Have you thought about a fuel leak? The reason for my asking all the questions about the exhaust condition as to identify rich running vs. fuel leak. However since there is silence about the exhaust condition, asuming exhaust condition is OK, I'd suggest to look for fuel leaks. It is not necessary that the fuel is dropping on the floor, it might be evaporating as well. BTW, what is the distance over which you are calculating the fuel economy?
I usually put Rs 1000/- worth of petrol at each re-fill. If I calculate the amount your car would run with 1000/- petrol, it is just about 30km. WOW! I am thinking that perhaps your car's engine is healthy that is why the Honda scan picked up nothing. Engine heath is indicated by the exhaust condition. My car's exhaust tip colour is rusty brown. Spark plug colour also is another indicator of the engine health. Below are two pictures of plugs. The correct fuel air mixture ratio is about 1:15. Means one part fuel to fifteen parts of air. If the mixture is lean (too little fuel compared to air) the plugs are whitish. Too rich mixture leads to black soot fouled plugs.
Healthy engine showing balanced combustion. Plug colour is ash brown
Engine running very rich. Too much petrol.
You have to go about finding the problem by eliminating one cause at a time. Just shooting lots of bullets and hoping one might hit the target is wasteful of both time and money plus it is frustrating for the owner. The cost effective way to do this is to eliminate the cheapest cause and work your way up to the most expensive one.
First thing is to identify if the engine is ACTUALLY running rich.