2001 Jeep Cherokee Sport - 104000 miles.

For well over a year we drove around with our check engine light lit. At the moment we happen to have a pretty good mechanic. I don't remember the code but it was 'small evap system leak'. In the end it turned out to be a small hose near the fuel tank. The check engine light was out for months. Came back on earlier this month - this time 455 - large leak. The mechanic said he really couldn't find anything wrong but he changed out the little evap box (??) under the hood near the windshield on the passenger's side (at no charge for parts or labor) - and reset the error code. The check engine light has stayed out.

BUT

We live in a climate where we very very rarely turn on the air or heat in the Jeep. We had a particularly cold morning and I turned on the heat. It was shortly thereafter that the check engine light came on. The mechanic pointed out that currently the heat only comes out the vents at chest-level regardless of where the dial is set. He feels that this is somehow related to the 455 error code - that the hot air is not being routed properly and causing the error. What do you think ? (I'm hesitant to turn on the heat in the meantime).

Thanks and Happy Holidays !