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Catalytic Converter & MAF sensor

I’ve been having issues with my car, i took it to Orielly’s & they ran diagnostics, one of the codes said a recommended fix was replace the MAF sensor.
Today I took it by an auto shop for them to look at, and the guy said it was my catalytic converter, and I mentioned the MAF sensor, he said fixing the CC would take care of the MAF sensor malfunction. So I have a couple questions…

  1. Is what he said correct?
  2. If you have a bad CC, does it ever NOT smell bad? Bc I’ve never smelled anything other than stinky exhaust, never any rotten egg smell.

Make, model, year, miles?

Describe the issues the car is having.

How did the shop determine the car needed a catalytic converter? Was it a chain, dealer, or independent mechanic?

A third opinion by a trusted independent mechanic may be in order.

Ed B.

My advice is to stay away from that auto shop. Get a third opinion. You are most likely 0-2 in finding your real problem. Find a good independent mechanic because chain shops like Orielly’s don’t have a good track record in reliable repairs.

O’Reilly’s doesn’t run diagnostics. They pull fault codes. Diagnostics is like getting an X-ray and medical exam. Reading fault codes is like taking your temperature. Unfortunately a number of auto shops don’t understand the difference either. Anyone that relies on the list of “recommend fixes” for a particular fault code is not a competent mechanic.

I can easily come up with a situation where a faulty catalytic converter can and will cause a fault code for a Mass Air Flow sensor, it’s not unusual. As often as not a bad catalyst doesn’t have any smell.

But first year, make, model, engine and a detailed description of you complaint about your car is in order.

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MAF sensor usually is easy, they get dirty and most parts stores sell MAF cleaner. Its a simple job to clean the MAF sensor. They can go bad, but that is pretty rare and replacement sensors are very expensive. I had one go bad not too long ago and I got a cheap replacement sensor from Hong Kong, $40. Reviews for the sensor were about 50/50 on whether they worked or not, so I pulled the battery cables while doing the change so the computer had to do a relearn with the new sensor and it worked just fine.

Post the actual code here though, you will get better opinions if you do. Oh one more thing, any leaks in the intake ducts after the MAF sensor will throw a code for lean fuel, this is the code most often interpreted for MAF sensor.

First of all, how about telling us what your problems are?

What is the car doing?

What kind of car, what engine?

Exactly what codes did O’Reilly retrieve?

I agree with @asemaster . . . most of the bad cats I’ve encountered smelled perfectly normal

Now, how did the shop arrive at the conclusion that the cat is bad? Hopefully, he performed some diagnosis, and isn’t basing it solely on a fault code. And if the car is indeed bad, hopefully it isn’t the symptom of some other root cause

Such as excessive oil consumption, to name one

Ignition misfire, to name another

A few years ago, a colleague of mine . . . not a mechanic . . . asked me to check out his truck, because it sounded funny, and it wasn’t running right

Anyways, it was quite obvious that he had a gigantic vacuum leak, because it made a whooshing sound. I hooked up my scanner and retrieved a bunch of misfire codes, as well as lean codes. But there was also a MAF code, saying it was out of range, or something like that. I quickly verified my suspicion that the intake gaskets were flat. No coolant running through the plastic manifold, by the way

We agreed that I would fix his truck in my garage, at my house, on the weekend. The real problem was the flat intake gaskets. It was obviously causing the lean codes. But it was also causing misfires with a cold engine, before they expanded somewhat when hot. But never enough to fully seal. The out of range MAF code was also caused by the intake leak.

After fixing that, and verifying the repair, I threw a new set of plugs at it, because it was due by mileage.

The truck ran great afterwards

The funniest part, was that when I removed the intake, a bunch of acorns fell on top of the valves. I used a vacuum cleaner and needle nose pliers to retrieve them. I spent quite a bit of time, with a flashlight and mirror, to make sure I got all of them. No matter how well you vacuum an engine before removing an intake, something always seems to fall in. There is always those few seconds when you remove the manifold, before you have a chance to cover up the ports

One more thing . . . I decided to be proactive. While I had the intake off, I replaced both knock sensors and the subharness. It seemed like a good time to do it. Not to mention that the sensors looked pretty crusty, and the harness had gotten brittle over the years

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Now THAT’s “shade tree”! {:smiley:


But it seems to happen every single time I remove an intake manifold . . . at least on a v engine

Maybe all the vehicles are parking under that shade tree you mentioned . . . lol

But I think it’s more a matter of rodents finding a place to store their food

Rim shot for @the same moutainbike !

On cat/O2 sensor/MAF codes, you need a good mechanic to figure out – before the parts swapping begins – which one is the actual problem. The shop manual for the car will have a procedure, a series of tests to follow, which will make that determination based on the diagnostic codes.

It used to be the case – before electronic fuel injection and O2 sensor feedback – you could do the parts swapping thing and eventually you’d fix the problem. But these days you’ll likely run out of money before you find the problem if that method is used.

