6 Symptoms of a Bad Catalytic Converter (and Replacement Cost)

Updated

Catalytic converters (not cadillac converters as some like to say) help remove carbon monoxide (and other toxic gasses) that a car produces from entering the outside air that we breathe. While it’s likely you will never need to repair or replace yours, sometimes they fail.

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So what are the symptoms of a bad catalytic converter? Can a catalytic converter be repaired? And how much will it cost to replace a catalytic converter?

Keep reading to get a basic understanding of catalytic converters, know what signs to look for in a faulty cat, and what your options are as far as fixing the issue.

What is a Catalytic Converter?

A catalytic converter, also commonly referred to as a “cat”, is part of the exhaust system. It’s positioned between the exhaust manifold and the muffler.

Most cars on the road today have one catalytic converter but many with dual exhausts have two (one unit for each set of pipes). Some higher end vehicles now have two cats in-line which further reduces harmful emissions gases.

Its job is to filter out or “convert” the harmful gases (nitric oxide, carbon monoxide, and unburned hydrocarbons) a vehicle produces before they enter the atmosphere. These toxic gases almost single handedly ended the classic muscle car era in the 70’s and are responsible for much of the smog found in large cities.

How Does a Catalytic Converter Work?

After a vehicle is started up, the toxic gases produced by the car’s engine pass through a catalytic converter. The internal structure of the cat consists of a honeycomb design made up of precious metals such as platinum, palladium, and rhodium.

At a high temperature that a catalytic converter is designed to function at (about 800 degrees F), a chemical reaction occurs where the end result are safer elements that come out of your muffler such as oxygen, water, and carbon dioxide.

What Causes a Catalytic Converter to Go Bad?

failed catalytic converter causes

As mentioned earlier, most catalytic converters will last the lifetime of a vehicle. But in some cases, a cat will go bad and need to be replaced.

Almost all problems with a catalytic converter are because of an engine issue. It is most often caused by excess fuel entering the exhaust system due to an incorrect air/fuel mixture, bad spark plugs, incorrect engine timing, failed oxygen sensor, or other issues where fuel leaves the engine’s combustion chamber unburned.

When this happens, the catalytic converter reaches a temperature that’s too high and actually begins to melt the internals of the cat or break apart the honeycomb material. The result is a non-functioning cat.

In addition, failed gaskets, bad valve seals, or worn piston rings can cause oil or antifreeze to enter the exhaust system and coat the ceramic catalyst within the converter with a thick carbon soot.

If this is allowed to go on long enough, these carbon deposits will clog the catalytic converter so it can’t do its job. Worst case, exhaust flow is completely blocked and backpressure builds back up to the engine which can cause overheating and other problems.

Finally, physical damage to the cat can occur. Though not common, rocks, road debris, or potholes can all damage or crack the outside shell of a cat or the hangers that support it.

A dented catalytic converter often means that parts of the fragile ceramic catalyst have broken off inside and functionality of the cat is reduced.

Bad Catalytic Converter Symptoms

Here are 6 common symptoms letting you know your catalytic converter is clogged or bad.

#1 – Rotten Egg Smell From Exhaust

If you smell something that seems like rotten eggs, it’s a tell tale sign of a failed catalytic converter. Gasoline contains a small amount of sulfur which converts to hydrogen sulfide during the combustion process.

A catalytic converter that’s working like it’s supposed to, converts the smelly sulfide into odorless sulfur dioxide. When the cat is bad, the conversion doesn’t take place so some of the unburnt gases containing the smelly hydrogen sulfide exit your exhaust.

#2 – “Check Engine Light” is On

check engine light

If your vehicle’s catalytic converter is failing or has gone bad, the “Check Engine Light” will illuminate on the dashboard. Modern vehicles contain oxygen sensors which are able to monitor the catalytic converter’s efficiency by checking the exhaust’s gas levels.

If these sensors detect the exhaust gases are not being catalyzed properly for whatever reason, the “Check Engine Light” will come on. The driver will then know something is wrong, although they won’t know exactly what the problem is because the light can mean a lot of different things.

The best thing to do is retrieve the trouble codes from the car using a diagnostic scan tool. P0420 and P0430 are two common fault codes that indicate a catalytic converter failure.

#3 – Poor Acceleration

One of the most common symptoms of a clogged catalytic converter is a lack of power from the engine, particularly when you try to accelerate such as when you need to pass another vehicle quickly.

When carbon buildup within the honeycomb design gets to be too much or when the internals start to melt due to excess heat from unburned fuel, a partial blockage inside the catalytic converter takes place.

Your engine depends on good exhaust flow to operate at peak power. When the flow is restricted due to a clogged converter, you’ll notice a lack of power when accelerating, going up a hill, or towing a heavy load.

