P0012 error code issues

I need some more suggestions please.

Had to change the Head gasket out on my 2012 prius(400000+miles) and since then have had the p0012 error code. Have changed the oil etc what seems like a billion times, sometimes with cleaners too. Have replaced all the potentially affected sensors out. Have replaced the timing chain(which was slightly stretched) and the vvt gears etc. Just had to replace out coils too as was getting misfire codes but that was just a recent thing.

Engine runs "ok" and when I clear the dtc codes for the first drive it doesn't come back on but it always does on the second drive!

Also most likely unrelated passenger airbag remains illuminated "off" even with passenger needing one sitting in seat?

Any suggestions please