Replace camshaft position sensor ?

Check the wiring first. It isn't a common part to fail, conversely, rodents frequently chew up wires.

If you still need to replace it you could get a techstream subscription for a day, then find the instructions in the appropriate manual.

If I had to guess though, the air filter box would come out, and maybe that coolant hose, and you would then access it from that side. The repairpal cost estimate is only $240-$324, which would indicate not much of a job. (Except it doesn't say which Prius, and it might be much easier on a later model.)

 

The service manual says to remove the inverter assembly first, then the sensor.

What codes do you have? I haven't seen a cam sensor go bad. More often there is a problem with cam timing chain or Variable Valve Timing control.

Usually if the cam sensor is completely dead, then EVERY time the engine tries to start it will: 1)run "ok" with a check engine light, or 2) "try" to start (spin at 100 rpm for 10 seconds) then not run.


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and here is how to remove the air filter assembly

Toyota Prius: How to Remove the Air Filter Assembly - PriusDIY.com

I can't tell from those instruction sets if or how either gives you access to the part you want to reach. For the air filter assembly there is still at least one hose in the way, but maybe with the inverter out the sensor can be reached from above with a low profile ratched/socket, or maybe use a wrench to loosen it? I searched a little after reading this thread and couldn't find a single picture of that sensor going in or out. Sorry.

Read this thread, there are some people who have changed that sensor. Contact them, maybe one of them has some helpful tips.

P0016 Error Code | PriusChat

 

This shows how to move the fuel injection wiring around.

Toyota Prius: How to Replace the PCV Valve - PriusDIY.com

Perhaps with that loose, and the air filter assembly off, you can reach the sensor by going between the inverter and that coolant hose?