For cases of clogging due to carbon buildup, using a good catalytic converter cleaner can help remove the blockage.

#4 – Failed Emissions Test

Most parts of the Unites States require your vehicle to pass a smog test every few years to make sure your car is properly burning fuel and not expelling dangerous gases into the atmosphere. One of the most common reasons for failing an emissions test is a bad catalytic converter.

Depending on your vehicle, diagnostics will be done through either the OBD2 port on your car (cars made in 1996 or newer) or with a hose connected to the end of your tailpipe that directly measures levels of toxic gases (cars made before 1996).

#5 – Rattling Noise

catalyst honecomb broken noise

If parts of the honeycomb material inside the catalytic converter break apart due to excessive heat or damage, you will likely notice a rattling sound coming from under your vehicle while idling or driving. The noise will often be the loudest when starting the car.

You’ll want to replace the cat as soon as possible as those chunks of dislodged material can actually make their way further down the exhaust system and into the muffler. This blockage can then cause the car to stall and even prevent you from starting your car back up.

#6 – Reduced Fuel Economy

bad fuel economy

When you have a blockage in your catalytic converter, the lower airflow can cause your car to burn more fuel than it needs. Related to the poor acceleration symptom above, when you don’t have proper exhaust flow, you are forced to step on the gas pedal more since acceleration is affected.

That correlates to the engine injecting more fuel into the cylinders which causes the car to have a much richer fuel mixture than needed. 

Since a decrease in your gas mileage is a symptom of many other issues, it alone doesn’t mean you need to replace your catalytic converter. But combined with one of the signs above, it may be a good indicator.

Catalytic Converter Replacement Cost

For replacement parts, we recommend: PartsGeek.com

catalytic converter replacement cost

Because of the precious metals used in its construction (ie: platinum and palladium), replacing a catalytic converter is not cheap. Things to keep in mind are the age and make of the vehicle, number of catalytic converters on the car, and whether the cat is an OEM or aftermarket part.

Gasoline Engines – 1981 and Newer

Almost all gasoline engine cars build since 1981 have a three-way catalytic converter. This design is more complicated than those in older cars which means the parts cost is also high.

On average, expect to pay anywhere between $500 and $2,200 to replace a catalytic converter in newer cars. Parts alone will cost $400 to $2,000. Labor costs will set you back $75 to $150 for the estimated one hour of labor needed for replacement.

Gasoline Engines – Before 1981 (and Diesel Engines)

Gasoline engine cars built before 1981 and most diesel engines (any year) have two-way catalytic converters. Since the design is fairly simple, replacement costs are usually cheaper.

Total cost of a catalytic converter replacement in these older cars will run you about $175 to $750. You’ll be paying about $100 to $600 in parts and another $75 to $150 in labor. Keep in mind that the older or rarer a vehicle is, the more expensive a replacement cat due to scarcity.

Can You Repair a Catalytic Converter?

It depends. If you believe you simply have a dirty or clogged catalytic converter, adding a catalytic converter cleaner to your gas may help clear up the clog. It may take more than one use. An alternative and much strong method of cleaning a catalytic converter is to remove the cat and soak it in a solution of citric acid for 6-8 hours.

Another option is using sodium hydroxide. Here’s a good video explaining the process:

If there is damage to the catalytic converter including pieces of the honeycomb breaking off inside, it’s simply not fee-sable to attempt any type of repair even though you’ll find people online saying otherwise. In these cases, replacement is really your only option.

How to Help Prevent Clogging

If you regularly drive short distances with your car, the hydrocarbons may not be getting burnt away completely because the catalytic converter is not getting the opportunity to get hot enough. To reduce your chances of having a clogged catalytic converter, try driving your vehicle on the highway every now and then for around 10 to 15 minutes.

This will create the necessary heat inside the cat to effectively burn off these hydrocarbon deposits so your cat is running as efficiently as possible.

 

Comments

    • Unfortunately, they’re just giving you an excuse for not having it done. Yes, bolts in the exhaust system are notorious for getting stuck or rusted but every mechanic knows how to deal with them even if it means snapping, grinding, or torching them off.

      Reply
    • It depends on what bolt(‘s) are not coming off and where it is/they are located. As a 24yr BMW mechanic I had my fair share of nuts and bolts that would not come off. I got very good and creative in persuading most to come off. On occasion one would break off before I realized it was stuck. It all takes time, whether heating and soaking with lubricant or cutting, drilling and tapping. It takes a seasoned and experienced mechanic to resolve the problem without further damage.

      Reply
  1. My mechanic said the bolts are rusted on my catalytic converter and that they will need to drill them out. He said it may take several hours.

    Reply
    • While rusted bolts/nuts on a catalytic converter are some of the most difficult to remove, it does not take several hours. Cutting or torching them is the usual method. I’d look for a new mechanic that has more experience with this.

