Is it unhealthy to drive with bad catalytic converter?

I don’t know that I’d trust a shop that claimed the cat is bad without setting the CEL.


Here’s an article that contradicts the ‘it’s a broke cat’ diagnoses on the web:
Mazda Tech Tip: Sulfur Or Rotten Egg Smell/Odor From The Exhaust (

It says that high sulfur in the gas can cause the smell, not a bad cat. I tend to agree.

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Technically incorrect. Catalytic convertors were first installed on carbureted cars in 1975. None of them had O2 sensors. The first O2 sensors appeared on Volvos in the late 70s but were not commonly used until the early 90s.

But any 2007 vehicle like this Sentra will have O2 sensors

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If no code, I wouldn’t rush into replacing the converter. It might be OK and this isn’t a cheap part. Also, a cheaper aftermarket could be worse than what you are dealing with.

Here is another thought… We had this when I was a kid. The car STUNK of rotten eggs all the time for several days. Finally my dad realized it was the battery. I guess there was an internal short but a sulfuric smell was coming from a bulging and oozing battery. Maybe have the battery checked at a parts store to see if that could be going out. It was still starting the car but obviously had a major problem.

I dunno. I replaced one cat and it was because it rusted apart. I think I had about 400,000 miles on the car. I think a lot of cats are replaced needlessly. Correction, one was replaced under warranty after the thing glowed red hot. Pretty rare though.

Agree that is a good possibilty I had a battery do that one time.

My '86 Taurus had a smelly cat problem from day one, but only with certain brands of gas.

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True…BUT I was really referring to 2007 vehicles in other countries (like Chile) that may or may not have Catalytic convertors. They don’t have emission requirements. But if the 2007 vehicle does have a Cat, then it will have an O2 sensor.

Good catch. My late 70’s VW Rabbit had a cat, but no o2 sensor. But for a 2007 nissan w/obd II technology, sold in usa, that car has a bevy of o2 sensors.

OP, if I had that problem I’d ask the shop to check the entire exhaust system for an exhaust leak. If nothing found then I’d ask them to check the cat’s performance using the standard method, by comparing the signals from the pre and post cat o2 sensors. If that test passed, cat is probably ok, so I’d try switching gasoline brands first, before considering replacing the cat.

Checking for exhaust leaks, replacng exhaust componets, most any muffler shop can do that.
But suggest to have the cat’s performance tested by an inde shop that specializes in Nissan’s or at least Asian cars. Or a Nissan dealership could do it nicely as well. Requires special expertise and special diagnostic equipment.

A bad idea, imo

Meineke has a vested interest in selling you a new catalytic converter


If your car has a gasoline leak, a new catalytic converter won’t help. Have someone check out this “odor”, some people are not familiar with gasoline or exhaust odors.


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You got the inside joke.

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I ended up taking my car to a 5 star mechanic and he said that what was most likely causing the rotten egg smell was a bad alternator, since it was also apparently making the battery leak from the liquid compartments (battery acid/water?) And it did seem to have that residue near it.

But he said not to worry about that hard fuel smell so much but I personally don’t think that’s right to be smelling that all the time but IDK. But like I said, the rotten egg smell went away once I changed only to Valero gas it seems and only the hard fuel smell remained.

But he did check the catalytic converter (albeit not run his hands over narrow underside/unviewable part of the pipe) but where it is viewable once he raised it, and it seemed there was no cracks to where any gas might be leaking at least. And he said, it wasn’t black either as some that are bad (oxidized? he may have said?) or have gone bad, usually show these signs.

But he said there was no real way to know for sure, unless you took it apart. He also said I may have been smelling the rotten egg smell from the inside the cabin since it’s close to where the battery is or something to that effect. And like I said I only ever seem to smell it when I was parked.

Also I noticed the coolant has been slowly draining over the course of maybe 5 months or so from the max level to the bottom. But the mechanic that refilled/flushed my coolant said there was no leak and I didn’t see one on radiator hose or anywhere else. Where could this leak be coming from or is it normal to fill it up this much? Is it related to the alternator or some other part; bad/slipping drive belt or catalytic converter, I’m describing that might be giving it issues?

And I did notice that the drive belt does seem to kind of have a sheen on it, so maybe that is why I’m hearing it squeaking (if that is what’s causing the squeaking I’m hearing to begin with) since I read an article that has something to do about that and how it can affect the alternator too or vice versa I forget…

Also sometimes I’m hearing some light squeaking coming from my brake pedal when I go in reverse, but maybe doesn’t happen all the time and I just got brake parts worked on not long ago and don’t remember experiencing this before, other than getting a clicking noise when I’d press on the brakes, which the dealer fixed with new brakes shoes I believe? What might be causing this?

Anyways what would you guys do think I should do, if you were me? Should I get a new alternator, drive belt and catalytic converter or what? like I said my car has a 100,000 miles on it, and I hear that a drive belt and catalytic converter only last to about this long. While the alternator can last up to 150,000 miles. Is this just a rough estimate for the most part or can they last much longer and what would this depend on?

After explaining my situation to a rep at O’Reilly’s, he said it was probably best to get a new alternator since this is what he thinks is giving the battery problems or he thinks it was a bad battery to begin with (he said the multimeter showed it had a bad cell), since I got it tested there with a multimeter, and get a new battery since it’s under warranty. But I don’t really know how much he knew or was guessing on, but the mechanic I saw that I just mentioned, sort of confirms more or less it has something to do with the alternator.

By the way the multimeter readings on the battery when he tested it were shown as 13.15 Volts when the car was not on and would flux between 14.3 & 14.2, when I turned the car on and used the headlights, radio and other electrical components if this may be of help. My battery he said was 12 V.
Can a multimeter really show if a battery has a bad cell by the way?

