Honda Civic Oxygen Sensor Keeps Malfunctioning

I would greatly appreciate your valuable opinions and expertise concerning an ongoing issue with my Honda Civic 10th generation. Over the past two years, I have been consistently facing problems with the oxygen sensor malfunctioning, and unfortunately, replacing the sensor hasnt resolved the situation.

Recently, I sought guidance on this matter from a Honda engineer based in the USA. The engineer provided me with some symptoms to check in order to identify the root cause:

Radiator coolant frequently decreases without any visible leakage.
The car feels underpowered, particularly when the AC is turned on.
The cars mileage has noticeably decreased.
Persistent error codes related to the powertrain, indicating a potential wiring issue.
Occasional errors related to the air-fuel ratio sensor.
Upon inspection, the wiring seems to be fine, but the fuel average varies significantly – it shows 15/16 kmpl on long journeys and s to 8/9 kmpl in the city. Additionally, the coolant level remains consistently low, which contributes to the sensor malfunctioning.

Based on this information, my engineer friend suspects that the problem could be related to the head gasket. It is possible that the burning coolant might be causing the frequent sensor malfunctions.

Now, I find myself at a crossroads and would greatly appreciate your advice. Should I proceed with the head gasket replacement as suggested, o r do you believe there might be other underlying problems that should be diagnosed first? One concern is that I am from a small city, and I am hesitant to take the risk of opening up the engine without a clearer understanding of the potential issues.

I eagerly await your insights and experiences. Your help in this matter would be highly appreciated!