Car knocking, changed spark plugs, problem persists

So I've had a knocking sound for the past several weeks. I replaced the spark plugs but that did not take care of the issue. Today, while knocking, the engine light came on so I had it plugged up to read the code and it read that the 2nd cylinder is misfiring. I'm considering replacing the ignition coil next (a $80-100 fix), but I've seen videos on YouTube of people just replacing the boot (a $10 fix) and that solving the problem. I can do the work myself, but what I am not sure about is how to go about finding the specific boot replacement. As far as I know, the ignition coils are original.

 

It's a 2012 and has 173,000 miles. I've not cleaned the EGR valve since I've owned it (I bought it at 154,000 miles in 2017). It does not have an oil catch can that I am aware of.

As for ignition coils, are there preferred brands? If looking at brands like Delphi, Denso or NGK they are about $90 a piece. However, on eBay I'm seeing them in the $40 range for complete sets, but this seems suspicious to me... I'd certainly love to save money, but I don't want to put junk in my car either.

 

So last night (30 degree weather), I was driving to town and had the heat on (it takes a lot longer to heat up the car than it use to - now it takes a good 15 minutes to feel any warmth, at least), the coolant temperature warning light came on. The coolant level is slightly lower than it was when I filled it to the full line about a week ago (though it was warmer when I filled it). It's still just above the low line (it's awfully difficult to see between the full and low line, due to the casing.. not sure why they did it like that). Perhaps I have multiple problems? I'm looking up videos on EGR cleaning.. will update everyone when I finish that. I'll also look into an oil catch can at some point. Will change the ignition coil if EGR cleaning doesn't work. Thanks

 

It is a 2012. I just checked the coolant levels and it has indeed dropped (about an inch and a half below low line) since I topped it off about a week ago (although it is night and I'm shining a light on it - visibility is low).

I put my brother's code reader on it the other day and it has P0300, P03001 and P03002, so apparently it is more random rather than localized (O'Reilly's, earlier, told me it was the 2nd cylinder). The coolant temperature light has not come on since, though it still takes a long time to heat. I've not cleaned the EGR yet as I am a full-time student + working, so I do not have much time to devote to this.

Not sure if it's worth noting but since November or December of 2018 I've been using Top Tier gas from Shell. The knocking issue began in late January. I wouldn't have thought that would be related, but it's still worth mentioning nonetheless.

 

I checked my oil to see if there was any mixture of coolant. From looking at the dipstick, it looks normal to me. I'll have to investigate further in the morning, to see where the leak is at.

 

Wouldn't the fact that the oil on the dipstick is not milky indicate that perhaps the leak is elsewhere? I wish I knew exactly when the coolant leaked. Sometime in the last week.

If it is -- how difficult is it to replace a Prius head gasket on your own? I can't pay 2-5k for repairs.

 

Update: I just ran a block test on the coolant system and the liquid did not turn yellow, so I am assuming then that the head gasket is okay? Seeing as how you can't continuously run the engine on a Prius, as the block test directions call for, I just put the heater on full blast and ran through 3 engine cycles and then performed the test for another 2/3 cycles. I guess now it's just a matter of finding where the coolant leak is precisely.

I'll clean the EGR valve and cooler and see if that resolves the issue.