2008 Prius – Low Power on Highway & Uphill, Loud Engine, P1121 Code, Throttle Already Replaced

how many miles/km on her?

 

Are you beyond the weight limit of the car while your going up hill? The weight limit is in the drivers door jam. The car really doesn't carry much weight - 4 adults and it's near or over capacity. Stomping harder on the gas pedal going uphill isn't going to make the car go any faster uphill - the eCVT transaxle is trying to determine the correct gearing. All you'll accomplish is blowing up the engine by over-reeving it.
Something could be choking-off your engine and ECU is probably engaging safeties. Check the exhaust pipe for clogs, the CAT internal honeycombs can fall apart when a car is abused like that.

1. There probably isn't a throttle body issue, since it runs fine on the flats. Fix the flow control valve issue - may cause engine overheat.
2. Take a picture or write down the error codes BEFORE erasing them. Check cooling fan operations.
3. Install an engine coolant temperature gauge and ease off the engine when it hits 102C; normal operating temperature is 85C-100C; Higher temperature operations will eventually cause major engine damage.
4. Change the ATF in the transaxle, if that hasn't been done.
5. That 4-year old battery, was that a new OEM battery or rebuilt. If it's a rebuilt that could be your problem. Also make sure the battery cooling fans are working properly. I know it gets really hot in Jordan.
6. Check the MAP sensor on the air intake, that also provides barometric pressure readings so the ECU can adjust mixture based on the thinner air at altitude.

That's all I got, until you can get us error codes and mileage of the car.

 

It's often a disaster when people mix-n-match meanings for manufacturer-defined trouble codes like that.

All P1 codes are manufacturer-defined: different manufacturers can use the same code in the P1 range to mean completely different, unrelated things.

P1121 in a gen 2 Toyota Prius means coolant flow control valve position sensor circuit stuck, period, end of sentence. The code has nothing to do with any throttle anything.

Somewhere else in the world, there is some other model of car where P1121 has something to do with a throttle/pedal position sensor. When somebody working on a Prius happens to see that in some generic listing of codes somewhere, that's a pure distraction at best, and that's when it doesn't lead to spending actual money doing stuff with the throttle or pedal.

I wish there were a way we could warn people about stuff like that more prominently.