It’s a 98 Nissan Maxima, 6 cyl. it has 265K miles.
It usually runs “okay” when it’s cold, but once it warms up, it sputters, has problems idling, black strong smelling exhaust, it gets maybe 10-15mpg, when you accelerate sometimes it sputters.

So far we have changed the front 2 O2 sensors, all the spark plugs, 3 coil packs, and cleaned the MAF sensor. We’ve even tried fuel injector cleaner.

The last time we took it to Orielly’s (I assumed it was diagnostics since OBD stands for on board diagnostics) the codes were…
p1320 - Ignition control signal circuit malfunction
p0100 - MAF sensor circuit malfunction.

The other ones were for the O2 sensors & coil pack (like, p0134 heated oxygen sensor Bank 1 sensor 1).
The shop I took it to was an independent, the guy had me start it, and he went to the rear & put his hand in front of the muffler, and of course there was black spots all over his hands) and then told me its the CC, and that I probably need to replace the other 2 O2 sensors along with the ones I have already replaced bc they are probably clogged up again.

You are running rich. I would replace all the plugs. One or more of the plugs may be black and this will tell you which coil is bad. A multimeter may help by reading the coil resistance, but the coil may be failing under high load. Also, bite the bullet and replace the MAF.

You can also unplug the MAF and see if this makes the car run better–It will not run great with the sensor disconnected, and some cars may not start or run with the sensor disconnected, but if this improves things, the sensor is probably at fault. With the sensor missing from the data stream, the computer will substitute its best guess based on other data.

Plugs, as in spark plugs? We;ve already replaced those. We also had a guy come out to look at it a while back, he’s an independent mechanic, has his own high tech computer gadget stuff, he told us about the coil packs, one on top & bottom. We replaced em, then checked the voltage & they were all fine.
So is there any chance my cat could be goin out?

Yes i meant plugs, but since it was done your ok on that. Is the Check Engine light still on? You can unplug the MAF a stated, but it may or may not run. It would not hurt to try it.

Yes, I don’t know if it’s ever NOT been on…
I think my husband has tried unplugging the MAF, I don’t remember why, think we were having problems getting it started. So he unplugged it, we took off, and it started running ROUGH. So we pulled over & he hooked it back up, we made it to the store a block from our house, and couldn’t get it started again. It’s been a while so I don’t remember exactly what the problem was then…

I have a random question. I’ve had people tell me that there is no particular code for O2 sensors, it won’t tell you exactly which one it is. But how come with the OBD codes, like the 2 i got, one of them being p0134, that’s what the description is, is for O2 sensor, bank 1 sensor 1.
And I’ve also noticed there’s OBD I & II?


Those people are 100% wrong

The code will tell you exactly which sensor the PCM is having a problem with, so to speak

That said, the code will not tell you where bank 1 sensor 1 is located on the car, for example. You have to determine your cylinder bank layout. Not every manufacturer uses the same layout

I can tell you that bank 1 is upstream, meaning it’s before the catalytic converter

Check this out . . . I’m merely assuming it’s for a Maxima

Your car is OBD2 compliant, because it’s newer than 1996

If you have a code for a MAF sensor and cleaning it doesn’t work, it needs to be replaced because it has burned out. The Hitachi sensor from Nissan is very expensive, over $400, but if you go to eBay or Amazon, you can find a lot of Chinese MAF sensors selling anywhere from $20 to 80 each. If you read the reviews, they are split with about half saying they worked fine, the other half saying that they didn’t work at all.

I bought one for my Nissan PU for about $40. I disconnected the battery while I installed the MAF so that the computer would have to relearn the engines profile. After a minute, the truck started running smooth again and it hasn’t missed a beat since. I suspect that those for whom the sensors didn’t work did not disconnect the battery.

A fault code tells you that either a sensor is reporting a condition that is not normal, or that a particular system on the car is not functioning as designed. A sensor just takes a measurement of something and reports that measurement to the car. The car sees that information and uses it to keep running properly. If the sensor reports something that is out of the ordinary the car turns on the engine light to say “Hey, something’s not right.”

Your O2 sensor code (no sensor activity) could be caused by a blown fuse, a broken wire, a brake booster leaking vacuum, a faulty mass air flow sensor, a worn out sensor, a coolant leak inside your engine, or any number of other things.

Look at it this way. You have a fever (your warning light is on). You check your temperature and the thermometer (oxygen sensor) reads 104. Do you replace the thermometer (O2 sensor) or look for the cause of the thermometer reading high?

Last week a car came in with a check engine light on. Scanned for codes, found P0420 indicating the catalytic converter was operating below efficiency. Before even opening the hood I determined that there was nothing wrong with the catalyst but that the car needed a coolant temperature sensor.

Too many people rely on fault codes instead of actually testing and diagnosing the car.