Gen3 recurring P0302 when going uphill

Hi guys, I got this 2011 prius with 200k on it in early March. I love it. The first thing I did was EGR air intake and throttle body cleaning, replace spark plugs and PCV valve. It was fine for a few months, then started rattling real bad upon startup and finally turned on CEL, I stopped driving it and then one day decided to take to mechanic because I was concerned it may be the head gasket, mechanic checked the oil and said no, on the way to drive home the car died a block and a half from mechanic (just shut off entirely, engine wouldn’t turn on, all dash lights on.) I had it towed into the dealership (mechanic I took it to wouldn’t touch it because of the hybrid system warning light that came on along with every other light) who gave it a diagnosis of bad gas, drained the gas tank, cleaned the fuel system and replaced spark plugs. I asked about the head gasket, the dealership assured me that was not the problem.


All was good for a few more months. Then, going up a large hill one day, the car began misfiring and flashing CEL at me. I drove it around for a while, this didn’t happen again, at one point my overheat light came on tho and I immediately pulled over and let it sit, the coolant was a bit low so I refilled it then drove home. After this, I let it sit for 2 weeks while I drove my other car on a trip, came back and tried to drive the Prius again, the CEL and misfires happened yet again while going uphill. I acquired a Bluetooth code reader which showed me P0302 and a water pump code, I replaced the water pump a week and a half ago, cleared codes, it was driving perfectly and had a lot more power than before. Well, it was fine for only 2 weeks before I started it the other night, the engine was misfiring really badly (I could hear/feel it skipping a beat) and again when going uphill, the check engine light was flashing. Previously, the misfires only occurred when going uphill, now they are constant but at their worst when going uphill.

Today I replaced the coil on Cyl 2, that did not fix the issue. The spark plugs are new (couple months old) from when the dealership replaced them due to “bad gas”. The only code I am getting is p0302; no other codes besides that after new water pump was put in. It is not rattling on startup like many I have seen that have head gasket issues (no “Prius death rattle”), but the engine has a very severe misfire that makes it basically not drivable at the moment. No coolant loss since new water pump. It seems unlikely to be a bad head gasket as the problem presents most often when the engine is hot, which does not point to coolant seepage into the chambers overnight. I am at a loss as to what to do next- some people seem to have fixed similar issues with a new MAP sensor? Does anyone have advice as for how to go about diagnosing and fixing this issue? Thank you.