is a dealer is responsible if the check light turns on after changing sparks plugs and fuel injector service?

I don't know what mileage you are complaining about or how many miles are on your car. The spark plugs do not need to be replaced until after 120,000 miles. Did you have the fuel injectors cleaned or did the dealer perform an "induction service". I have seen more than one Prius have the catalytic converter killed by this process. If you had an induction tract service done and a P0420 code appeared right away, they killed your catalytic converter. Talk to your dealer about this. If your service adviser cannot help you, talk to the service manager. If he cannot help you, talk to the store's general manager. If he cannot help you call the Toyota customer service hot line at 800-331-4331. Good luck, the catalytic converter is an expensive repair.

Checking the spark plugs won't damage the injectors. Chances our that once it used to be eliminated it used to be commonly easy off a bit of and reinstalled. Thus inflicting an extra load on that detailed cylinder. Or might be simply that it was once going bad to with and it happend at a time, correct after having the plug removed. The only harm that might viable occur is if the spark plug was once eliminated from the cylinder head and reinstalled at the same time the engine was once warm on an aluminum head engine. Dont see that happening to an injector although.

The check engine light can turn on for many reasons. A common cause is that the gas cap is not on tight. You need to get the actual codes and post them in the Yahoo Prius group so folks can tell you what is happening. Without the codes, it's just guesswork.

You should spend a minute to proof read your question before sending.

So you noticed the fuel mileage was bad and you aren't saying anything about that as a cause the cat is bad are you. In fact your not even sure it is bad you just want someone else to pay for it.

Sorry Steve, it is not their fault your car is having issues. Bad fuel mileage results from improper combustion which in your case is caused from worn spark plugs. This resulted in raw fuel going through the exhaust system (through the catalytic converter) and ruining the catalytic converter.

this is a tough one..... a bad spark plug replacement or poorly cleaned injector........... could cause a sporadic misfire that would cause a catalytic converter code...instead of a misfire code......but it is very very ....unlikely the most likely cause is that its just a coincedence.......