2005 Buick La Crosse check engine light

First of all thank you so much for your donation. Most of the time that code is caused by a bad catalytic convertor the good news is it is probably covered by your emission's warranty. What engine do you have the 3.6L or the 3.8L?Let me know so I can get you the correct information.

There is the trouble tree for that code it's most likely the cat even with the low mileage i have seen cat's fail with even lower mileage then that.When your car idle's it has less exhaust going thru the cat and that mean's the cat has a colder temp so the code is more likely to set at idle.

If you have 2 point's that's all you have to give someone you earn point's by someone giving them to you or by posting response's question's etc. What your going to have to do is go to the dealer and authorize a half hour to an hour diagnoses and if they find out it's the catalytic convertor or something else that is covered under your emission's warranty then buick will pay for the repair.I wouldn't let them play let's try this if the can't tell you it's going to fix the code P0420. But most of the time it's the cat for that code.