O2 Sensor before or after the catalytic converter?

The O2 sensor after the catalytic converter is the converter monitor and this is the one that would generate a faulty cat code.

I guess you have code P0420. That doesn't mean the catalyst is bad necessarily, just that the O2 sensors are reading off. You said codes, not code - which codes. If you have O2 sensor codes, check the sensors. If you have lean or rich codes, then the sensor readings (and therefore the catalyst readings) are probably off due to the underlying engine problem. First thing you need to do is figure out the exact problem. The muffler dampens the sound, not the catalytic converter, so that should not be a problem, however the vehicle check engine light will always be on throwing converter/O2 sensor codes unless you get O2 sensor bypass that gives the O2 sensor readings the car expects. The way oxygen sensors work is by reading the upstream reading (the O2 sensor before the catalyst) and comparing it to the downstream reading (the sensor after the catalyst) then comparing the readings to detect the efficiency of the catalyst. The upstream O2 sensor also responds to the PCM to adjust the fuel trim if it detects too much unburned fuel in the exhaust. Since what you are doing should not affect the upstream O2 sensor, it is unlikely to cause engine problems, but the downstream O2 sensor may go bad from being stuck high for so long. Good luck. If you want a better answer, please post the exact codes you are reading and any symptoms the vehicle is experiencing (e.g. rattling from catalyst, catalyst glowing red, stalling from excessive backpressure).

Oxygen Sensor Before Catalytic Converter

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RE:

O2 Sensor before or after the catalytic converter?

I think I need an O2 senor replaced on my 02 nissan altima because the P0420 (Catalyst System Efficiency Below Threshold (Bank 1)) code came up.

I need to know what what sensor to get, the sensor before or after the catalytic converter?

You want the O2 sensor that is after the catalytic converter though that might not be your problem. You may need a new catalytic converter. O2 sensors that are after the cat rarely fail so early. I suggest that you take your car and do a few full throttle runs and let it shift at red line. This will heat up your catalytic converter and hopefully clear it out and make it work good again. after that clear the codes(disconnect the battery for about 15 minutes) and see if the problem comes back before you buy anything.

This code is generated because of the relationship between the pre and post O2 sensors. Your system is not seeing the correct ammount of "efficiency" from the catalyst.

I would not recommend you replace the sensors. Have it properly inspected where your technician can monitor the readings. If needed, you may find the catalyst is still under warranty.

You need to understand WHY this O2 sensor is acting up. You may not need to replace it, at all. FIrst step is to LOCATE and remove the IDLE AIR CONTROL VALVE. Clean this guy and your throttle body to SHOWROOM cleanliness. Use Gum-out spray and Amsoil Foaming Cleaner, if you can find it. When these two things get dirty with time, they cause fuel mixture problems that create carbon and clog things up. When you are done with this cleaning, use the Gum-out to run through your intake manifold and throttle body. It eventually will clean the tip on your converter O2 sensor. You will have corrected the CAUSE of the O2 sensor failing, and get more time out of things. Good luck!! Remeber to use ONLY top tier fuels.

LeAnne is correct.

But remember the O2 sensor is just the messenger. You need to start back testing the whole system from the sensor.