Leaderboard

I don't think it's supposed to report that it's bad when it's just not being used. Quad drivers are open-collector output when "off" and are grounded when "on", so the diagnostic probably checks to make sure the output of each quad driver is 0V when commanded "on" and if the output is >0V, that shows up as bad. A break in the circuit won't fry the chip, but a short in something it controls would. For example, a quad driver that controls a solenoid that normally takes 100mA of current that gets a shorted winding in its coil and ends up consuming 10A of current would overheat and eventually fry the quad driver. I've heard of shorted injectors overheating a quad driver causing an ECM to shut down entirely (stalling the car) and then not being able to restart until after the quad driver has cooled down.

Quad driver is nothing more than a transistorized solid-state switch. Basically, it's a chip soldered onto the PCM's motherboard. Bad quad driver = bad PCM. That's all there is to it.