Traditional mass-flow Oxygen sensors respond rapidly, require the simplest of circuits to measure the sensor, they require no power and are very stable over time. However, eventually the internal lead is completely oxidised and the sensor then needs to replaced after one, two or three years, depending on the sensor.
The lead-free Oxygen sensor does not have a fixed, limited lifetime so 5 years or more are quoted for its lifetime, which theoretically is unlimited. However, this advantage is countered by the need to power the sensor continuously, even when the gas detector is off, requiring a permanent power supply.