      Reply
  2. Can having your serpentine belt switched and the air compressor switched on a 2009 Nissan Marano calls the Cadillac converter to go out

    Reply
  3. I have a Bmw 2009 528 XI my mechanic says i need a cat but he has not tried other options such as i read you can do and labor he want $300, I feel he’s doing something to keep me coming back, and my car idle is too high I can hear it plus it goes without foot on pedal need suggestions

    Reply
    • The mechanic is recommending a new catalytic converter because the car is idling too high and accelerates without your foot on the pedal? Unfortunately, it’s time to find a new mechanic.

      Reply
    • The first line of the article says the cat converter removes carbon dioxide. It doesn’t: it turns carbon monoxide and hydrocarbons INTO carbon dioxide.

      Reply
  4. 2019 Hyundai Elantra. Being told catalytic converters is expanding. Noticing a loud noise when driving. They replaced something in electronic board and the noise isn’t there right now but am concerned I have a bigger issue coming up with cat.

    Reply
  5. I have a check engine light P2099 on a 2010 Acadia. Need to fix this to get the emission test completed. What is the usual cause of this and can the average mechanic diagnose it and repair it? Or should I take it to the dealer?

    Reply
    • P2099 essentially means your car is running rich as detected by the O2 sensor. Likeliest causes are your sensor could be bad, you may have damaged wiring to the sensor, or you have a bad MAF or MAP sensor. You can try looking at those things yourself or take to a local mechanic (avoid the dealership for something like this).

      Reply
  6. Exhaust came loose . My son welded it back .that was a couple of months ago. Now the car backfired and won’t stay running because it’s idling so rough . what do you think is wrong

    Reply
  7. i had the car 2005 pontiac grand prix 3.8 tested and it says that the oxygen sensors for the catalytic converter its the problem, are the oxygen sensors needs be replace with the cat or is just the sensors and how many sensors are they talking bout. naomi

    Reply
    • The O2 sensors are separate from the cat. I believe your car has 2 sensors but you can contact your local GMC dealership to confirm.

      Reply
  8. 2010 Accord engine light went on a month ago. Mechanic used diagnostic and got catalytic converter code. He reset code and told me to drive to see if comes on again. Just came on again. He told me my car has 3 cats, all over $500 each. Is cat for sure the problem? do all 3 need replaced?

    Reply
    • It’s highly unlikely all 3 cats need to be replaced. It may not even be one as they rarely fail. What code did the computer throw when the tech scanned the car? This will tell which part of the exhaust system is affected. The problem may actually be a bad O2 sensor or even a catalytic converter that’s starting to clog up. I usually recommend the cheapest fix first where in this case would be to try using a good cleaner. See https://cartreatments.com/best-catalytic-converter-cleaner/

      Reply
  9. I have a 97 Acura CL. I was told by a muffler shop that it would take an hour to an hour and a half to replace exhaust and catalytic converter. It took 35 to 40 minutes. Does that seem enough time to replace items?

    Reply
    • Should be fine. Nuts and bolts in the exhaust system are notorious for being seized or rusted, especially on semi-old vehicles such as your Acura (I know, it’s crazy 1997 was so long ago). Sounds like they ended up not having too much trouble with them.

      Reply
  10. Hello my check engine light went on and I would get a stinky smell through the vent when driving the car sometimes. So I took it in and was told that 2 of my 4 catalytic converters were bad. He is quoting me over 5,400. Does that sound about right? I have a 2011 QX56 Infiniti.

    Reply
    • While you have one of the more expensive vehicles to replace cat converters on, $5,400 seems about $2k too high. I assume you went to a dealership? I’d recommend getting a quote from an independent shop.
      Also, it might be worth investigating whether there was any recall related to your year of QX56. I remember there being a known problem causing the catalytic converter failure but can’t remember which years were affected.

      Reply
  11. I just had all 3 of my cats replaced on my 03 ford explorer along with the 02 sensor and after driving for a few hours it rattles,feels like it’s about to shut off or stall but noise is still coming from the cat area,so what should i be looking or asking them to do when i take it back tomorrow morning?

    Reply
  12. Hi I have a surbu Forest 2010. My check engine light, skid, cruise control lights came on. I took it to auto parts store and they put it on the scanner. The code was p0420. I had just put gas in the car , I use the same gas station every time I fill up. This happened about 6 months ago and I don’t know if it was the same code or not.

    Reply
    • P0420 is usually a bad catalytic converter or O2 sensor. You can still drive the vehicle but you’ll need to get it taken care of before your next emissions test (if your area requires it).

      Reply
  13. I have a Nissan frontier 2012 , had a oil leak on the pan , the p20 code on the engine light came on , after fixing the oil leaks and steam cleaning underneath also switched to a higher octane gas and a bottle of seafoam and driving to heat up the cats , the light went out , I guess now do I keep driving with higher octane ?

