Can anyone here tell me what code P0540 is? got rid of CAT ran fine for first 1000 miles. Got up in altitude towing trailer Service light went on. Had it cleared and read, next day service engine light was back on (not towing) still in altitude. Taking it to dealer next week, just want to know if this a result of removing CAT or something else. Before I deal with the dealer. Thanks in advance.....

Intake air heater problem. Will clear in three ignition cycles unless it needs to be fixed...

Thanks....hasnt cleared in a week of driving. So I guess it needs to be fixed. Is it any cause of the CAT being removed?

Its not because of the cat removal. Most likely the IAH relay has failed. They shouldn't even look underneath to inspect it, the IAH relay is on the top of the engine.