We're a Volvo family (850 wife's, 940 mine, 740 Son's), but this is wife's car so if I don't fix I'm dead meat !

Hers: 1997 850 non-turbo, auto, 52,000 miles, always garaged.

Symptoms:
a) Blows black soot out tailpipe, but drives OK
b) Bad gas milage (about 16-17 mpg, when 27 is normal)
c) Sulfur smell
d) Check engine light sometimes comes on
e) Using Alex Peper's OBD tool, I got a scan code P0410, "Secondary air injection, incorrect flow detected" (O2 sensors say OK)
f) Also reset that damn "Service" light - YAY !!!!
g) Ooops...Weird part: this problem hapened before a few months ago then went away. Sometimes even now the gas mileage will be OK for a while.

I read the posts re SAS valve, air pump, check valve, etc. and am a little confused where to start.

Is this a 1) SAS bad valve , 2) Air pump check valve, 3) Air pump bad, 4) Other ?

I'm pretty good with tools, but need to know where to start with this one.

Thanks

Glen

(Now gotta get the kid to help replace the head gasket in his 740 turbo. My 940 wagon runs just fine, thanks ! )