P3190, P0A0F, and a few days later, P0301 pending

One of the most common causes of engine hesitation and misfire is the ignition system. Could be worn out spark plugs, a weak ignition coil, or sometimes on older Gen2 Prius, water can leak onto the engine and get into the spark plug bore for cylinder #1.

Usually I would replace all the spark plugs and at least the coil on the problem cylinder as a first step to "baseline" the engine. If the budget allows I do all the coils.

Only use NGK or Denso plugs (and Denso or Toyota coils) from a reputable source. That would be dealer, a big brick and mortar store, Rockauto, etc. Counterfeits -esp online- are a constant concern.

Another common fault is a dirty MAF sensor. Use the search function to read up on removing and cleaning this.

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There are three possible causes for a misfire. Electrical/spark (spark plugs, coils, etc like stated in post #3. Fuel (fuel pump problem or dirty/bad injectors, etc). Air (dirty air filter, MAF, vacuum leak, etc).

You seem to be competent working on cars so I would start with the spark plugs and coils. How many miles on the car? When were the spark plugs last replaced?

I live in York and own a 2011 (gen 3) Prius. I have no experience working on a gen 2, but if you need help I am available for the next few days.

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Call your coils up that just involves pulling a cover up and undoing some 10 mm and see if you see rust colored vapor all over the coils. If you do you need to pull them out have a look at why that vapor is there and especially if it's in the number two you'll need to pull that plug and see if you see any mild misty water drops and then we're pretty much done here I think we'll see.

 

Drive the car hitting the plastic cover will not hurt the plug hitting the pavement on its side pretty much won't hurt the plug unless it's from 5 ft or higher generally but if you don't see any cracks in the insulation and it looks as new let it rip you think there's something wrong change 3 and 4 and see if it follows I doubt it

 

Just because the coils look gray that means you don't have any vapor and rust forming in their butt they are old I didn't follow your thing so I don't know if you've changed them or not if you have it and you're at 200,000 mi it's a good time especially if you've got a lot of missing going on.

 

Might as well drive it from a cold start and see how it does. The ceramic insulator on number 1 plug didn't "burn away", it cracked and part broke off (& went out the exhaust port).

The question is WHY? The leading possibilities are detonation (too lean? too much oil consumption?) or possibly thermal shock (coolant leak?) Cross your fingers and hope it doesn't do it again.

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Another possibility is the plugs were counterfeits. It is hard to tell from the picture whether the iridium cap is still there on the ground electrode.

The center electrode should not be that worn after only 40,000 miles if it were a genuine iridium-tipped electrode.

I'd make sure to get genuine Denso or NGK plugs and replace them all.

You also need to entertain the idea that the plugs have damaged one or more of the coils. In any case, the coils don't last forever. If you do decide that you will replace any of the coils, use only genuine OEM (or Denso) coils if you can afford them – they will work out cheaper in the long run.

 

Who was the manufacturer of the old and new spark plugs installed?

I am glad that you easily found the reason for the misfire.

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If you didn't replace those ignition coils during the rebuild; I'd budget for a new set and replace them sometime this year. You've got 300K+ miles out of them and in my experience they don't just die, they just misfire a lot. If you have to make a left turn, into on-coming traffic - this would be an inopportune time for it to start misfiring!!!!!
I had a bad coil on my GMC truck but I had 5 additional cylinders to rely on. That bad coil lasted 18 month, before it finally died and I was able to track it down. I didn't get a CEL until it died, only a pending P0300 - which was no help. That was around 243K miles. Six months later, the same problem started again - that's when I went with a new set of iridium plugs and coils. OEM only calls for platinum plugs; but I take this thing into the back woods; so I wanted to make sure I didn't have to walk 15 miles out to get a cell signal. It's been two years, no more issues or intermittent misfires...

Hope this helps....