Is the engine light not covered by warranty?

We bought a 2007 Mitsubishi Pajero, and after driving locally the engine light came on. We rang the dealership and were told that the engine light was not covered by warranty. So therefore, the light indicating a problem with engine is not recognised.

How can this be fair if it's telling you something is wrong with engine itself! Now we are being treated unfairly because we paid for a diagnostic test and it shows that the DPF needs replacing.

Can they brush this off; as we think they have just been resetting it so it goes off long enough til you're out of their yard.