tanderson22

I've got a degree in Automotive technology; Thats not how it works..... sorry. The post cat sensor does not monitor oxygen consumption; nothing in the emission system does this. The cat converter consumes nothing. The post cat signal should not fluctuate because the catlytic converter is not creating any fluctuation/ increase in voltage due to a steady, clean emission gas that is stochiometric, clean, or balanced (fully burnt). (Lambda) Steady voltage tells the computer the catalyst is efficent and keeps the monitor out. If it is not doing this, the cat is not being efficient. The exhaust mixture from the engine must switch steadlily from rich to lean (what the upstream o2 controls), which is the minimal requirement of catalyzation, and if this does not occur, the catalyst does not work. Too much O2 and it overheats and the monolith melts down to a slug and when it cools; you stall. Too much CO and HC and you loose the catalytic action if it goes on too long because the platinum and palladium are saturated with these chemicals and have no o2 to combust them to CO2 and H2O with. Sometimes a lot of 02 will revive this type of catalyst. Catalytic converters do not wear out unless they are poluted. Catalyst means it aids in the chemical reaction, but does not take part in. There are no moving parts, and platinum group metals do not wear out. Simply. if the Post-cat sensor sees anything but a steady output, the cat is not efficent and the PO420 or P0430 code will be set, because that is the parameters per OBD specs for the trouble code to occur. IF the voltage fropm the post cat sens were to fluctuate, it would indicate either a rich or lean condition from the converter, which would be "pollution" so the MIL lights up. Differences in reading with the upstream o2 has nothing to do with it; You would not get any codes from any non-emission monitored parts failure, and most P0420 related causes are not monitored. The #1 misunderstanding of CEL diagnostics is to believe it has anything to do with diagnosis or helping a tech out. It has everything to do with Emissions and federal laws passed. It is the evil work of politicians. You are correct in one aspect; it is a mixture probelm. Understanding what the output voltage of that post o2 sensor is; and why that value would occur; is the first step to the solution. From this point you can pay me shop rate for the rest these are trade secrets and the fruits of experience which are seldom shared.

*** SOME XT6 covers don't have the lower plug to access this. Use the vise to hold it while you work on it.:headbang:and listening to AC/DC helps too.

- ALMOST right..... a rear O2 sensor looks for stochiometricy from the converter - indicating a complete combustion/catlization, and it looks for 14.7 or about 0.5 volts in a short band (0-1.0v) sensor and DOES NOT FLUCTUATE when the cat is working. Putting the O2 sensor in a sleeve or non-fouler can mimic this condition; however; you have a mixture problem somewhere else, and you will pay for it either in fuel or damage to the motor. You compound your problem by "moding' your rear o2 because the computer will always have erronious fuel trim information because it "thinks" the cat / mixture is fine. Like every computer, garbage in, garbage out, and you will see the results when you have to rebuild your motor, or why you can't seem to get it to run "just" right anymore....because the fuel trim matrix is packed with erronious information because you fooled the computer, and the correct mixture is never output. You have to find the cause of the mixture inefficency, fix it, and then you will have a real repair.

- Stupid Idea. Does nothing. just putting a "shield" over the o2 sensor so it's not in the flow of the exhaust. Poor idea built from misconceptions of how an emission system works.

What are you smoking????? Spark Temperature??? OHM resistance checks are a waste of time in most any repair scenario. The whole suppression ignition system is based on high voltage discharge through high resistance... The simple test there would be.... is it misfiring??? Yes, or NO??? And, no a new cat will not fix any P0420 problem unless the problem already took that cat out, and the new one will soon follow suit, because only the problem and not the cause was fixed.

When you get a P0420 code, it is because the cat is not getting the correct fluctuation in exhaust gasses (lean to rich & back) to work. It is a mixture problem and no matter how you fool the computer or how you get the light out, there is a problem that needs to be corrected. You can replace all the 02 sensors you want, most likely the o2 sensor is working and picking up on a problem, like 95% of the time a code for an o2 sensor is. If you think you can attack this problem without A) knowing the 5 critical inputs understanding the OBDII / IM240 programs, your in for a ride. The only thing I can say for sure is I have an extensive emissions background and I didn't read a post here that sounds like anyone has the foggiest......

I just put the tenioner in a vise, cut a length of trusty old mechanics wire, retract the tensioner with a screwdriver, and then secure it in the retracted position with the wire around the tensioner body. You have to judge where to run the wire so you can bolt it down to the motor. Cut the wire with dykes when it's in position with the belt on. It's not exactly easy, but I had to figure out how to get it on in a jam one day, and after I figured out the position, I haven't used any other method. It just works quicker than the mail.