CEL- P0420 OBD 2 code

[bangbg]

My 2000 Tacoma Prerunner Xtended cab has 255,000 miles on it.

This CEL code is Catalyst System Low Efficiency and comes up after about 500-1,000 miles after a reset. The reason is usually 1 of 2 most often. The bank 1 O2 sensor, the one downstream of the Cat is faulty giving an incorrect reading of the post treated exhaust gases and requires replacement. The 2nd possibility is the Cat is not functioning correctly and needs to be replaced.

Has anyone else gone through this scenario and have any words of wisdom for me. It is possible to test the O2 sensor to establish if it is functioning correctly by using a multi-meter and reading the voltage.

255,000 miles is a lot, I would just be inclined to replace the Cat.

Any suggestions?

[Brian R.]

If you have access to an oscilloscope, ideally you should compare the waveforms of the two HO2 sensors, before and after the CatConverter. This will tell you if the CatConverter is bad and if the problem lies with the HO2 sensor.

The only test for the HO2 sensor by itself is measuring the resistance between terminals +B and HT of the sensor. Resistance should be 11-16 ohms cold and 23-32 ohms at operating temperature.

If I were you, I would bring it to someone with an oscilloscope to avoid wasting money.