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Mine is at the dealer now for P0301 and P0300; running rough and stalling out at low RPM. The first theory was the coil pack for cylinder 1. Replacing it didn't fix it. The spark plugs were replaced about a month ago. They checked a bunch of other things and couldn't find the cause. They kicked it up to Toyota and after a week got back a new theory; cylinder 1 exhaust valve spring is weak and/or damaged. They are about to start tearing down the engine to find out and maybe fix. So, yeah, it seems it can be really hard to figure out the cause. I'm not sure they've figured out mine yet.
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