06 Mazda 3 2.0 P0421...think the cat is cooked

My friend brought her 06 Mazda 3 2.0 over last night for some diagnosis and I'm pretty sure the cat is toast.

Backstory is it was apparently low on oil, her friend added too much (maybe way too much from the sounds of it), thinks it puked a bunch into the exhaust, and started throwing a CEL. So she took it to a shop, got charged like $100 for them to Seafoam it for her (!!) and was quoted like $1000 for a new cat/manifold. That's the best I can surmise from her description of the situation.

Anyways, hooked it up to the reader and it's throwing P0421. Checked the voltage output from the 02s and they appear to be good, both reading somewhere in the .700V range. I'm thinking the shop is right and the cat is cooked. Does the hive concur?

If so, the question becomes then what? She can't really spare the $1000 the shop wants to do the job (of which I think she said $700 was in parts and $300 labor) and pretty much wants the cheapest possible option to pass inspection. Rock Auto looks to have the parts for around $250 or so and she's asked me to do the labor. I'm hesitant to volunteer my time if it's much more than a lazy afternoon of wrenching, but I'd be willing to help her out if it's easy enough.

So, anything that makes this job a particular PITA? Anyone parting out a similar car that I could snag the cat/manifold from?

The front O2 sensor should not be a steady .7 volts. If it was a NB sensor, it should swing fully back and forth from about .2 to .8V. Is that a WB or NB sensor up front?

The back one- well, it's at least alive.

BTW, P0421 means that it's not warming up fast enough, not that it's dead. If the catalyst was dead, it would be P0420.

Definitely replace the front O2 sensor first if it's sitting at .7 volts.

What's the mileage on the car? How much does it get driven? Short trips?

If everything comes apart easily, it's probably not that hard. But it's an 11 year old car, and you're in PA. It's not gonna come apart easily.

BTW, flooding the cat with oil CAN ruin one- I've had it done to me- when a testing system tried to go full blast on the flow, which pulled a lot of oil through turbos, and killing a pretty expensive cat system... bummer.

not to say the one your friend has is dead for sure, but it happens.

In reply to alfadriver:

Right, I know I've heard of it happening before, which is why I'm more apt to believe that may be the case. Front 02 isn't a steady .7, sat around there at idle but it moves around as expected with throttle input. Didn't drive it, just ran it in my driveway for a minute or two.

In reply to NEAL SMO:

IIRC, it's high mileage, like over 200k. Get's driven daily I'd assume and probably fair amount of short trips.

In reply to cmcgregor:

Yea, that's basically what I'm thinking, rust is gonna be a bitch . Just not at all familiar with these cars and didn't know if there was any kind of gotcha to the job. My biggest fear in agreeing to do it is collateral damage in taking stuff apart, she really doesn't have money to throw at it and makes it a sucky job for me.

In reply to Furious_E:

It shouldn't do that at idle, either. It should switch all the time.

Again, the code is for delayed warm up, not that the catalyst is totally done.

In reply to alfadriver:

I mean it's moving at idle too, just hovers in that vicinity. What should I be looking for to indicate a failed o2 then? In the past when I've had a bad sensor it will either read way low (near 0) or way high (≥1) all the time.

Would be great if it's just the o2, but again I don't want to just throw parts at the thing.

Well....At 200k, you could replace both o2's and the catalyst and still be completely justified. All are well worn by now, oiled or not. If oil got to the cat, it also coated the front o2.

You said she's broke, but a rock auto catalyst and front o2 can't be much over 300. Figure another 20-30 in needed gaskets and bolts. Start with the o2, hope for the best, plan for cat replacement if the code persists.

Take a look before you start...the stock system doesn't have flanges but is all slip joints and redneck D-clamps. Have fun getting it apart.

In reply to Furious_E:

I'm not sure what you are saying it's doing.

It should still swing between roughly .2 and .8 V at idle. Not move and hover at a voltage.

The fact that it's giving you a realistic voltage suggests that the sensor is alive, but it's also quite possible that the oil poisoned it enough to bias the reading high.

For the most part, the only parts that will be damaged will be the front part of the first catalyst and the first O2 sensor. So for a first try, changing the front O2 is a good step- it's likely to be damaged anyway, and should be replaced in the catalyst swap. With a new one of those, you'll get a better idea of the state of the catalyst.

I seafoamed my e36 m3 and it smelled like rotten eggs after that. Wouldn't pass smog and I had to replace the exhaust/cats with another used system.

I, personally, won't be sea foaming any car that has cats/has to pass emissions any more.

alfadriver wrote: BTW, flooding the cat with oil CAN ruin one- I've had it done to me- when a testing system tried to go full blast on the flow, which pulled a lot of oil through turbos, and killing a pretty expensive cat system... bummer. not to say the one your friend has is dead for sure, but it happens.

GM bounced a warranty claim on us for a converter, saying that oil cannot harm a catalytic converter. Everyone in service says otherwise

NickD wrote:

alfadriver wrote: BTW, flooding the cat with oil CAN ruin one- I've had it done to me- when a testing system tried to go full blast on the flow, which pulled a lot of oil through turbos, and killing a pretty expensive cat system... bummer. not to say the one your friend has is dead for sure, but it happens.

GM bounced a warranty claim on us for a converter, saying that oil cannot harm a catalytic converter. Everyone in service says otherwise

Need to bounce that claim up to engineers.

First, the phosphorus in the oil does chemically poison the active sites. Which is why the industry oil consumption targets are now clear of 12k miles/qt- so that the cats can survive 150k miles and still provide the lowest of the low emissions.

Second, coat it up enough, and it will die enough. BTDT. (I'm glad I wasn't working with my money...)

alfadriver wrote:

NickD wrote:

alfadriver wrote: BTW, flooding the cat with oil CAN ruin one- I've had it done to me- when a testing system tried to go full blast on the flow, which pulled a lot of oil through turbos, and killing a pretty expensive cat system... bummer. not to say the one your friend has is dead for sure, but it happens.

GM bounced a warranty claim on us for a converter, saying that oil cannot harm a catalytic converter. Everyone in service says otherwise

Need to bounce that claim up to engineers. First, the phosphorus in the oil does chemically poison the active sites. Which is why the industry oil consumption targets are now clear of 12k miles/qt- so that the cats can survive 150k miles and still provide the lowest of the low emissions. Second, coat it up enough, and it will die enough. BTDT. (I'm glad I wasn't working with my money...)

Oh, I totally agree with you. It's kinda like the time we got a claim for an engine bounced after following their bulletin stating that on certain year Traverses, if you find coolant/oil mixing, do not pass Go, do not collect $200, go straight to replacing the engine and heater core and radiator. GM Technical Assistance Center approved us to do the repair, we installed everything, sent the engine back and then GM Warranty told us they were rejecting the claim because they tore the engine down and didn't find anything wrong.

You could do it for her, if it is the manifold/primary cat it's not hard to get to, liquid wrench the exhaust nuts and o2 sensors and swap it out not that big a deal. slip jointed cat is the secondary cat. This job is just like putting a header on it. two flange bolts from under the car and like 8 exhaust manifold bolts and you are done.

Another vote for start with the upstream O2 sensor, at 200k regardless of whether or not this incident hurt it the thing is most likely out of whack enough that a new sensor will quickly pay for itself in fuel savings (tired O2s read leaner than reality, causing the car to run rich).