at my shop, we charge $150 for MIL diag, i always ask customers if they got gas a week before cuz it could be the gas cap, some of them yell at me and tell me thats absurd and gas cap cant cause the light to come on, $150 later, i tell them its just the gas cap and we just tightened it up and reset the mil, they try to ask for a discount, i tell them its too late, we did our checks, you gotta pay