Ok, here's the scene. 2003 Sonta 4 cyl, 105k miles, check engine light on, no overheating. Dealer runs diag and comes back with a P0421 code (Warm up Catalyst Efficiency Bank 1 Below Threshold). Dealership tells me I need to replace the cat and might as well change the 02 sensor while they are in there. After reading up a bit on this code it seems possible the cat is ok and it's the 02 sensor that need replacing or even the plugs. There could also be an exhaust leak. Total cost for replacing the cat and the 02 sensor at the dealer is $1300 . After checking around other local shops Merchant's Auto can do the same work for $800. I then inquire with the dealer again and he says the other shops are using after market parts and doing so will cause problems down the road and it will fail emissions. Any truth to that or is that a scare tactic on the dealerships part?

So, what should I do? Start with plugs and a new 02 sensor or just grin and bear it with a new cat? If the problem was indeed an 02 sensor wouldn't a different code get thrown than the 421?

Thanks in advance!