2006 Honda Accord emission warning light 06 Accord

Thanks for responding. :) Turns out light refers to an O2 sensor upstream from catalytic converter, near the manifold, feed mix error. Its only been 37,500 miles, outside warranty of course. Shops say they will only replace. It triggers a second downstream O2 sensor error too. Shops all quote fairly high costs to replace the one, bank one sensor one; Could it be too much salt spray in sloppy winter? To test, I replaced air filter, light came back on anyway. So just what happens really if I don't replace? More, different damage to come? I did a sample calculation if its just more fuel cost only: iif overall I lost 4 mpg (don't know what real would be), if use $2.50/gal, the extra fuel cost is 2 cents/mile, then extra expense exceeds cost to replace that one sensor in about 12-15,000 miles anyway. Thought I'd drive 100 miles to see what mileage loss is, but it's snowing again. Will it cause real damage or not? Or cascade to new damage or more over time? They say this car has 5 of these sensors total, this is just one sensor. Significant mileage loss, or not? Engine sounds a bit throaty but could be my imagination. You never know who to trust, and I just discovered this forum. Thanks.