Help!!! 2004 got codes: P0505, P0790, P0993. Scary!

The continuous high pitch squeal is actually a desperate cry from your car that you need to stop driving it and get help as it has a serious problem.

Your code reader is probably only capable of reading general engine codes and is not reading any of the specific system ECUs in the car. There should be codes set by the ABS ECU that you're not reading. The codes you detailed are not really meaningful.

First priority is to get all the codes read, and then it will be more clear where to go from there.

 

12 years and a quarter million miles, you may want to consider looking for another car.

 

P0505 - IAC (Idle Air Control) System Malfunction
P0790 - Transmission mode selection switch -circuit malfunction
P0993 - Transmissionn fluid pressure (TFP) sensor / switch F circuit range/performance

If you hadn't said your car was making a sound I would say check the 12v battery. In this case two of the codes point to the transmission. These are generic codes. A factory scan tool will probably give you more information. You could take it to the dealer for a diagnosis. While there you could even stroll over to the show room.

 

1. You need Mini VCI to read the codes.
2. It appears your car has several issues: brake system, throttle body, perhaps a transaxle issue.
3. The repair cost is likely to exceed $2K so tow the car to your local salvage yard and walk away.

 

That's scary!

 

The car probably needs a new brake actuator pump. The Prius does not rely upon engine vacuum in the braking system. I suggest you need a better-trained mechanic to service your car.