Since I got it about 2 months ago, my 93 Jimmy seems to have some problems while idling - the engine runs a little rough, surges once in awhile, and produces a lot of fumes. Runs perfectly while I'm driving though. The SES light goes on and off seemingly at random, but almost always comes on after idling awhile.

While I had it in the shop for some other work, I had them read the codes and they told me it was showing 43 (spark control) and 44 (O2 lean). So, before I go and replace the spark control module, where should I start trying to figure this out?

EDIT: Also, where exactly is the spark control module located? I can't seem to find it, and the repair guide is no help...

 

43 is not spark control:

43: Knock sensor signal was either above 3.5 volts or below 1.5 volts for at least 20 seconds.

44: Oxygen sensor voltage was under 0.25 volts for up to 4 1/2 minutes of closed loop operation.

These are from here:
http://www.troublecodes.net/GM/94-95_43W.shtml

There are 2 knock sensors. One on the driver's side, down between the spark plugs. The other is on the back of the passenger side head.

 

According to my book (and the shop), it is in fact the spark control, so that is still where I am looking.