My idle is rock solid, but I'm wondering if I should replace my 18 year old ISC anyway after reading this FAQ on DSMISC's web page.

"2. Why should I replace my ISC, besides improving the idle?
There is a far more important reason. If your ISC has shorted coils, it will eventually damage your ECU. Shorted coils will fry the drivers in the ECU; it doesn't take a fully shorted ISC to exceed the current specs on the M5269L drivers Mitsubishi used. They are only rated for 2A 10ms 10% duty cycle pulses max. A closed short (meaning low resistance) ISC coil will easy cause it's corresponding ECU driver to load ("sink") many more times than it normally would; eventually the driver exceeds its rated duty cycle, heats up and shorts. As resistance goes down, current flow increases. The old ISC coils are supposed to test at 28-33 ohms. On those, anything below 14 ohms or above 50 (indicating leaning toward an open coil) would be a problem.

Once an ECU driver goes as a result of shorted coil, several ECU IC circuits will be next in line to be damaged. Why risk your $375 socketed ECU by running with a bad ISC?

3. My ISC tests ok, but I know it's not working. What gives?
Sometimes, the ohms reading can be in spec but the ISC is still bad. Testing with a 6V battery is a better test (tests the ISC motor), but even then you can't be 100% sure. It could be a mechanical plunger problem as opposed to an electrical coil problem, see below #8. Or, your ISC O-ring could be missing, or leaking."

Makes sense to me, but maybe they're just trying to sell ISC's. $131 bucks might make me sleep better, but it'll also buy a lot of cigars.