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P0420 OBD Code

My 2005 Nissan Altima 2.5 L has a P0420 code error (Catalyst System Efficiency Below Threshold Bank 1). So far I changed the spark plugs, upstream and downstream oxygen sensors and the Catalytic Converter. I turned off the Check Engine light after each repair and two days later, the same P0420 code turns on the Check Engine light.
I have no idea what to do next. Any help would be appreciated.

Have you checked for exhaust leaks?

Actually looked at the signals from the O2 sensors? Checked the wiring for the O2 sensors?

The P0420 code, despite being labeled as an inefficient cat, really means a downstream O2 sensor that is too active relative to the upstream sensor. A bad cat will cause that. An O2 sensor issue will cause it. So will anything other than the cat (such as an exhaust leak) that actually throws off the exhaust content.

It would also help to give a complete run down on the car’s general condition and maintenance history. Also note what kind of new converter was installed - direct fit OEM from Nissan? Aftermarket, universal or generic?

There is a service bulletin for a software update that may apply;

Classification: EC10-014

Reference: NTB10-083

Date: July 8, 2010


APPLIED VEHICLE: 2005-2006 Altima (L31)

QR25DE* Engine Only

  • 50 State emission specification ONLY


DTC P0420 (Catalyst System Efficiency Below Threshold) is stored in the ECM.


Determine if this bulletin applies by checking the ECM Part Number.
If this bulletin applies, reprogram the ECM and perform P0420 DTC Confirmation.

Sounds like it might be a software problem not a hardware problem…

Here’s another one to add to Nevadas that applies to California SULEV : Service Bulletin NTB 09-046.

I had the same issue except that I almost changed the cat (but did not). I too changed all spark plugs, did the engine clean service, dumped several Cataclean systems into the car, changed both oxygen sensors and nothing, so drove it around with the code for months. But then one day, I got a misfire on cylinder 3 code (P0303). I replaced the ignition coil just to get rid of this new code…Lo and Behold the 0420 was also gone.

I have an OBD monitor (not just a code reader). I watched my O2 sensor performances, during driving, idling, idling at 2000rpm over a period of time, watched fuel trims, compression ratio etc…and everything was perfect. My gut feel was that the cat was good and the system had a leak…but looks like the ignition coil was just low on efficiency.

Try changing that if you can. You can get some good ones at

Check Vapor Canister Purge Valve / Solenoid. I had this problem with 2006 Sentra and I did everything you mentioned. Finally, went to another mechanic and he figured this out. Since replacement of this guy,no P420.

Edit: Didn’t notice @Shashi revived 7 year old post. Either way, hope it helps someone that comes looking here. It was a royal PITA to deal with this. Spent upwards 1000 to fix it, just to spend $450 more to get rid of it finally.

What model?

I have the OBD device linked to my android phone. Works excellent!

I have this one:
INTEY Bluetooth OBD2 Car Diagnostic Scanner for Android; Windows 7 8 Devices OBDII Car Check Engine Light Code Reader Scan Tool with 5ft Extension Cable

Here is the link: