Ack! Hybrid needs a new computer. Really?

We’ve been told our 2001 Prius needs a new computer, so they can decide if it really needs a new catalytic converter. This means several thousand dollars. The car has ~150K miles on it. Isn’t there any way to test the exhaust to determine if the check engine light is really a catalytic converter problem or just a computer problem?

How fast can you run? At least, walk away as briskly as you can from that shop (even if it’s the dealership).
Worse case, say, it does need another computer. The computer doesn’t have to come from the treasury vaults of Toyota parts. You can use a used computer, or go to ebay etc.
Find a shop which know its stuff better. Hopefully, there is one near you.