AT seems to run like new after I overhauled the PNP switch, dropped pan, replaced filter (perhaps could have been cleaned and reused to save $35), flushed with Shell ATF, and put in a Magnefine in-line filter. OBDI and Volvo manual diagnostics check out OK too. So I flushed again with Mobil 1 and put in another new Magnefine filter, hopefully good for a couple of years. Shell ATF flushed out clean and pink. I unscrewed the top of the Magnefine I had put in with Shell ATF and found a small amount of unpalpable fine soot like dark grey stuff adhering to the magnet.

As far as I can tell, the PNP switch missed on the highway, AT went into limp home mode (fourth gear in D and third L), ATF then overheated, orange arrow and E,S,W lights flashed,and ATF smelled burnt. OBDI read missing or shorted PNP signal and overheated ATF.

To avoid such headaches and AT replacements at a cost of several thousand dollars, perhaps maintenance should include PNP overhaul (cleaning inside with electrical parts cleaning and regreasing, and perhaps replacing wire loom or patching it with electrical tape) when we flush ATF every two or three years. I found I did not have to touch exhaust pipe or ATF line to get to the PNP switch. So it can be overhauled for little trouble (particularly if the car is on a lift) and expense. This would be a good time to clean and repack with dielectric grease AT electrical connectors on the driver side of AT by ATF filler tube.