AsSalamOAlaikum
Santro back fire CNG per bohat he common problem hai,
Its occurs usually when CNG ratio is excess and current is too much weak .
For best accuracy usually scanning is recommended and codes can help out to clear the problem easily.
For best ratio setting to avoid backfire plz contact
Cartech autos
Syed Sadat Masood
03224450322
Johar town
Pia road
Lahore