I don't know how others see a car with an error, but for me I would like a car for which I am sure what the costs are.
As I experienced some problems with mechanics starting replacing parts and not fixing the problem, I know that error codes can mean a very simple thing like replacing a sensor or replacing the catalyzer (talking directly about your case).
For one car I had I cleaned the air intake (cheap and good anyway), the throttle (cheap and god anyway), replaced knock sensor (not needed, but cheap), replaced spark plugs (did not fix the error, but needed as they were very old), spark plug wires (did not fix the error but needed as they were very old), tested some induction coils.
In the end the problem was where I suspected from the beginning: the EGR valve.
My advice (could be wrong for USA, I can tell what works in my country): if you want to sell the car with a good price make sure it does not have any errors reported, as that would drag the price down.
USA might be different, as I see that having the car in top shape seems to not be as important here (my subjective impression).
I am constantly looking for a car although I will not buy one this year.
I want to get used with the prices.
I can tell you that if I see mentions of engine check lit but the car is working perfectly I do not look at that add anymore, because I don't know what will be the real cost.
Some say that they know what the problem is and almost everytime they say it is cheap to fix it but I know from my own experience that they could be right or wrong...
What would I do:
1. If it works fine, use it like it is (I used my car for around 15000 miles with the EGR valve blocked before replacing it as it removed the problem. I only blocked it after talking with my mechanic). Replace the O2 sensor and then the catalyzer when you have the money. That is what I would do, but I like to have my car in "perfect" shape. I feel "pleasure" when I fix the problem and the car works perfectly - I had two used cars until now, both sold in better shape than when I bought them.
2. If it really does not bother you, use it like it is right now.
3. If it does not work fine, replace what is needed to make it work fine. Continue using it or sell it.
4. As I understood the car is not appealing anymore to the driver, I think you should really think of getting another car. But in this case you might need to fix the issue to get a good price. If you spend a lot, this could mean you will have to keep the car in the end to at least use the new part you install :).
5. Sell it with the error code present, but expect a tough negotiation.
Regarding fixing the issue:
Try to find a way of testing if a new catalyzer will fix the problem without having to buy the catalyzer - perhaps talk with a mechanic to test a used one.
If this fixes the problem you at least know what you have to do to fix it. If not, you will only pay the work to put it in place and remove it...
Please let us know how things work out.