Is it safe to drive around with these should it be safe to drive around with these parts considering the condition they might be in? I hear the battery can even explode after some time if leaving it with a bad alternator.

The only thing is I don’t experience any kind of stalling, any warning lights or anything other than the things I’ve mentioned to you all.

Edit: Also I forgot to mention sometimes my friend says that feel this vibrating on the front passenger side under her seat when the car is turned off. Does this have something to do with the alternator maybe?

A “5 star” mechanic would charge you $100 or more for a diagnosis.
Why did you then go to O’Reillys for an opinion from a clerk?
A bad catalytic converter would illuminate your CEL.

BTW, what is “hard gas”?

Me thinks this 5 Star mechanic needs to return 3 of those stars . It really sounds like you need an alternator, battery and serpentine belt . I also have no idea what a ’ hard gas smell is '.

I presume you mean by “car on” that engine was idling. If so, those voltage measurements are good, they do not indicate any problem with alternator. Auto batteries contain sulfuric acid so could conceivably produce a sulfur odor, but in 40+ years of diy’er car maintenance & repair, I’ve never noticed that. I’m skeptical of the battery/alternator theory as the cause of the sulfur odor myself. Ask shop to thoroughly clean the outside surface of the battery and the connections. If the battery cables are very corroded, ask shop if they should be replaced.

Ask shop to assess braking function. The sound is probably just the pedal bushing squeaking, or the brake master cylinder rubber boot rubbing.

re: cat

Cat function is tested by a standard test on OBD II cars like yours, w/shop mechanic comparing time waveforms from pre- vs post-cat o2 sensors. That’s the only way to assess the cat function. Uusally done w/a pro-level scan tool. Your car’s drivetrain computer does that test automatically as you drive, and if there was a problem it would usually produce a diagnostic code (P0420 probably). I’m skeptical of any problem with the cat function. If there’s a sulfur odor from the cat, that would be caused by the gasoline used, not the cat. By switching to a different gasoline brand it seems like you’ve already solved that issue. Odors from the exhaust system more likely caused by exhaust leaks or oils and fluids (from engine & transmission leaks) dripping on hot exhaust components.

Unlikely to be related to alternator or cat. My guess, your water pump seal has a very small leak, which evaporates from the engine heat so isn’t visibly noticed expect by a slight loss of coolant. Suggest to monitor the coolant level and top off as necessary. If more than one cup of coolant loss per 1000 miles, report back. Next time at shop ask them to inspect water pump for signs of leaks. Often can spot white residue in leak area, even if no coolant drips seen. I had a leak of this sort on my Corolla, but could never see where it was leaking from until I eventually cleared stuff out of the way enough to replace the water pump.

Me, I wouldn’t do anything until an actual measurable symptom appeared. If you plan to continue driving this car, keep it to 150k, 200 k miles, no harm done to replace those parts pro-actively, as long as the replacements are to oem spec. Note that most parts-store replacement parts are not oem quality; oem versions often only available at dealership parts department. Replacing a working oem alternator with a non-oem version, not something I’d do. Likely to end up with a worse alternator that you started with.

If you are looking for something to replace proactively on a 100K miles widely sold sedan like the Sentra, these would be my first priorities, depending on what an inspection showed about their condition:

  • tires
  • brake pads/calipers/master cylinder
  • radiator & water pump replacement
  • exhaust leak test
  • timing belt (probably not applicable to your car)
  • fix engine oil and transmission fluid leaks. Most common engine oil leak is valve cover gaskets, relatively inexpensive job

You mentioned getting some tools. I’d suggest that the first tool you buy is a simple code reader. They cost around $40, O’Reilly’s should have one. There are more expensive OBDII diagnostic tools ($200 and up) but I don’t think they would do anything for you.

O’Reilly’s will read your codes for free unless where you live has an ordinance against it. With a code reader, you can check for pending codes. Most codes require detection in two successive drive cycles before they turn on the CEL. If detected on one drive cycle, a pending (PD) will be stored.

If any PD codes appear, post them here and we can help you interpret them, if a parts store reads the codes for you, they will give you a list of parts to replace, we don’t need that, just the actual code.

The fuel smell should give you a code for the evap system unless the fuel leak is in the fuel line or rail or in one of the injectors. To check this, a mechanic will have to hook up a pressure gauge to the fuel line, charge the line by turning the ignition key to the on position (no start) for a few seconds then turn it off. If the pressure bleeds down rapidly, then there is a leak.

You could be smelling gas when you fill up if you spill any gas or you overfill the gas tank by clicking the pump after the first time it clicks off. If you do that, STOP. Do not add fuel after the first click.

About that coolant loss. Check you coolant level every time you fill up. If you are losing a little coolant, say one cup or less every 1000 miles, but are losing coolant, you may be in the beginning stages of head gasket failure. Yes it could be the water pump but if you don’t see any evidence of a leak around the water pump, then assume a head gasket.

Nissan uses an open deck design on its engine blocks, like Subaru. And like Subaru, they have a higher failure rate of the head gaskets than normal, especially if drive hard on occasion. Newer Nissans use a conditioner in the factory fill to help the head gaskets last the life of the vehicle. I do not know if Nissan sells this conditioner to the public, but Subaru does. Its called Subaru Cooling System Conditioner and you can get it from a Subaru Dealership or from Amazon. It’s about $8/bottle. It is not made by either Subaru or Nissan, but a company in England.

Anyway, it is a stop leak that works on a leak that is just beginning. It will not work on a blown head gasket or one that is leaking more than about a cup every 500 miles or so. If you are still losing a tiny amount of coolant from an undetermined cause, then an $8 investment would be a very wise choice. This will not stop up your cooling system or do any damage like a stop leak or “Head gasket repair” will do, but only use one bottle. The bottles are quite small.