I have just found out that my 2001 Nissan Sentra has TWO catalytic converters. One front and one rear. Is this true? And if you need a new one, how can you tell which is the bad one and can you change only one?
BIZZARE!
If the cat is bad because it rattles bang on both till you can tell which one it is. If the vehicle is failing emissions maybe a code generated by the upstream or downstream oxygen sensor. I seen them fail mainly by rattle (the cats I mean) maybe other code can point to a inop cat. Plugged is not so hig tech to find just open the exaust system. Hope it bolts up and not welded
The engine computer relies on the oxygen sensors to tell it if the catalytic converter gets sick. The oxygen sensors, themselves, can become sick and the engine computer may think that the catalytic converter is sick.
When the engine computer gets “sick calls” from the oxygen sensors, it will turn on the check engine light and set DTCs (Diagnostic Trouble Codes) in its memory.
If there is a check engine light on, the engine computer is scanned, with a scan tool, for the DTCs (like P0430).
Then, the oxygen sensors signals are checked with a scan tool, or with a digital voltmeter (multimeter). It’s known what the signal should look like for healthy, or unhealthy, oxygen sensors and for the catalytic converter.
There may be audible indications of a catalytic converter fault; but, most often, not. The expensive way to find out is to just start changing parts, willy nilly. The wiser way is the checking with the scan tool, or digital multimeter.
Some Ultra Low Emmission cars had two. one right up at the ex manifold and one further back.
It woudl have several O2 sensors, the downstream ones to monitor the CAT performance this shoudl tell the ECU which if any may be bad.
However it is a good assumption that the leading one, (dong most of the conversion) is the most probably to fail first.
Hellokit, you know your stuff! My engine light came on with a P0420 code. My mechanic suspected the cat at the time and also the O2 sensor. He did check everything and even installed this gizmo on my car’s computer to monitor for a few days. He ended up changing the front O2 sensor and my light came on a week later. Although his scan shows the cat as OK, he still thinks it may be a small crack inside or ???. Visually it looks OK. But do these cars really have 2 cats?
Many BMWs have 2 cats. The only reason I ever had to replace one was because they rattled. Cut it off with a saws-all and BMW provide a neat sealing sleeve for the new one,make sure you cut the old one off at the correct point. The v-12 engines had electrically heated cats wouldnt want to pay for that one.
A lot of what happens in the catalytic converted is affected by what happens 'way before the exhaust gases get to it. I hope your mechanic is checking the MAF (Mass Air Flow) sensor flow rate at idle and at higher engine power settings. How healthy is the tps (throttle position sensor), the iac (idle air control) valve, the egr (exhaust gas recirculation) valve flow? The scan tool is used to determine this, and the mechanic’s knowledge of the systems.
Clean the MAF with MAF Cleaner to get its best readings. Use Carb/Throttle Body cleaner on the throttle body & throttle plate, and the iac.
If you have to replace one, you should do them both. The cost difference is not huge, especially if you buy an aftermarket unit instead of OEM. Check out prices on eBay Motors for examples of the cost.
I also have two cats. They are both Siamese. 
You will get a factory warranty claim rejected for replacing parts that cannot be shown to be defective. I know OP is a 2001 probably out of standard warrantybut what about emission related warranty? At BMW we went 6years 80,000 miles
Two years ago I had the mass air flow replaced with a new one. It was still under warranty and the codes at that time showed this as the problem. The Nissan dealer replaced it. Where is your shop? I wish it was right around the corner… I will pass this info on to my mechanic. Also, if it is my cat, can it harm my car if I don’t have it replaced now?
Federal emissions allows 8 years or 80k miles IIRC.