2002 sable, DOHC, last year the compressor locked up and I replaced pretty much everything in the system minus the evaporator. Put it all back together and there was a slow leak - lost most of the refrigerant over a month or so. There would always be about 10 or 15 lbs left. I put some dye in but could never see it. That only leaves a few options so I bought a sniffer from harbor freight and sure enough it alerted very strongly when brought next to the evaporator drain. I know that's a big job but it's gotta be done so here it goes!
Started by removing the radio and plastic piece with the adjustable vents on it, glove box, ash tray. Gotta remove the steering wheel, remove bolt above the flex joint near the firewall to disconnect the steering linkage. Remove the multi switch for turn signals/headlights, unplug harnesses. Four nuts and the whole steering wheel assembly comes out.
I crawl under and disconnect all the wiring harnesses near the driver side left wall, only a few connectors and it's all free. Then two big bolts on each side, front of the door sill, the whole dash will come out.
Now we're getting somewhere! At this point I removed the center console, that's only a few bolts and it comes out giving more room to work. Now the whole black plenum box has to come out. Unfortunately this means disconnecting the heater lines and AC lines from behind the engine. At least on my DOHC, this is pretty hard to get to so my trick is to remove the upper intake manifold, wiper motor and plastic cowlings around the windshield. This gives you pretty good access to the hoses on the firewall as well as a couple nuts you remove to loosen the plenum inside.
Drained radiator, disconnected heater hoses from the heater core. Make sure no pressure in the AC system, then disconnect the AC hoses from the evaporator core. Cap immediately to keep out moisture.
Now the plenum box can come out inside the cabin. There's a duct on the floor that catches it and I had a hard time getting it loose. See the arrow in the pic below, that duct goes way under the carpet and I didn't want to take all the carpet up. The duct is thin and somewhat flexible, I was able to bend it around but difficult. Not sure how it'll go reinstalling.
Finally the plenum box can come out. I removed the bolts around the middle and slowly worked apart the halves, lots of little plastic catches around it. Be careful, there's a blend door linkage thingy that has to be snapped out of the top part. I could now finally get to the evaporator core and pull it out. Thankfully, the leak is pretty obvious:
There's a hairline crack you can actually see looking at it close. I'm 6 or 7 hours into it at this point, now waiting for a new evaporator core. Here's hoping for long lasting, ice cold AC this summer!