It's happened about 6 times now in the past three days, every time it's when I'm doing /light/ braking, and every time it's downhill (not really a steep downhill either.) Can't replicate it when going uphill or on a fairly even street. The problem light turns on and I get a very quick beep and then, without any change in my braking, it goes away. Any help would be greatly appreciated =D
Check your oil level. If low, it can cause a warning when the oil shifts due to turning, braking downhill, or accelerating uphill. JeffD
I have a similar "problem." Recently in the last week or so, when I come to a stop sometimes or when I make a turn, the red triangle warning light comes on and "Problem" is displayed on my screen for only a second or two. However, I am not aware of any symptoms caused by a problem. My 2008 Prius has 174,500 miles on it.
I assume we are not talking about the case where the HV battery is full green, at which point regen breaking stops. Checking oil sounds good first step.
I checked the oil level, and it did not appear on the dipstick. I added one quart, and it now covers the twisted end of the dipstick. So, I added another quart. I've seen no evidence of smoking or leaking, so I'm not sure where the oil went unless the dealership didn't put enough in at my last service. I replace the oil every 5,000 miles.
The oil was consumed by the engine. It doesn't have to produce blue smoke to be burning some oil. It is OK to replace the engine oil/oil filter at 5K mile intervals. At what frequency do you check the oil, in between oil changes? If your answer is "never", that is why you are now having a problem with excessive engine oil consumption. 174K miles is a lot of miles and the Prius engine has developed some wear by then, which allows increased oil consumption. If you allow the oil level to drop to the point where a warning light indicating low oil pressure is registered, that causes accelerated wear to the engine. If you would like to log substantially more miles on your Prius, it would be a good idea to start that practice, replenish oil as necessary, and hope that permanent damage was not caused to the engine by allowing the engine oil to get ~2 quarts low.
Add more oil until the level is at (or just below) the top (full) mark on the dip stick. There is precious little oil in the sump as it is (when it is full) in these engines and it needs all that oil to ensure it can lubricate the engine well and also so that the oil does not break down due to there being insufficient quantity in the sump. As others have said, check the oil regularly (every 1-2000 mi) and top up when it is 0.5 qt down (~1/4 the way between top and bottom marks).
Another old Gen2 killed by dealer oil changes. Your motor is done dude. Start saving up. Drive it till it explodes but try to catch it on video.
It appears to me that the issue is the owner (of a Prius which had logged 174K miles) apparently did not check the engine oil level at reasonable intervals in between oil changes. 1K miles would be a reasonable interval for a car with that odometer reading.