Request for diagnostic advice to pinpoint least expensive solution to resolve persistent P0421 OBDII DTC

On 18 Nov 2020 13:17:51 -0000, Scott Dorsey wrote:

> What you want to know is in the graphs for the
> two O2 sensors and how they track.

Hi Scott,

Thanks for that gentle reminder, as I will borrow the vehicle today, but I
have to check it in their garage, as I only get one chance a day to test
warmup conditions (I think, at least, at Silicon Valley temperatures).

> But you will not figure the problem out just by looking at the plots, you will
> figure the problem out by thinking about how the emission control system works.

Well, to tell it to you bluntly, I'm not omnipotent.
o I didn't know anything when I first posted this thread

I didn't even know the _range_, which I've since learned is basically about
~100 mV for lean conditions, to about ~900mv for rich conditions.

Nor did I know when I opened this thread, what the expected differences
were for the upstream versus downstream lambda sensors, where the front
should be changing far more frequently than the rear (which should read, as
much as possible, a stoichiometric ~450mv under steady warmed load).

What I didn't know at first was why voltages bounced all over the place.

But now I've boned up, a tiny bit, on the fact that the bouncing around of
the voltages is "normal", at least for the upstream side, since the engine
computer is constantly adjusting the ratio (within seconds) depending on a
whole bunch of factors, not the least of which is engine loading.

Further honing what I didn't realize at first is that there are two states:
1. Steady state (which is where most of my prior measurements lay), &
2. Warmup state (which is where I need to focus, as you had advised me).

What I need is what most sites don't provide, which is I need to learn more
about this warmup process of the cat, since it's a 'warmup efficiency' DTC.

I only get about a minute (or two at most), right?

>>When do I take the readings once the engine is warmed up?
>
> All the time. You take them before it warms up, you take them after it warms
> up, you take them over the course of driving around. What you are looking for
> is trends and correlation more than specific values.

Yes. But.

I belatedly realized I need to focus specifically on 'warmup efficiency'.
o My main focus now revolves around how to diagnose 'warmup efficiency'.

Obviously I need to figure out what 'warmup' even means, which I need to do
as my homework, since I'm not even sure how long warmup of the cat lasts
(the sensors only take ~20 seconds to warm up, but I'm not sure how long it
takes the cat to warm up, nor, if a cat can even warm up twice in the same
day given Silicon Valley typical warm nightly & daily temperature ranges).

I'm gonna borrow the vehicle today but it's wet outside so I'm not sure if
I'm gonna crawl underneath to check the rear lambda sensor; but I will at
least check the live OBD data during the first 20 seconds of the lambda
sensor warmup and then the first few minutes, where I'm guessing a cat
warms up in, oh, I don't know, within the first five minutes perhaps?
o I'll look at an engine temperature reading (coolant perhaps?)
o Then I'll look at the front & rear sensors (and what they do over time)

I may need to buy an infrared thermometer to check how the cat warms up in
the first five minutes with the vehicle on jack stands; but no sense in
doing that until I learn more about how the cat _should_ warm up, as the
numbers won't mean anything if they're measured under the wrong sequence.

>>I don't have the necessary equipment to save or see graphs.
>>o I can only see the live data as it happens.
>
> Well, that's useless. So you're going to have to plot it on semilog paper
> by hand. Your scanner does not provide ANY plotting functions?

My OBD scanner _does_ provide plotting functions; but it's second hand (I
picked it up at a garage sale for a few bucks). It works fine for live
data, but I don't have the cabling or software to view the graphs.
o Harbor Freight item #60693 Cen-Tech OBDII/EOBD Deluxe Scanner

It also does 'freeze frame' but I haven't even started to look at what that
means to me, in terms of how I can use that to diagnose warmup efficiency.

In summary, I started out with the wrong assumptions, and I ran the wrong
tests, and I didn't know how the system worked... but I'm reducing my
ignorance by reading up on how the system works, and I'm gonna focus on the
all-important warmup process, where I think I only get one chance a day.
--
Experience is knowing which measurements to take, when, where, & why.