# 2006 02 sensor/catalytic converter problems



## myvols2002 (May 29, 2016)

My mother-in-law has a 2006 Nissan Altima 2.5 auto with 180000 miles. It popped up a code P0420 "Catalyst system defective (Bank 1)" about a month ago. I cleared the code to see if it would come back on and so far it has not. Yesterday it popped up a code P0300 " Multiple cylinder misfire detected". I cleared that code and done some checking. The upstream 02 sensor reading is a flat .300 to .275 not osculating hardly any at all. The downstream 02 sensor is a flat .800. I got these readings after driving it for about 15 minutes kicking it a few times. Not driving it much in the past I couldn't tell if it had a loss of power or not but I thought it should have a little more than it did. It also had the smell of sulfur from the tailpipe while I was at idle doing the test. At first I thought that it was a bad cat but now maybe a bad upstream 02 or maybe both. Any thoughts. I hate for her to be out for a new cat when all it was a 02 or maybe something else. Thanks for any help.


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## cj1 (Oct 21, 2014)

If the only code you have is P0300, the multiple misfire can cause lean exhaust( O2 sensor reading low).

Test of O2 sensor- some snap throttles should drive voltage high. If so then sensor responding and not the cause of misfires.
If not then confirm test by adding combustible to intake( maybe brake cleaner into brake booster vacuum hose going to intake)
If no response then sensor bad( note should have had error code if sensor not responding) 

Test for multiple misfires- try to determine when misfire happening( freeze frame data when error code set)

Are fuel trims normal? If fuel trims are high at idle and don't improve at 2500 RPMs then suspect vacuum leak.
If fuel trims remain high thru RPM range then suspect MAF or fuel delivery.

Finally at 180,000 miles have plugs been changed? Correct OEM ones?

Fuel pressure normal? Injectors clean?
Engine compression good enough?
Exhaust back pressure normal?


Regarding P0420 - cat may be going out but first verify engine parameters are normal. Misfires won't be good for a new cat.


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## myvols2002 (May 29, 2016)

Thanks for the response. I only see her once every week or two so the next time I see her I will do some more text. I did spray some carb cleaner in her brake vacuum and got a slow response but not sure if it was the 02 sensor response being slow or my wife who was watching the reading while holding the code reader for me. I replaced the spark plugs with NGK iridium plugs around 100000 miles.


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## cj1 (Oct 21, 2014)

Considering mileage and not having vehicle available to test. Replacing the coils when you get the chance might not be a bad idea. 

Let us know what the outcome is.


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