I have a similar issue. My 03 Accord’s little engine light lit up on the dash, indicating a problem with the emissions system. I went though a couple tanks of gas, hoping it was just a loose gas cap. So after a month I take it to the dealer, they tell me the code is a loose gas cap. I say no way, so they check it again and surprise surprise! Now the code says I need an O2 sensor and the part costs $189.
Am I right to be suspicious? I am due for an emissions test in the fall and I will fail with that light on, but I have six months to deal with this.