How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Dan Your Own Question
Dan, Technician
Category: Nissan
Satisfied Customers: 24443
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
Type Your Nissan Question Here...
Dan is online now
A new question is answered every 9 seconds

Code P0420 on a 2006 Nissan Altima 2.5 SE. 101,000miles. Is

Customer Question

Code P0420 on a 2006 Nissan Altima 2.5 SE. 101,000miles. Is there a way to diagnose whether this is just a bad O2 sensor or a the catalytic converter itself? Would you recommend replacing the O2 sensors first? I do most of my small repairs at home and have always had good luck with Thanks for your time!
Submitted: 1 year ago.
Category: Nissan
Expert:  Dan replied 1 year ago.

Hello, my name is***** almost always means the converter is problem and it is pretty hard to test without a scan tool. Monitor the front and rear oxygen sensors while driving at a steady cruise. If the front O2 sensor is operating normally (switching rapidly 0.1 to 0.9 volts) and has a good pattern, then look for the rear O2 sensor to be steady and not switch rich to lean. It is the job of the catalytic converter to use up the oxygen to clean up the emissions. So, if the rear O2 sensor switches like the front one, the catalytic converter is bad and will need to be replaced. If you do not have a scan tool to monitor sensors you can try running long wire from o2 sensor signal wire to monitor voltage too. Let me know if you have more questions or if there is anything else I can do to help. Thanks.

Customer: replied 1 year ago.
Hey Dan,
Thanks for the input, sounds right. I have a ELM 327 Bluetooth code reader and scanner. Can these perform the O2 sensor tests you're suggesting? It gives me a decent amount of real time data
Expert:  Dan replied 1 year ago.

If you can monitor oxygen sensor voltages on scan tool it should work fine and I think it probably will give the voltages to both. Let me know how it goes or if you run into more questions. Thanks.

Customer: replied 1 year ago.
I will give it a try. Thanks again!
Expert:  Dan replied 1 year ago.

No problem, keep me updated. Thanks again and have a good weekend!

Customer: replied 1 year ago.
Hey Dan,
These are screenshots of my o2 sensor test. The upstream sensor seems to be perfect. You can see on the graph where I stepped on it. RPMS went from 2500 to 5500. The lower sensor is giving some weird results. It isn't really mirroring the front sensor, but definitely isn't staying at/around .3v. You can see on the sensor 2 graph where I accelerate the same way (it goes above .8v) but when I let off the gas, it goes to 0.0v. When i looked at both readings in the menu, #1 was much lower than #2 (.31v versus .78v). Could this be something else (ecu reprogram?). Let me know what you think. The images will say in the bottom left which sensor was being tested (o2 1x1=upstream, o2 1x2=downstream). Whatcha think?
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Customer: replied 1 year ago.
I know it's the weekend, hope I'm not bugging you... really hoping to get this diagnosed and fixed this weekend so I don't lose a day next week. If possible, please respond asap. Thanks again Dan
Customer: replied 1 year ago.
P.S. disregard the request for a phone call. That was an accident
Expert:  Dan replied 1 year ago.

Because oxygen sensor isn't being clear about converter having failed oxygen sensor would be place to start but converter is still the most likely cause. Let me know how it goes, thanks.