On the W201, the odometer tripped the O2 sensor light at approximtely 60K miles. Symptoms of a totally failed sensor: fuel fouled spark plugs (wet); really bad gas mileage; running extremely rich. The sensor doesn't really affect start up much, since it only comes into play once it reaches operating temp in the exhaust system, so initially the car may seem fine, but a minute or so later, you should notice. For a sensor that's marginal, the symptoms may be less dramatic.