If you are selling a car that you know has some emission problems, you have a couple of options. It is best to confront the problem directly. Do not try to sell the car without disclosing the issue. Not only might you have problems selling the car, but it could have legal ramifications.
The first word of advice is to spend the money getting the problem checked out and possibly fixed. Granted, you don’t want to go this route, spend the money and find out it will require an enormous cost to fix, but it may just be something relatively simple. If, for instance, it is the catalytic converter, spending the money to have it replaced has benefits. First, the emissions problem gets solved and second, you can ask more for the car knowing you just made a somewhat costly repair and have the receipt to prove it.
Disclose the Problem
The other tip is be completely upfront about the problem. It will likely result in you getting less money for the car, but nobody can fault you for not being honest. If it’s either that or spending a ton of money that you may not recoup, disclosing the problem is by far the best choice.
Emissions problems can make a car hard to sell, but if you are either willing to spend the money to fix it or disclose the problem completely and sell the car for your asking price minus what the buyer has to pay to repair it, you should still be able to make the transaction.