Symptoms that a catalytic converter is needs to be replaced

Have a 2004 Nissan Murano with 149K miles. Service engine light came on about 2 months ago and the dealership said the catalytic converter needs to be replaced…for $1200 parts/labor. Car starts fine, but runs very rough and I get a thumping noise from under car for the first 10-15 miles. Also noticed a drop off in MPG from normal 20 to about 17.5. I’m hoping to keep this car awhile, but need assurance the Cat converter is really the problem. Saw several similar posts make reference to the O2 sensor. How can I be sure what’s needed to fix this? Appreciate feedback…

The only reason that I know of that would cause a driveability problem stemming from the cat would be if it were plugged up. There may be other reasons for it to cause driveability problems but I don’t know any. Google this: ‘Troubleshoot Catalytic Converters’. Skip this ads and hit the top link. Read that first. Then hit the second link and read that. Since you know more about how your car is driving than anyone here your best bet is to educate yourself on the symptoms of a clogged cat and see if they apply.

If you are handy with tools and know where your O2 sensor is, this advice will surely tell you something:

“A sure way to diagnose whether a catalytic converter is clogged is to remove the oxygen sensors temporarily from the exhaust pipe. If there is an obvious change in the performance of the vehicle then you can safely reach the conclusion that the converter is clogged. As catalytic converters are not repairable, it will have to be replaced.”

The hole left by removing the O2 sensor will give the exhaust gases a secondary outlet. If the cat is clogged this secondary outlet for the exhaust gases will improve your car’s driveability.

I don’t know how thorough a diagnosis was done but the running rough and thumping part leads me to think there are other issues involved that a converter will not cure.

In other words, the converter (if bad) could be a symptom rather than a cause.

I’d recommend taking it to a reputable independently owned and operated shop rather than the dealership. Get a second opinion. Firstly, you might find out it isn’t the converter, and secondly you’ll save money if it is. Dealers typically charge 2 to 2-1/2 times the price of aftermarket parts for the parts that are needed, and their shop rates are generally substantially higher as well. With something this expensive, an independent could save you hundreds.

One other suggestion: if the car does need a cat converter, and you plan to keep it for many more years, have him install a stainless steel component rather than “aluminized steel”. Stainless lasts far longer, often the life of the vehicle.

I would also recommend to turn to someone who is good in car repair for the diagnosis, so you could be 100% sure what the reason was that caused this problem. And then you would definitely know what and how to fix. Try to look here to see the most affordable options for you, as the prices may vary thoroughly.

Additional note on Jack Dak87’s suggestion to pull the upstream oxygen sensor - it is going to be REALLY loud when you start the car with that sensor hole empty. Do not do this at 6 AM if you have neighbors within a quarter mile.

If you have a single exhaust pipe (or one dedicated exhaust per converter) you can check for an obstruction in your exhaust by putting your hand at the exhaust opening with the car idling. You should be able to feel the individual bursts of exhaust. If the flow is smooth, there is an obstruction.

We still have no theories about the cause of the thumping, other than it may be related to the engine running rough.

At 149k miles, they will absolutely replace both oxygen sensors if they replace the cat. You have nothing to loose by replacing them first to see if that corrects the problem.

If the cat has failed, there is likely some additional problem with fuel or ignition. Make sure that is corrected if you invest in a new cat.

A couple other ideas, besides the good ideas above. Better equipped shops will often measure the pre- and post- cat O2 sensor signals. Those are what the ECM uses to decide when the cat is bad. So the shop should be able to verify that what the ECM is seeing is actually due to a bad cat or not. And something can usually be learned about the degree the cat freely passes the exhaust gasses with a simple intake manifold vacuum test.

@Manolito: >>You have nothing to loose by replacing them first to see if that corrects the problem.

Remember when I said that how many regulars dived on me for bad advice?