Buying a CNG-converted car: what to check

CNG makes a lot of sense for a car that does real daily kilometres in West Bengal, where the fuel is widely available and the running cost difference against petrol is large enough to matter on a monthly budget. But "runs on CNG" covers two very different situations, and a listing rarely says which one you're looking at.

Factory-fitted versus retrofitted

A factory CNG car comes from the manufacturer with the kit built in, under warranty, engineered for that specific engine. A retrofitted car had a kit added afterwards by a local fitter, quality varying enormously by who did the work. Neither is automatically the wrong choice, but they need different questions. Ask directly which one you're looking at — the seller will know, even if the listing doesn't say.

The cylinder certificate is not optional

A CNG cylinder has to be hydro-tested periodically and carries a certificate proving it. This is the single most important document on a converted car and the one buyers most often forget to ask for. A cylinder past its test date is a real safety question, not paperwork for its own sake, and it's also usually a condition for the car's insurance to actually pay out on a CNG-related claim. Ask to see the certificate and check the date yourself; don't take "it's fine" as an answer.

What a retrofit does to the rest of the car

A kit fitted by a competent shop is invisible in daily use. A cheap one shows up as rough idling when switching between fuels, a noticeable power drop that the seller will call "normal for CNG" when it isn't always, or a boot that's lost most of its usable space to the cylinder. Ask to switch between petrol and CNG during the test drive and feel for the transition — it should be close to seamless. A hard jolt or stall when switching is the kit, not the car, and it's expensive to sort out properly.

Insurance, specifically

A retrofitted CNG kit needs to be declared to the insurer and often needs its own endorsement on the policy. A car running CNG with a policy that only mentions petrol is a claim that can be refused on exactly the part of the car most likely to cause damage if something goes wrong with it. Ask to see this on the policy document, not just hear that it's "covered."

Resale, honestly

A well-documented factory CNG car holds its value about as well as the petrol equivalent. A retrofitted one, even a good one, tends to sell for a bit less than the same car without the conversion, because the next buyer inherits the same uncertainty you're weighing right now. That's worth knowing before you buy, not just before you sell — it should be reflected in the price you're paying today, the same way it would be if you were cross-shopping against an electric car instead.