If the 'service engine soon' light comes on in your used 2001 Sentra SE with 81K miles on it is this serious?

Answer 1

It means take the car to the dealer for its scheduled maintenance.

You can also check for a reset button for your vehicle. In a certain number of miles the check engine light will come on you you can take it to the dealer and they can charge you $50 to $100 dollars to reset it. They won't tell you your vehicle has a reset button they will tell you to bring it in.

its probably just the oxygen sensor. have it looked at/ its not that serious

Mine came on at 105K kms (~ 65K miles). I bought an OBD II Code reader off of Ebay for $50 and found out that the 'System was too lean' So it's not serious in my case. But I haven't taken any steps to fix it. A friend recommends I take care of it since a lean system can cause wear to the valves.

Mine came on at roughly 80k, the engine would stall while I was breaking, and excelleration became difficult. I had the code read and it was I believe bank 1 sensor 2...system too lean. I had it diagnosed and its the mass airflow sensor. You can take this part out and have it cleaned or you can spend the 500-600 on the part from Nissan (not including labor time) because they insist the car will chew up and spit out generic brand sensors (nissans are nitorious for it ANYWAY!)

A code reader for your car is a pretty good investment. Two or three uses will generally pay for it. If your code indicated something like a defective oxygen sensor or a mass air flow sensor, you would probably want to get that fixed because most of the errors you see will affect gas mileage and at todays prices, can you afford to ignore it?