(New) Code 21 and (less new) bucking/surging at idle while stopped (related?)

[rallycar_jepsen]

Hi TVT! It's me with the 91 Previa with the engine I pulled and refreshed down to the block while replacing the head gasket.
Now I'm 8,000 miles in the future. She's been treating me very well for the most part. Still so happy to have this van back on the road.

I have two situations to tap into the collective expertise about, one new and one ongoing, and they may or may not be related to one another.

Situation #1:

This afternoon driving home (~60 min cumulative drive, mostly highway and state routes, 50-70mph, nothing unusually taxing), about halfway through the trip, I stopped for gas and filled the tank about half full.
About five minutes afterward, I noticed that my check engine light had come on, solid. For about 15 seconds, and then off. After about two or three minutes, the same. On for about 15 seconds, then off for another few minutes.
It did this the rest of the drive home. Didn't seem to correlate with engine load or RPM or uphill or downhill or anything I could feel or hear.
This is the first time the CEL has been on for non-diagnostic reasons since the rebuild.

Got out the old paperclip and jumped my TE1 and E1. Reading error code 21, which according to Tim in this post from 2014 is related to the main o2 sensor, likely a bad o2 sensor itself.
Troublecodes.net confirms this - Code 21, Heated oxygen sensor (H02S)- front – circuit.

By the way, is this diagnostic table in my factory Toyota repair manual somewhere and I'm just not looking in the right part of it? Is it in the owner's manual?
(I'd like to know because in diagnostic mode my oil level light is also flashing once every four seconds and I should also figure out what that's trying to tell me, if anything.)

I didn't get down in this evening to do a visual check on my o2 sensor to make sure nothing's frayed or loose, that's tomorrow's job.
Is there anything telltale that I should look for while I'm down there? Does anyone have recommendations from experience for or against any particular replacement o2 sensors?



Situation #2:

This one's been puzzling me but it's been minor enough that I've let it be until the weather warmed up enough for me to want to mess with diagnosing it. For the past month or two, sometimes but not always, when I'm moving very slow or stopped while the engine is running (eg. at a stoplight), I'm experiencing a sort of "surging" or "bucking" type action where the van feels like it's wanting to move forward for a moment, in conjunction with what feels like a momentary hesitation (or spike?) in the normal idling pattern. When it's happening, it will happen once every few (3, 5, 10) seconds, until I accelerate again, at which point the irregularity either goes away or becomes imperceptible, I haven't figured out which. Occasionally there is a slight hesitation accelerating again, usually there's not.

At some point I noticed that when I'm in Park instead of Drive, even though I don't get the "surging" or "bucking" action to the same extent (I assume because the irregularity isn't being transferred down the driveline in Park), if I let the engine idle for long enough on a day when it's doing it, I can feel the same periodic intermittent hesitations (or spikes) in Park. It doesn't stall or feel like it's at risk of stalling. I wish my Previa had come with a tachometer, it would be great to know what the RPMs are doing more precisely when this is happening.

I checked out the exhaust once I knew I could hear/feel it in Park. Each hesitation produces a sputter and a visible vibration from the exhaust. That's probably not surprising or illuminating. No smoke of any kind.

At some point I noticed that when I'm in Reverse, on a day when it's doing it, I can feel the hesitation happening in Reverse as well. Always intermittent, just one spike / moment of hesitation every handful of seconds.


So, that's the symptoms of my Situation #2. I ruled out a clogged air filter by getting a new air filter. Not air filter.
I ruled out spark plugs by putting in the new set I already had waiting to go in. Not spark plugs. Tested spark to each cylinder, each cylinder is getting spark.
It "feels" a little bit like an intermittent misfire feels. I haven't ruled out injectors, but the set of injectors in there are only ~8 months old. Not that new parts can't fail.
I did keep my original injectors around in case I decide to send them off to be professionally cleaned and tested. I haven't had the current ones out to give them a once-over since I installed them.

I also have not yet done a compression test. I'm not ruling out that a cylinder might be misfiring very intermittently, and I'll wind up doing a compression test as part of my spring tune-up checklist anyway, but it just seems to me like the problem would be showing up a lot more regularly if I had a bad cylinder, and it also feels like I'd feel it more than just when I'm stopped or going backward. Perhaps not.

My amateur assumption has been that it's airflow or exhaust related. I did a cursory inspection of the intake boot that connects to the throttle body and there is one very small beginning of a tear on the edge that meets the throttle body, but it doesn't look like it even makes it as far as the hose clamp, so I don't think it's actually in the sealed part. But that might be a sign that the material could be getting brittle in other, harder to see places too. This is the original 32-year-old boot. I picked up a replacement boot anyway because I'm sure that little edge tear will find its way past the clamp someday if I let it. And you never know when what parts are going to get harder to find. I haven't installed the new boot yet, pending further inspection of the current one.

Then I'm thinking, judging by the kind of situations the airflow is in when the hesitation is noticeable (very slow, stopped, reverse), maybe the airflow is not being regulated correctly? I don't know much about the intake/air control system in a practical sense. Idle Air Control Valve? Mass Airflow sensor? This is the part where my lack of experience catches up with me. Could it be an exhaust system issue and have something to do with the failing or failed o2 sensor?

Does this situation sound familiar to anyone? Does the description imply anything about potential causes to someone who's better attuned to troubleshooting these engines than me?

Would love second opinions or thoughts here. Much appreciation for reading, best wishes.