Check engine light still on in my 2006 Holden Rodeo Ute

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.