Faulty o2 sensor but no performance change

[Ukhaysum]

2 weeks after passing a mot my 40k mile 2016 diesel lit an amber engine light. Left if for 2 months as was running fine then took for diagnostics which showed faulty o2 sensor and was told to give it a good run as it has hardly done any mileage in recent months- lots of short journeys. Week later engine light again and same diagnostic and garage is replacing o2 sensor. What gets me though is that it’s been driving perfectly - no change to fuel economy, efficiency and emissions must have been fine when it passed mot. Can o2 sensor fail with no change to driving performance on a low mileage car? Would have suspected dpf considering its mileage is now all short journeys.

[calnorth]

The O2 sensor is electrically heated which will raise an error (MIL) if faulty. The sensor works at a very high temperature as such. It may still work if the heater is faulty...to some extent? Likely the ECU may default to a basic air/fuel mix ratio. The amber MIL will be an MOT fail.

This sensor is in the exhaust manifold and may get sooted up being a diesel particularly...hence the good run advice I suspect. Anyway, lucky you my petrol engine O2 sensor failed at 17k miles last month.

[Ukhaysum]

Appreciate the reply thanks!