i have done a lot of research on the "Check enginer light issue" for I-DSI model showing only on CNG and i want to share my findings with all of you.
If the check engine light is only showing on the CNG and not on Petrol, it means that there is nothing particularly wrong with your car. Infact, there is one Oxygen sensor installed in this model and when mechanics do the CNG installation, they mess with that sensor. So, the ECU is getting the signal that the mixture settings of air+cng is abnormal. That is why this light show up. There is no way to get it fixed, perhaps the interval of appearing this light can be variable but it should come if u have CNG installed. Also note down 2 pionts
1)if u r going on same speed and dont vary ur speed, it will not show up.
Rather check
2) whenever u refill CNG after that whenever u applying brakes the air mixture goes out of proportion for ECU and this check engine light glows up.
Once, my friend has the same issue with his I-DSI, and we tried everything to get it fixed, even we got it scanned from scanner, but it is just Oxygen sensor setting that is getting this light to glow up.
For confirmation, whenver u get this light on CNG, just turn off the switch and drive on petrol. If oxygen sensor is causing the issue, it should not come on petrol drive.