O2 sensor or catalytic converter?

Have a manual 2003 nissan sentra with over 80,000 miles on it. For the past month or so, the check engine light will come on and go off on it’s own. Took it in, was told converter was failing. Spoke with someone else who says symptoms point more to needing an O2 sensor replaced rather than new converter furthermore my car doesn’t have sufficient miles for the converter to be failing. Has anyone else had this problem with their sentra? Usually how old is a car before the converter goes out?

What you were told, for both things, is partially correct. The signals from the oxygen sensors, which take measurements in the catalytic converter, go to the engine computer. The engine computer reads these signals, and decides if the catalytic converter is doing its thing properly. (The engine computer, also, decides if the engine is running rich or lean).
The problem, with SYMPTOM BASED assumptions of “troubleshooting”, is ignoring the fact that the oxygen sensor indications can be ERRONEOUS. Oxygen sensor output can be read with test equipment; but, what mechanic/shop will do that?
Though there are troubleshooting charts which the mechanic could use as a guide to diagnosis, many (most?, few?) do. So, it’s EXPEDIENT, for the mechanic/shop to just change the oxygen sensors (or, catalytic converter) and SEE WHAT HAPPENS.

Check the price on an O2 sensor and a catalytic converter. See which one it makes sense to try first.

There is no computer code that says, “Replace catalytic converter.”

The O2 sensor should be tested before anything is replaced, but I’d try a new O2 sensor before I paid all that money for a new cat.

It could be the cat converter.
It could be O2 sensor(s).
It could be something else upsetting operation of the converter & sensors, like:
Vacuum leak, exhaust leak, dirty MAF sensor…

First, find out what the error codes from the computer are (look like P0123)and post them here.

FWIW, I recently sold my '88 Accord with it’s original converter and O2 sensors fine at 219K miles.

I’d get a second opinion. The cat efficiency fail protocol isn’t an on again off again situation in any vehicle I know. It’s a given number of like states between the upstream and downstream sensor. The closer the count is to zero, the better the cat efficiency. The higher the incidence of like states, the lower the cat efficiency. Once the threshold is reached, there is no return to normal (again, in any vehicle I know of). The code will remain set until it’s it’s reset …where it should return and stay set again.

That is, get the real code. I think that they’re “telling” you CAT …which then opens the door for "Oh, Mr. Whileyou’re here, can we offer new O2 sensors “while we’re at it?” and turn an already super high bill into an astronomical one for the sake of “peace of mind” (mostly theirs so that there won’t be a comeback due to not figuring that it ONLY needed ONE sensor).

Also check the year/mileage limita on your emissions warranty.

There is no shortage of mid-range (in technical degree of difficulty) test procedures to check out both 02 sensors and catalytic converters

The code is P0420

In your case, the first thing to check, according to the FSM, is for water in the rear O2 sensor connector. The connector is located in a bad location and it wet weather, it can get wet.

Dry it out and then coat the outermost seal with silicone grease.

Technical Description P0420 Catalyst System Efficiency Below Threshold (Bank 1)

What does that mean?The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emmission control. Causes: A code P0420 may mean that one or more of the following has happened:

?Leaded fuel was used where unleaded was called for
?An oxygen sensor is not reading (functioning) properly
?The engine coolant temperature sensor is not working properly
?Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
?Retarded spark timing
?The oxygen sensors in front and behind the converter are reporting too similar of readings
Possible Solutions:
Some suggested steps for troubleshooting a P0420 error code include:

?Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
?Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
?Inspect the downstream heated oxygen sensor (HO2), replace if necessary
?Replace the catalytic converter
One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it’s out of it’s bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It’s worth checking into.
All the above info came from this website: http://www.obd-codes.com/p0420

This is an expensive and unreliable method of troubleshooting vehicles. Today’s vehicles are way too complex and replacement components are not always cheap. You can find multiple possible causes for any given symptom and most components have a way of being tested i.e. oxygen sensor,tps sensor, maf sensor, fuel injectors, etc. Spend the $30-$70 or more you would spend on an O2 sensor on an electrical multi-tester and a service manual and do your troubleshooting properly.

Skypilot, that’s a great post. Anyway, all in my mind when hearing about o2 sensor is the leakage. Might as well check your dealer or auto shops. Maybe oxygen sensor relay has to be replaced.