I'm currently troubleshooting my A/C which works perfectly well some of the time, but not at all at other times. My latest is that I think I've isolated it down to either the temperature sensor(s) or the ECU. Here's why: When it works, it works well. It blows nice and cold. During my last major, the shop verified the A/C worked (I'd complained of it not) and topped off or otherwise checked the refrigerant. With the engine NOT running but the ignition on, I ducked under the dashboard and watched the servo move in and out as I adjusted the climate control from Lo (coldest, 64 degrees) back and forth to Hi (hottest, 80 degrees). Interestingly though, it moved all the way to one direction when it was at Lo, and all the way to the opposite direction at any other setting (66 through Hi). It never stopped part-way. In the past, when I've been driving it around, basically the AC works when it's cool out...and I don't really need it. And it DOESN'T blow cold at all when it's hot out, when I actually need it to. So it either has bad input...or it's making the wrong decision given the input. I'm going to do some more driving around, testing its behavior in different ambient temperatures at different settings while watching the servo. In the meantime, does anyone have any advice on where to go from here? Precisely what to check and how? It looks there are a pair of 68154200 temperature sensors under the dash, and they run $40 each. The cabin air temperature sensor is 63934200, at $100. Maybe I could test that by blowing hot air on it with a hair dryer to see how everything responds. The external air temperature sensor 65889400 ($50) seems unlikely, but I guess that's one more thing to check. And I don't even want to mention the price of the ECU, with the hope that that's not the culprit.