Dear Tom and Ray:

Do you think it's OK for me to put spark plugs in my 1991 Chevy Caprice Classic? My dealer says only he can do it without mixing up the cars computer.
Roger

RAY: Of course it's OK, Roger. He's worried that you'll switch the spark plug wires and mess up the firing order of the cylinders. That would make the car run poorly, but it won't do any permanent damage. Once you put wires back on the proper plugs, it'll run fine again.

TOM: And you can avoid that problem entirely by simply labeling each wire with a piece of masking tape as you take it off.

RAY: Or, you can use the absolute-idiot-proof method: Do one plug at a time. Take off a wire, change the spark plug, and put the wire back. You can't screw that up. That's why we still make my brother do it that way.