When in doubt pull out.
Customer: "I think our printers broken? I've tried two brand new toner cartridges and it won't print."
Engineer: "Do you want us to come out and look at it? As you know it's chargeable."
Customer: "You might as well get us a new printer."
Three days later.
Engineer: "Hey Pete, you know that printer for xyz? I've installed the new one. I also found out what was wrong with the old one."
Me: "What was it?"
Engineer: "She hadn't pulled the safety tab off the side of the toner."