    Reply
    • Did you mean code P0420? The octane rating of gas should have nothing to do with it. It’s possible the oil leak and excess oil consumption caused the fault code from the O2 sensor in the first place. Seafoam may have helped also. I see no reason to use a higher octane fuel than what Nissan calls for.

      Reply
  14. Hi
    I have a 2007 Hyundai Santa Fe. My engine light is on. Failed inspection. Mechanic says cat needs replacing, however a friend hooked up sensor detector thing to it and only O2 sensor showed was bad. Can I try to just replace o2 sensor or should I get cat replaced?
    I am just about to trade in car for new one and I have to deal with this. Is it worth dumping $1800.00 into??
    Thank you

    Reply
    • Without knowing the exact code that was scanned it’s hard to say if replacing the O2 sensor will resolve the issue. You may try replacing it since it’s a fairly cheap repair (especially if you can do it yourself).

      Only you will know if it’s worth it to keep the car. Sometimes it’s better to stick with your current car when you know its history instead of buying something with a whole new set of unknown problems.

      Reply
      • Yes in regards to the car you have with it’ s problems is better than the used car you can purchase with unknown problems. Buying a brand new vehicle is a different story entirely if you can afford it.

        Reply
  15. Hello my check engine light just recently showed up I went to auto zone to see what was the problem and they said it was the catalytic converter. I have a 2008 Honda Accord I was just wondering how much should it cost to replace and fix this issue? And can I still drive my car without replacing it or getting it fixed I plan on going on a 6 hour trip soon will it cause my car any problems if I don’t get it fixed before I go on my trip?

    Reply
    • Hi Ernesto, catalytic converters are pretty expensive and generally cost $800-1500 to replace. Without knowing the exact check engine light it’s hard to say, but generally catalytic converter codes are safe to drive on as long as there’s nothing obstructing the catalytic converter; a blocked catalytic converter could catch fire and is very dangerous!

      In many cases the vehicle will run with increased emissions and/or reduced efficiency, and it won’t pass an emissions test until the problem is addressed.

      Reply
  16. I have a 2008 Chevy Impala that is coding P0172. So far over the past couple years we’ve changed the O2 sensor, MAF twice, fixed an EVAP vacuum leak, and it is still coding. My brother (a former mechanic) hooked up his scanner and it came up as being O2 sensor, EVAP leak, (both have already been replaced/repaired), or catalytic converter.

    I’ve put almost $800 into getting this car ready to pass an emissions test so I can renew my license plate (which expired in November of 2018). I’m wondering what the possiblity of it actually being the cat and if the cost is worth replacing and if that will turn off the check engine light long enough to be tested. THe last time the car was at the mechanic, they told me the next thing to check was the fuel pressure regulator…estimated at $500. I can’t afford to put much more money into this car and I still owe too much on it (bought it two years ago) to trade it in for something else.

    Reply
  17. Hello, I have a 2014 Chevy Cruze LTZ. I haven’t had the car that long and have so many problems with it already. Recently took my car to a dealership to get my exhaust manifold fixed (had a crack) total around 500. Engine light came back on the very next day, took it back now they are saying catalytic converter needs replaced total 1250, took it took another mechanic and he said he would check the O2 sensors first and wouldn’t worry about the converter. Why would I not worry about the converter it’s a car that I plan on keeping for awhile given the fact I haven’t had it that long. Car shows no issues of driving besides when I’m stopping my RPM slightly jumps up but I’ve been told that’s not from a catalytic converter. How quick should I worry about getting this fixed before it causes anything else to go bad?

    Reply
    • I think it’s wise to check the O2 sensors first, since they’re much cheaper to replace than the catalytic converter. Sometimes a faulty O2 sensor will throw the same check engine light you’d see on a bad catalytic converter.

      The second mechanic may have told you not to worry about the catalytic converter knowing the car is still safe to drive despite the fault it detected, especially if you live in an area that doesn’t do emissions testing. Driving with low catalyst efficiency will usually just increase emissions, but may also reduce fuel economy depending on the root cause.

      If you have a problem with your catalytic converter, you will almost certainly fail an emissions test. In my area, you will actually fail smog if there’s any check engine light, no matter the cause.

      Reply
    • Hi Joyce, there’s no way to know what the malfunction was from without getting the code scanned. If you take your car to a parts store, they should be able to scan it for free.

      Reply
  18. My check Engine light just came on my 2006 Acura TL its the cat (P0420 )About how much is the average cost of repair?

    Reply
    • It depends on the root cause. It could be a bad O2 sensor which would be about $200-500 to replace. A bad catalytic converter costs in the neighborhood of $1000-2500.

      Reply

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