Glad to STILL have an expert on PW because techie-pw'ers are not active much these days
Ok i will test when i return home (i.e 45mins of driving so should engine should be fully warmed up)
Afaik No oxygen sensor in this model of civic. I have no idea about POT. The only time car has been to local mechanic is when cng was installed... Well so far i have only ruled out ECU re-learning so other 4 parameters are still possible suspect.
In G-10 there is an enercon shop which has exhaust probe and other stuff like that which analyses the gas level, may be i should visit them first before getting into the engine components.
Just a thought (i might be wrong), when i did ecu re-learn, the air filter was new (2 day old (throttle body serviced too) ) but as the filter gets old, it gets clogged and hence the air flow is reduced. BUT the settings saved in ECU are when the air-flow was higher. Could it be that ecu is sending more fuel because of saved values but in real the air is less so the fuel is not burning completely? :o
This whole theory can be thrown away if ecu keeps learning values in real time