P0420 diagnostics on Prius Classic

All,

there are a bunch of threads about the Check Engine Light (CEL) and P0420 fault already in existence, but I would like to offer one on diagnosis of not only P0420 but other emissions-related faults which may occur... and possibly offer some hope.

The popular level of knowledge and response when a P0420 shows up seems to be:
--change the cat converter and/or the O2 sensors
--Less popular solutions are to find the pinhole leak in the exhaust system between the O2 sensors,
--tune the car (replace plugs and recheck valve gap)
--clean the MAF sensor.
--change fuel types (go to no-ethanol regular or premium)...I have seen suggested to change gasoline types, as an excessive ethanol level in gas is suggested as confusing to an emissions control system designed in pre-ethanol days.

I have also fixed a couple of P0440 (evaporative sytem fault) issues where the fuel fill port lip becomes corroded/rusted, and rust accumulated on the rubber of the fill cap, preventing a good vacuum from being pulled. Cleaning the fill port and cleaning and restoring the rubber on the fill cap solved those problems, but only after getting real dirty under the car looking at the rest of the plumbing of the evap system and deciding that it showed no obvious defect.

In common: these are all guesses based on symptoms.

It would be better to have a more direct indicator of the problem which set the CEL.

Much has been published on use of ScanGauge instrumentation to help out with traction battery and other Gen I hybrid-specific issues, as well.

Now, I have recently learned a tiny bit about an OBD standard "Mode 06" which, for many high-end OBDII tools, gets at the raw data of the on-board tests for cat efficiency, evaporative control, and so forth... the very problems which set the CEL but have no directly-testable single point of failure.

Has anyone on this thread had experience working with OBDII Mode 06, particularly on Gen I Prius, and can point to a good learning module? Or have expereince where Mode 06 (or other OBDII diagnostic modes) don't conform with how the Gen I behaves?

I intend to search, but I thought to ask first.

<edit #1> early learning results from Wikipedia:
OBD-II PIDs - Wikipedia, the free encyclopedia
not sure how much of the entire standard is implemented in the Gen I's...

 

additional information: in perusing a variety of other boards, there seems to be a number of automotive brands in which the "senior citizen" vehicles (more than 6 years old / more than 80k miles) begin to show P0420 codes at the onset of winter. Since it is about this time of year that the refiners switch to "winter blend" fuels, with higher volatiles content to reduce incomplete combustion (reduce pollution), it would make sense that vehicles with reduced operating latitude (older catalytic converters/sensors/etc) MIGHT trigger the P0420 code, which in itself is indicative of a "system" failure as opposed to a "component" failure.

So, the "low cost" approach to a P0420 code (which may depend on how much time you have until the next mandatory emissions vehicle inspection) is to run the tank down to nearly empty, then fill with Techron and high octane, run to empty, repeat, clear the fault code, then run with whatever fuel you can afford for a couple hundred miles and see if the code comes back.

Next, if not already done, a tune up is not a bid idea: replace the air filter, clean the air intake with lots of carb cleaner, change the plugs (as appropriate, if needed)... about $50 and about an hour of driveway time, all told, to restore some system performance latitude.

Alternately, invest in an OBD II scanner which will graphically display both the upstream and downstream O2 sensor voltages. In a properly operating system, the upstream sensor switches voltage frequently (1x/second); the downstream sensor is approximately constant, altho it does twitch occasionally. I did the graphic scanner route (about $100), and it showed me that in one case, the cat DID need replacement; in another case (early November in the northeast), the cat was just fine; no lazy sensors, no temps or other indicators out of wack, so the decision was to clear the code, flush the fuel with Techron and high test for a bit, and drive on.

Nothing like genuine data when faced with a problem like P0420, unfortunately...