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My question is, how far is a dealer expected to go to sort a problem with a car if a fault code appeared on day two after purchase? He has been great so far, but I just want to know my legal position if he starts to baulk at fixing the car if this drags on, which is what I have heard can happen with P0420 cat converter/oxygen sensor issues.
I purchased my Prius 2009, Gen 3 with 112120km on the clock in late Feb.
The engine fault light came on at day two, and was diagnosed as a P0420. I had already driven hundreds of kms away from the purchase site so couldn't return immediately. But when I did get there, after it had been serviced at an AA service centre, the dealer was open to helping me fix this.
The dealer is also a used car wrecker and parts supplier, and so he replaced the bank 1 catalytic converter with one from a hybrid with 60000km on the clock. This did not fix the problem. He got a hybrid specialist to diagnose, and three faults were now detected (whereas the day before only one was detected at an AA service centre), the cat converter, a 12v battery code, and another that I forget but was related to the oxygen sensor near the air filter. The hybrid specialist also told the dealer straight-up that he may have installed a faulty/dodgy second-hand cat converter and that a new authentic Toyota part would be best.
The hybrid specialist cleaned the sensor over the air filter with compressed air/liquid, but this worked for only around 20 hours before the fault came back (yesterday afternoon). I will take my car back on Monday to confirm which code it is.
So again, if it is the P0420 code again as suspected, is it up to the dealer to take all measures to fix this, i.e., replace the B1 oxygen sensors, fit a new cat converter, and perhaps even check for vacuum air leaks or manifold intake seal leaks (which I've found out could also be the problem).
The dealer is an awesome guy, and I feel as if he will do me right, but it is hard to tell. I paid $9,800, so not high-end, but enough for me to be annoyed, and perhaps not enough $$ for him to keep me happy forever and a day trying to solve this problem, especially if it is a case of how long is a piece of string.
Finally, are you aware of costs for this kind of work, any other potential causes, and any simple fixes to try first?
Thanks in advance =)

The CGA guarantees mean the vehicle should be:
•of acceptable quality – reasonable in look and finish, durable and safe
•fit for purpose – either generally or to meet a specific purpose you told the dealer before you bought the vehicle
•as described – match the description in advertising or anything the dealer said at the time.

If the problem is minor

The dealer must get any repairs done within a reasonable time. Be sure to tell them when you need the car back by. Ask for a 'courtesy car' while they repair the vehicle, although a dealer is not legally responsible for providing one.

The dealer may take a while to nut this problem out,

Looks like there is a sensor at the beginning of the catalytic converter and one at the back, This fault could relate to multiple parts,
It appears like the dealer will sort it out but I would keep a bit of pressure on the dealer he may need to invest in a few more parts to sort it out, If he cant you may need to take things further i.e. if he is unable to resolve the issues.