I've gotten three check engine lights (P0161 - O2 sensor) in the 60K miles that I've owned my VX. The first time was under 20K and I took it to the dealership, where they replaced the O2 sensor under warranty. The second time was at around 30K. I owned a code reader by then, so I just cleared the codes and went on my way. I got the last one at around 45K. Didn't give it a second thought; just pulled over, cleared the code, and continued to work. All three lights occurred shortly after starting, and were probably related to slow warm-up of the sensor. I would only be concerned if the code comes back immediately, and/or cannot be cleared.
Gregg
2001 Proton Yellow #1379