muffler shop $100 to bend a pipe and mount it to a specific bolt pattern. However, I would still have the CEL code 420 issue. Anyone had any experience overriding this error code by tricking the sensor (I measured 2.66kohms@~750f temperature, so theoretically i could jam a resistor in there at that rating and it would always read 750f, which I believe is the minimum operating temperature of the warmup cat.

So I guess I'm asking...has anyone done this before or should my car be the labrat?

[davens]

I had that code last summer but the dealer wrote it off to a single misfire. My timing was off a bit. I took it home, reset the ecu, now it's fine.

I want a header but no one makes them for the 1.5. So I'll be doing the same thing, replacing that pre cat with a straight pipe.



Got the primary off Ebay and the downpipe from a mailorder place.

[labikesrcool]

i know this is slightly off the subject but is there any useable horsepower gains by taking out the pre-cat?

[Naomi]

there is power to be gained, but not as much by removing a pre-cat, compared to a cat. at any rate, they're both restrictive to some degree.

i'm not really looking for power though, im just trying to get rid of the f%(king engine light

with ur straight pipe, u just ganna stick the precat warmup sensor in that socket u made for it? u sure it wont return an error code...? if it does, i'd say we work together on hacking the electrical system and finding the right resistance to override the temp sensor. all sensors in a car are based one one of two things, voltage or resistance. o2 and temp sensors are always resistance. whatever their reading is, it is returned as a resistance in ohm to the ecu. we need to find out what the ohm value of the sensor is when its operating at 'peak' or normal condition to trick the ecu into not returning a code.

nice pipe. u have it welded or u actually buy it like that?

[davens]

The bung on the downpipe is for the second o2 sensor. Are you saying the precat has a sensor also? Worse case scenario, I'll have something rigged on the center pipe to make this work.

I know an electrical engineering student who is a car nut and a buddy. If it comes down to figuring something out, he's good. I'll be happy to share any info I come across.

[Naomi]

Cool. Share away There is a sensor in the precat that detects warmup time and temperature, which it reports back to the ecu. Although people may think there is a sensor in the primary cat, there is not. The car detects only when the precat fails, and assumes the primary cat is visually inspected to determine wether it needs replacement or not. Also, when a primary cat fails, you usually hear it, so theres no need for your car to remind you there is a leak in it.

[98zoomzoom]

...

where did you get the downpipe?

[davens]

ForeignAutoParts

Not bad prices...

[98zoomzoom]

thanks!

[loganss]

P0421 Warm up catalyst below threshold

I'm ressurecting this thread. I know some people hate this but the topic is related to my problem.

I've gotten a P0421 Warm up Catalyst below threshold.

Does the 2nd Gen even have a Pre-Cat?

Damnit I need to figure a lot of crap out.

[loganss]

Well I answered my question after searching the 1.5L does have a Pre-Cat just after the header but not the 1.8L. Why??? I have no idea.

The pre-Cat shouldn't need replacement though would it? My car has ~93000 miles

I've already replaced both oxygen sensors, got a high flow cat from Magnaflow, and a new cat back exhaust system. WTF this car stuff is expensive.

I reset the code today and I'm waiting to see if it comes on again.

[Naomi]

Well I answered my question after searching the 1.5L does have a Pre-Cat just after the header but not the 1.8L. Why??? I have no idea.

The pre-Cat shouldn't need replacement though would it? My car has ~93000 miles

I've already replaced both oxygen sensors, got a high flow cat from Magnaflow, and a new cat back exhaust system. WTF this car stuff is expensive.

I reset the code today and I'm waiting to see if it comes on again.

Check your manifold. 421 usually means an exhaust leak. Pull off the manifold shield and see if its cracking between 2nd and 3rd cylinder outlets along the manifold. This is a very common stress point. If its cracked, buy a new manifold off ebay for 50 bucks, get it powder coated (or ceramic coated) for another few bucks and it probably will never crack again since its now protected from a lot of the heat produced by the engine.

Ceramic coating will also keep the manifold and engine bay cooler and just send all the heat through the exhaust where it belong.

[loganss]

Wow. I should call most of the people on these forums the Protege Gurus of the New Millenium.

Thanx for the help I'll look into the exhaust manifold cracking.

Is there a good way for me to notice small cracks or are my eyes the best thing for this job?

I'll keep you guys posted on the results.

[davens]

Uh, that's not a pre-cat, that's your actual cat.

Further downstream is just a resonator.

And if you're manifold is cracked, you'll see it. I've cracked two now. The stress comes from heating/cooling rapidly. Like when it's hot as hell and you splash water on it from a puddle.
Supposedly, the earlier manifolds didn't have enough nickel in the alloy. The new ones are a better material with a few enhancements to structure.
For extra insurance, hog out the mounting holes so the manifold can stretch a bit without creating stress at the mounting points.

[loganss]

Uh, that's not a pre-cat, that's your actual cat.

Further downstream is just a resonator.

Check out this post I went to (Pre-Cat thread)

I've attached the picture that shows the pre-cat in a 1.5L right after the exhaust manifold. Sorry I called it a header before.

I don't know where the pic was taken from so I'm not trying to say if it's right or wrong. Just thought u might find it interesting.