Gustavo. The problems may be the same as any other engine. It depends a lot on the use of the vehicle. It is a very good 140HP engine. Although it does not compete with 3.0, it is very good and its consumption is much lower. Another thing to keep in mind is whether the CNG equipment is factory or installed outside the factory. That can make a big difference. Keep in mind that the engine torque is lower than that of a diesel. This is noticeable when used for loading or towing. But if the treatment given to him is kind and not to "kill her", it goes very well. That said, I tell you that the things to check if it is used are the same as any other vehicle. That it has a neat maintenance, with possible original elements or of good quality, that the engine does not consume oil or coolant or work outside the factory ranges in pressures and temperatures. That the color of the coolant is ideal (without traces of rust or oils). And it also tests the transmission very well (box, differential and transfer if it is 4x4). That the changes come smoothly both up and down, that there are no hums or vibrations. Losses of oil usually mark poor maintenance or somewhat gross and careless use. And several other things that you will realize as you see it. And finally, the cleaning and care of the interior of ANY vehicle, lets see if this care is in general or not. A broken interior, dirty and neglected, shows that the user does not give importance and usually the state of the rest is the same. A hug and good luck with the purchase!