Possible airflow issue, trying to verify AI advice

2015 Prius, 350k miles. A few months ago, the hose for the engine air filter was improperly connected for about a month after a repair job. Car had been running great for a while, then one morning started running really rough, and threw off a check engine light, P0303, misfires on cylinder 3.

Took it to the dealership and got an oil change. Car was then running a lot more smoothly, and a kind of grinding or chittering sound I'd asked about a few weeks ago went away. (Think I'm going to switch to a 7,500 mile oil change schedule.) But they refused to give me any info on the engine code without paying a $99 diagnostic fee.

So I ran the car to Auto Zone and got the error checked, as I said above, two P0303 errors. Started checking with google AI to try and figure out likely problems. Explained how after oil change, things were much smoother. Was told that the oil change *might* have solved the problem, and if so, the check engine light would turn itself off after a couple of days. (It did.)

But it also said there was a good chance the airflow sensor might need cleaning, especially after that loose hose for a month, a little while back. It also said very common with the age of the car to develop heavy carbon build-up on the intake manifold or something like that. Said there was a three step process involved in cleaning, and that quite possibly, the first, easiest step might be enough to clear the problem. (Supposedly something about Prius engine design means it's most often the 3rd cylinder.)

Anyway, I was told the airflow sensor was easy enough to clean, and the four videos I have watched back that up. The air intake thingy is a more involved process, and is going to have to wait a month or so until my handyman heals from a hernia surgery, but I'm told that shouldn't be a problem, and that just the sensor cleaning might fix the problem.

For the sensor, I got the proper CNC or whatever it was airflow cleaner, and that ought to be a simple project for today.

Oh, yesterday, rough idle at startup, check engine light came back on, which is why I bought the cleaner for today.

Does all this sound fairly reasonable, and not completely insane?

 

I've had it since 40k miles, and for roughly 10 years. Tomorrow before I start driving, I'll check the coolant level.

The air filter hose was not properly rehooked up after replacing a water pump in March. (It was in position, but not tightened down.) Noticed it in May and tightened it down at that point.

With my next fill-up I'll add a bottle of Techron Plus (arriving tomorrow). After that, planning to start a 3k mile schedule with a half a bottle of Gumout Re-something during fill-ups to reduce future continued carbon build-ups. IF it is only carbon build-up and not failing gasket seal, am I probably safe waiting a month for my handyman's hernia to finish healing up before actually cleaning out the intake 'pipes' build-up?

If I'm showing coolant levels dipping, obviously it is much more serious and needs immediate intervention. Will report back after I get out to the car tomorrow morning.

 

OK, my car, and probably myself, are pretty much doomed. While the driver reserve is full, the passenger reserve is low, and I'm told that's the one that counts in this case. While I can try to say maybe Toyota didn't top it up while doing the oil change last week, I know they almost certainly did. So it looks like the gasket is going, and I'm going to have no way to get that taken care of. Damn!

[edit]OK, just talked to the dealership, and they assure me they would have filled it to the proper level, but that the proper level would NOT be to the full line. So can somebody look at the picture and tell me if it is at the proper level, or low?[/edit]

 

OK, AI says if there is a leak, currently it is a tiny one, because the car starts rough, then behaves for most of the day. As long as I don't run out of coolant, I should be fine for a shift of driving passengers, keeping an eye out of an overheating warning, just in case. Tomorrow I will check the coolant level again, and see where it is. I don't want to damage the car, but it seems I may have been overreacting, and it should still be driveable for the moment, while I look for evidence of whether it is leaking or not. If I'm wrong, for God's sake tell me quickly, before I head out to drive in about an hour.

[edit]Oh, and if Toyota says they wouldn't fill it to the full line, and when I had the water pump replaced in March, they filled it to the full line, is it possible they filled it too full, and that caused problems in the system somewhere? To me, that's the point of a full line...if you don't fill it to the full line, then having a full line seems very confusing.

And if it seems low, and I should fill it before monitoring the level, I could do so, as I still have a half-bottle of the coolant from when they changed the water pump.[/edit]