There is a preignition ping with a/c on at low rpm. It looks like others have had this problem. I just took my truck to the Toyota dealer and the representative spent the time looking through my owners manual trying to find where it told me that the condition was normal. I've already checked the spark plugs myself and they have no problem (other than a unusual condition of mixing plug brands in the two banks). Since there are no codes, the dealer can't (or doesn't know how) to check the vehicle for me. Is this a problem or do I have to live with using higher octane gas?