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Trouble Code

I have a 2001 Nissan Sentra, automatic. My transmission console lights up when I turn my car lights on. The other night, the console felt hot and now my light doesn’t work. The very next day, my check engine light came on. My mechanic tested it and said it was the oxygen sensor. The trouble code is P0420 (I think he said front bank). Can these two events be related? My console light out and then this?

It is unlikely that the console light being out and the o2 sensor problem are linked other than the age of the car. The console light gets its power through an interior lighting fuse and the multifunction switch. It is unlikely that the same fuse would power the heating circuit for the o2 sensor. I am pretty sure the o2 sensor in this car is heated.

If the problem was not with the o2 sensor heating circuit, the two problems are definitely not linked. It is just coincidence.

Hope that helps.

P0420: “Catalyst System Efficiency Below Threshold (Bank 1)”. This DTC (Diagnostic Trouble Code) does not say, “Change the oxygen Sensor”, to me. Which oxygen sensor does the mechanic propose to change, the front one, or the back one? Both? What test did he do? Maybe the catalytic converter is inefficient. If it is, changing the oxygen sensor(s) won’t change that.

Two years ago, the Nissan dealership changed my rear oxygen sensor (they didn’t need to and I did get my money back). The mechanic says it is either the cat or the front oxygen sensor. He plugged this little device in my car and said drive it around for the weekend and it will record everything that is happening. He will then plug it into his computer and will be able to see more of the problem. He does not want to change anything unless the problem is without a doubt the front oxygen sensor. He suspected the cat from the beginning, but because it is a more expensive part to replace, he wants to be sure. Also, my car smogged fine last year. Could the cat have gone bad since then?

Hellokit is telling you like it could be or is.

It is my belief that when you get a code suggesting a ‘Catalyst System Efficiency below threshold’ it means the POST-cat O2 sensor (the sensor after the cat) is sending the signal saying the cat converter is finished or is on its way out. NOT the O2 sensor.

AS to WHEN the cat started to deteriorate is anyones guess. It may have been on its way out when the last test was done but not far enough to fail the grade.

The health of the catalytic converter can be difficult to check. A quality digital multimeter might be sensitive enough to read the voltages on the oxygen sensors accurately enough. A better instrument is a scan tool with a graphing screen, or one which connects to a pc (personal computer) which can display the oxygen sensor voltage scans.

Here is a Web site, with an excellent article about testing for catalytic converter efficiency and oxygen sensor behavior:

The DriveRight CarChip is a nice recording tool; but, I don’t think you can get the best (or, needed) type of information with it.

The health of the engine is a major factor in the health of the oxygen sensors and the catalytic converter. If an engine is running rough, misfiring, burning oil or antifreeze, or has an overly rich mixture, it can damage the oxygen sensors and catalytic converter.

Thanks a bunch. I printed the article from the website.