98 ML320 -- check engine light is on today with no other change in the car's performance, driveability, mileage consumption, etc. It's not the fuel cap. Any insights as to what this might mean and what should I do?

 

took it to local indy -- shows code PO141; O2 sensor heater malfunction -- he reset it and recommended doing nothing -- wait to see if it comes back on then he would replace it. Any worries about this approach?