hat code doesnt necessarily mean you need an o2 sensor. it means the o2 sensor is reporting an issue- the cause of which could be many. hate for you to spend 150 on the part only to have the light come back on. bad gas, loose gas cap, etc. reset the light, use good gas, tighten the gas cap and hope it doesnt come back on. the code will tell you which bank and sensor e.g., bank 1 senor 2. On the altima, I think the first bank is where the number 1 cylinder is but double check that. Get the right part- rockauto is pretty good. get oem so it bolts on with the right connectors. the muffler is an afterthought- it is after the o2 system so it would not do anything. they are not hard to change if you are a little handy. you will probably have to get under the car and unplug the wire from the harness and unscrew it. it might be frozen in so you might need to heat it up first. could take a little doing but not a hard job, definitely a DIY project. Electrical parts are NOT returnable FYI.

Thanks for the quick replies. Since it could be my exhaust which is causing my Service Engine light to stay on, would it be better to have that repaired first? Or will it solve the problem completely. Because if I replaced the Oxygen sensor, if the exhaust is what caused it to go bad originally, wouldnt the new one go bad or the Service light stay on until it is replaced?

If your car is running fine the o2 sensor is not bad. You will know immediately if it is.

I paid 200 to have a sensor replaced. 2nd time I googled and cleaned it for nothing.

You'll need an exact replacement, as a wrong one will continue to mess the engine up. Also, most use a specially modified deep well socket to remove & replace them.

Get the exhaust fixed and erase the code. Chances are extremely good that your exhaust leak is causing the problem.

Which part should you buy, duh, an oxygen sensor. They make special socket wrenches for oxygen sensors, because of the wire.

Autozone gave me the code P0136 for my Altima yesterday. Said that the Oxygen sensor would need to be replaced. Which part should I buy, and what's the difficulty of replacing it? Also how important is it that I should get this issue fixed immediately??

The Service engine soon light came on two days ago, never happened before, but probably 2 days prior to that my Muffler cracked. Could this be the cause?