There are many, many things in a modern cat that can cause the check-engine light to illuminate. The cause could be anything from a sensor somewhere on the engine or in the exhaust to a problem such as low oil pressure, overheating, a dud fuel-injector or even a stretched timing chain that has allowed the crankshaft and the camshafts to become out of synch.

Basically, the car’s computer notices and logs anything that’s amiss and warns the owner that some form of servicing is required by lighting up the check-engine light. The other possibility, of course, is that whoever worked on the emissions control system you mentioned, may have neglected to manually turn off the check-engine light. It’s happened before.

There was a mid-model update released on the Rodeo late in 2005. It’s probably relating to that.

There are a number of LPG systems available for diesels, but they’re different from those we’re used to on petrol engines. In the case of the diesel system an amount of LPG is substituted for diesel, the engine never runs on LPG alone, and the effect of that is to increase power. The idea is then that you back off the throttle and save on fuel, but some people make full use of the extra power and can suffer engine failures as a result. I would urge caution and certainly urge you to talk to other owners who have done the same conversion you are considering to get their experience. It’s also crucial to have an experienced installer do the work on your car.