You don't negotiating payments, you negotiate the price out the door. Payments are easier to digest, $20000 or $350 a month.

I would contact a local muffler shop I had the same issue a few years ago. Dealership wanted 2k for replacement catalytic converter, local muffler shop was $350. Honda's are good cars and can last 250k miles. It will be cheaper to maintain for a bit longer, car prices are coming down.