P0301. Replaced coil & spark plug, now it's returned.

Just hoping I can get some theories on the most probable causes here.

My Check-Engine light came on with the P0301 code.

I got a new spark plug and ignition coil, and had them both replaced. For about a month and a full 1000 miles, the light stayed off. Also, it ran perfectly--not a trace of shaking or anything.

Now suddenly the light is back on again; exact same code: P0301. What is the most probable cause here? I know that paying someone just to look at it is the only way to know for sure, but mechanics have given the runaround when I've asked. They say things like "I guess it's not the spark plug/ignition coil," but they're glossing over what I said above ("For about a month and a full 1000 miles, the light stayed off. Also, it ran perfectly--not a trace of shaking or anything") -- if it had nothing to do with those parts, why would replacing them have solved it temporarily?

I know sometimes generic brands aren't the best, and I'm planning to replace them again with higher-quality parts. But I don't want to do this and have it revert back in another month.

Any theories? Thanks for any help.

 

You're right, I posted in the wrong section. The labeling has confused me nonstop from the beginning. It's a regular 2013 Prius...the grade is Four. Is that the same as 4th generation?

 

Just over 190K miles.

The plug and coil were purchased at a well-known and publicly trusted discount auto parts shop, in person. Of course Toyota would recommend their specific factory-made parts, but that's also to put more money in their pockets.

Don't have an up-to-date answer on the coolant level, but I will provide this ASAP. I know it has never overheated...but conversely, the air has pretty much stopped blowing, so this could be related.

I will make note of the procedure suggested:

• Check compression,
• Check fuel mixture,
• Check spark.

Is there somewhere I could find out the basics...or could you add a few more words to these phrases so someone as mechanically ignorant as myself can comprehend the basics of what this means (ie. what aspects of these things need to be checked)?

If it's a head gasket, I'm dead and buried. That's what happened to the 2000 Civic I had before this, and I had to junk it. But this one still has payments to go, and I couldn't afford the head gasket repair--yet the vehicle is a necessity for my income (UberEats, GrubHub, etc.)

Thanks everyone for all the help so far.

 

UPDATE: I topped off the coolant, but it was never very low in the engine unit or the power control unit--closer to "full" than "low."

 

Well...maybe that's some hope that this isn't a head gasket failure...

Same thing happened to my Civic when the head gasket blew. Had to keep pouring coolant after every trip.

 

That reminds me...my camera now only works about half the time. That applies to the screen in general, whether showing the reversal view or the selection screen. Sometimes it's fuzzy and invisible, sometimes normal. That's a different issue though--on top of the air not blowing anymore.