Santro O2 sensor issue

2 months back, while installing cng on my Santro, I had the O2 sensor tested for accuracy. This was important as the closed loop system, depended on the O2 sensor feedback to know if the air fuel ration was rich or lean. Turns out my sensor was running only to 70% of its range. I was curious why there wasn’t a sensor error / check engine light ON. A month back I had the car scanned (for a different reason) via Hyundai scangauge and there wasn’t any error. The CNG technician explained that in petrol mode the sensor reading is not needed to be 100% accurate but in CNG mode it has to be very accurate. In petrol mode the car’s FE is decent, but not good. 11 to 13kmpl in Mumbai, Andheri to Bandra traffic. I mainly drive in peak hours only. My questions are, 1. Does the sensor have to be 100% in petrol mode too? Would my FE in petrol increase if I change the O2 sensor? 2. How much does a new Santro O2 sensor cost? 3. How do I check if the replacement part is 100% working, at the service center or outside? I was thinking of fitting an AirFuel display gauge for this purpose even though I am not too much in favor of this. I am currently running a open loop cng system due to this O2 sensor issue. UM

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