Toyota Tacoma P0420 cat or o2 sensor

On Apr 14, 2:32 pm, "jcaru" wrote:

I'd definitely suspect the front O2 sensor before condemning the cat. The drop in gas mileage is another strong indicator of a lazy O2 sensor. The front O2 sensor monitors the oxygen content of the exhaust gas in the manifold and should normally be switching rapidly between about 200 millivolts and 800 mv as the computer rapidly adjusts the fuel/air ratio to optimize the fuel mixture. If the cat is working properly, the rear O2 sensor should output a fairly constant voltage of about 450mv. The computer compares the switching rate of the front O2 sensor to that of the rear O2 sensor. If the switching rates are similiar, it sets the P0420 code under the theory that the exhaust gas coming out of the cat looks very similiar to exhaust gas going into the cat and so the catalyst efficienct is below normal. But if the front O2 sensor gets lazy and starts switching too slowly, then the P0420 code is set and the problem is the sensor and not the cat. It is also possible that the front O2 sensor heater circuit is malfunctioning which would cause the O2 sensor to not get hot enough, especially at idle, to work properly. However, if that were true, there is usually a separate code for that. Before spending a ton of money on a cat, it would be very worthwhile to put the vehicle on a scan tool and watch the operation of the O2 sensors and observe that the O2 heaters are on. If you can't do that, investing in a $50-75 OEM front O2 sensor first before opting for the $1000 fix would seem to make good economic sense. Good luck. Al

Or, if you still have the old O2 sensor, take the new one and put it in front of the cat and put the old one back in the rear position.

It's cheap if you still have the old part.

Thanks for all the input. I have a good mechanic that will help the truck pass emissions if you know what I mean. Say it is the front 02 sensor. Will and after market Cat mess with my sensors at all. And also how do I place the voltmeter on the sensor to test them??

=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D your SECOND sentence..... yeah..i know what you mean...he's not a good mechanic, because a good mechanic would NEVER MAKE...a vehicle pass emisions.

regardless.....

here....more than you need to know, and more than you deserve because of your intent on be'n a stick in the mud about global warm'n... oh yeah..i know...there is NO global warm'n... sure...sure.

.

~:~ MarshMonster ~slaps you side the head....sips his crownroyal.....hopes you come to yer senses....or sensors...or convertors~

If you really need a cat, which I doubt, aftermarket catalytic converters are definitely the way to go. Federal law requires minimum standards for them. I believe, though I'm not sure, that they must be designed not only to adequetly reduce emissions but to last something like eight years. I don't recall the exact number of years. There's no compatability issue whatseover. Thanks for that very informative website with the primer on O2 sensors. If you want to test the sensor, a scan tool is really the only practical way to do it. The very informative website that another reply mentioned describes a voltmeter test but you'd have to tap into the wiring somehow and have a voltmeter with a fast response. It's also possible to bench test it with a blowtorch and a voltmeter but that's a bit of a Rube Goldberg approach. I presume the "good mechanic" can put the vehicle on his scan tool and not just conveniently look the other way during an emissions test.