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What do you mean by ES is flashing? Did you get the wires all repaired correctly? are there any other codes besides the 420? Did you ever have any codes for the sensors that you replaced?
As far as codes though you are only getting P0420? When you monitor the voltage of the rear or post converter sensor what is the voltage doing? Does it oscillate from .1 to .9 volts fairly frequently?
The only way the 420 code can set is if the converter has deteriorated and the rear O2 sensor will show this because it will be fluctuating rapidly ie .1 to .9 volts 5 or 6 times a minute similar to an older upstream O2 sensor. If it is then the converter is no good. The converters are notorious to go bad on this van. If you do not need to have an emissions test done I would not worry about the code. The converters are very expensive and the aftermarket ons do not last.
Do you have the engine computer number or have you compared it to the listing in the bulletin?
Most of these were done under warranty along time ago. If you have the VIN I might be able to check it.
What wires exactly were chewed?
Spark plug wires or electrical?
What sensors did you replace?
Do you have VSC and TRAC and are these lights coming on you said?
Still need to know what the rear O2 voltage is doing. The logic in the ECU is not going to affect the voltae out put of the O2.
I will check the vehicle identification number on Monday. If you can get me the number that is on the side of the computer that would be helpful also. It should start with 89661 then five other numbers. Let me know on Monday what you find on the O2 voltage swings and I will get back to you. Ideally if you have an oscilloscope that is the best way to monitor the voltage if you see a sine wave pattern developing rather quickly this means the converter is deteriorated and is not doing its job. Have a great weekend.
I am not familiar with that specific scanner at all life data code readers and scanners are about the same. You need to monitor the O2 sensor that is after the catalytic converter this should be listed as sensor two. This voltage should not be swinging from .1 to .9 V rapidly. It should have steady rises and steady decreases. If the voltage swings go from .1 to .9 V several times within a minute this means the deteriorated catalytic converter is not doing its job. They did change the logic in the engine control computer to help correct for this but if the converter is bad the computer cannot override it. Were you able to get the engine computer number? Is this the original converter? If this is the original converter you're probably going to have to replace it. If you purchase an aftermarket converter keep in mind that it will not last as long as the original factory installed equipment. It is cheaper for reason.
Normally aftermarket converters are only good for one or two years. If you're not planning on keeping the van longer than that it shouldn't be a problem. If you are see if you can find a converter that offers a lifetime or five year warranty.
Based on the information above if O2 S12 which is the rear O2 sensor or post catalytic converter sensor is fluctuating from .150 to .685 more than 10 times a minute the catalytic converter is bad. Your other sensors that are averaging between 3.1 and 3.3 V are the AFR sensors ideally to run 14.7 to 1 air fuel ratio these voltages should be right around 3.30. They will fluctuate up and down by .1 or .2 V on acceleration and deceleration this is normal but when the engine is idling they need to be right about 3.3 V. your computer is also not up to date as per the latest technical service bulletin. The technical service bulletin is EG047-05 according to that service bulletin the new updated number should be 89661-08081. Both the catalytic converter and the engine control module are only warranted for eight years or 80,000 miles. If your goal is to keep his vehicle long-term aftermarket converters are usually only good for a couple of years they do not last because they are cheap for a reason.
You need to start by replacing the catalytic converter. The problem is what comes first the chicken or the egg? These converters were notorious to go bad but no one has really determined whether or not it is the fuel calculations in the computer that prematurely ruined the converter or did Toyota change the logic in the computer just to keep the p0420 code from setting prematurely? That information will never be known. The computer front is going to be very expensive probably 800 or thousand dollars. There is a chance you might find the updated number in a salvage yard but that would be a long shot.
You are welcome. If you need any further assistance or any help in the future just ask for me Skyvisions, in the question and I will get back to you.