Re: P0420 code and PA state inspection

Bad advice. One should always CHECK the component identified by the diagnostic system. In the case of P0420, based on a signal from the sensor, the ECM determined that the cat is bad. The system cannot really give a false "cat is bad" code.

Think of the signal from the O2 sensor as an EKG. #1 sensor sniffs oxygen, sends a signal to the ECM to adjust injector duration, takes another sniff, and adjusts some more. When it is in closed loop mode, it is continually varying voltage.

Then, if the cat is working, it cleans the nasties out of the exhaust stream, and then sensor #2 takes sniffs. If the exhaust is clean, sensor #2 sends a flat line signal because sensor #1 has already done the work of adjusting the air/fuel ratio. If the cat isn't working, then the signal from sensor 2 would be the same as the signal from #1 - in other words, it is working.

If you think about the logic, it is theoretically to get a false 'cat is good' if sensor 2 were bad but because it takes an active sensor 2 to give a cat is bad signal, it has to be good.

I am not a fan of aftermarket O2 sensors because many of them require snipping and splicing wires, and that splice makes a big weak point in the circuit.

Again, there is a different code for a bad after-cat sensor.

Fortunately, the exhause shop allows me to purchase Toyota O2 sensors and installs them for me for a $100. I have ordered O2 sensors online(waiting for shipment now) and each costs me $100 which is much cheaper compared with the one offered by the shop($140 each). I tried very hard to rremove psot-O2 sensor, but I could not and it stucks and I don't have decent equipment to raise the car high enough I guess to apply full-force.

Agreed, but there is no check for the CAT. The only thing to do is check the sensor, or simply replace it and cross one's fingers that everything else is okay, and the sensor is simply not working. sure, one could test the resistance, but any test done at home will be a static test -- the conditions are constant, not changing -- and the error is related to a dynamic set of parameters -- it is looking for changing conditions. I can't think of a dynamic test that can be done at home, although I readily agree that theyr COULD be a dynamic test that may be accomplished in the shop -- although I have no knowledge that such a test actually exists.

Splitting hairs, Ray. P0420 _IS_ the after-CAT sensor. It is reporting that what comes out of the CAT is not what the system is looking for, by definition making it the After CAT Sensor. Anything reported in this range of codes is saying that either the CAT itself is bad or that the sensor's internal heating element is not working. Since we do not have a heatrer element error code, then the CAT is bad OR the sensor itself is bad. My vote is on the sensor.

There is a check for the cat. Use an exhaust gas analyzer to probe the exhaust stream before the cat and then after the cat, compare the readings.

In the case of DTC P0420, the error is related to a dynamic set of parameters, but not in the way you are suggesting. If the cat is working, the signal from sensor #2 should be a flat line - static. If the signal from sensor #2 is dynamic and fluctuating, then the cat is bad. If the signal from sensor #2 is fluctuating, then it is working.

A good code scanner can read and record the signal from individual sensors while the vehicle is operating. You could also do the same with a recording volt ohm meter, or have someone drive while you read the meter.

Not splitting hairs. There is a whole different set of DTC's for O2 sensors. P0420 comes up when there is a signal where there should not be a signal. I recommend that you go to vehicle computer controls class for a more thorough understanding ;-)

There is no check that can be done at home.

The exhaust stream before the cat is checked by the O2 Sensor(s), and the after-CAT exhaust stream is checked by the sensors that throw P0420 and P0430 if the car has dual exhaust.

Exactly, the codes for the O2 sensor(s) are in the P0100 family. The P0420 specifically says that the exhaust stream coming out of the CAT is out of spec.

If you cannot do a check at home, then IMHO, the smart move would be to bring the vehicle to a place that has equipment and trained personnel to do the check for you. Recommending that someone blindly replace parts without checking the part could very easily waste that person's money.

Then why are you recommending that the OP replace the after cat sensor without at the very least checking the sensor to see if it is good or bad?

Because he has limited testing capability, as I have, and the sensor lives in a VERY hostile environment that can cause a failure mode over extended periods of time.

The O2 Sensors report out of spec conditions bafore the CAT and report these errors with P01xx numbers (P0120 and P0130, if my memory serves me) and the P0420 code reports out of spec conditions after the CAT. Since the CAT typically costs several hundred dollars and is next to impossible to test at home, and the sensors typically cost about one hundred dollars (I've not paid more that $50 for an O2 Sensor, and never bought the after-CAT sensor, but it's configuration is similar therefore its cost should be in a similar ballpark), then I would not hesitate to consider replacing sensor that I can do in the driveway, and leave the expensive stuff for later.

To the Original Poster, Replace the after-CAT sensor before you waste another second on this problem. No matter what Ray suggests. DO NOT replace either of the O2 Sensors, yet.