Check Engine Light

Without diagnostic test codes (DTCs), we're simply guessing. A zero-cost option is to try getting a free scanner read at your local auto parts store--some will do that, call around. But those scanners do not read all Prius-specific codes--you get what you pay for. The dealer will charge about $100. Most DIYers here buy a MiniVCI for about $25 and use an old Win7 laptop for full capability.

Report back when you get something.

When this symptom showed up on my Prius, it was the three way coolant valve, a mostly non-essential item. If it were the inverter pump, you'd probably have noticed a harder failure by now. Many minor intermittent issues will reset themselves, but should not be ignored. A two- or three-digit cost will quickly become a four-digit cost.

 

Most posts we see here concerning an intermittent cel is because the engine has no oil in it. Its extremely low so when you go around a corner the tiny amount that it does have sloshes around and the oil pickup runs dry and the oil pressure which is already very low then goes to zero. It will then throw the check engine light. Usually resulting in permanent engine damage.

 

If I was guessing, I'd say bad catalytic converter

 

Yes, it certainly could be an issue. If you're not in the habit of checking the dipstick every couple of fill-ups, now's the time to start. It's not unusual for an aging Prius to begin consuming oil with no other symptoms, no other problems, and continue to work fine for years. Check that dipstick! Keep an eye on the coolant reservoirs, too.

 

And keep in mind many oil change places routinely over-fill the Prius. If you put in the rated capacity, the level is slightly high on the dipstick. So what's perfect now may mean you've already consumed several ounces. Again, keep an eye on it and make a habit of it. 5000 miles is a long way to go without checking the oil at this point in the car's life.

 

P1121 is the coolant control valve, not fuel air metering, and is a fairly common code, especially when coupled with low coolant. But I'm concerned about the confusion--something's wrong. Maybe the scanner or operator read it wrong. P1121 the code I referred to in post #2. You can live with this one, and the worst case is it'll affect cabin heat come winter. But the check engine light will probably come back soon, annoy you, and may mask another problem.

Keep a very close eye on coolant level. Check in the COLD radiator, not the reservoir, by removing the upper splash shield (six annoying plastic fasteners). If you can't see coolant in the radiator neck, you need to find the leak.