P0420


DTC P0420 Catalyst System Efficiency Below Threshold




CIRCUIT DESCRIPTION
The ECM compares the waveform of the oxygen sensor located before the catalyst with the waveform of the oxygen sensor located after the catalyst to determine whether or not catalyst performance has deteriorated.
Air-fuel ratio feedback compensation keeps the waveform of the oxygen sensor before the catalyst repeatedly changing back and forth from rich to lean.
If the catalyst is functioning normally, the waveform of the oxygen sensor after the catalyst switches back and forth between rich and lean much more slowly than the waveform of the oxygen sensor before the catalyst.
But when both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.



DETECTING CONDITION

HINT: U.S.A. and Canadian vehicles does the MIL light up when a malfunction is detected.

WIRING DIAGRAM



CONFIRMATION ENGINE RACING PATTERN


(1) Connect the LEXUS hand-held tester to the DLC3, or connect the probe of the oscilloscope between terminals OXR, OXL, 0XS and E1 of ECM.
(2) Start engine and warm it up with all accessories switched OFF until water temp. is stable.
(3) Race the engine at 2,500 ~ 3,000 rpm for about 3 min.
(4) After confirming that the waveform of the heated oxygen sensor, bank 1, 2 sensor 1 (OXR, OXL), oscillate around 0.5 V during feedback to the ECM, check the waveform of the heated oxygen sensor, bank 1 sensor 2 (OXS).



HINT: If there is a malfunction in the system, the waveform of the heated oxygen sensor, bank 1 sensor 2 (OXS), is almost the same as that of the heated oxygen sensor, bank 1, 2 sensor 1 (OXR, OXL), on the left.
There are some cases where, even though a malfunction exists, the MIL may either light up or not light up

 

I took the car to my mechanic to have him check the other two O2 sensors. He asked me to buy 'catalytic converter cleaner' and put it in the gas tank. That should clean the cat and the CEL should go away. Anybody have any prior experience with this cat cleaner? Does it work? Does it do any harm? I wouldn't mind trying it as long as it does not cause more damage.

 

This by far is the best thread on this forum for the P0135 ODBII code. My CEL just came on with this code. I'm going to change out the O2 sensor and see if it goes away.
Thanks for the VERY informative information!!! this forum rulez!

 

According to what I have read, if your car has more the 60k, it would be a good idea to change both bank 1 and bank 2 sensor 1. I personally replace mine after they hit 100k.
I would also recomend ohming the heater out before you start. 9 out of 10 times, it will be the sensor, but for that 1 time that it is not it is a good idea to check.
You did not mention the year, but if any of the hardware is rusted, and you have problems removing the sensor, you would be really pissed if it turns out the sensor is not bad. With that said, I realize that bank 1 is very hard to get to, I just replaced both my sensors (at 130k miles).

 

I have 143k on it. this is the only the second time the CEL has come on for me. I do like the idea of changing out both based on what you say.

 

143k, yes it is time. Now keep in mind that bank 1 sensor 1, and bank 2 sensor 1 are the ones that control fuel mixture. So you want to replace those. The senors #2 monitor the cat converter, and are nowhere near as important. If you would like to save a little money I would leave them alone till they cause a problem