The check engine light came on just as I was about to climb the "grape-vine" section of interstate 5 north of Los Angeles. At a certain point, I knew the tranny would shift for "a climb", but all I got was a cough and hesitation before finally correcting itself. I got over the mountain ok and drove for another two hours before it came on.

Checked the codes and found a 1-1-3 and a 1-5-3. The first is listed as fuel injectors (short term fuel mixture too weak) and the other is for a rear O2 sensor.

My question: since the 850 can set a code for a specific injector, should I focus only on the rear O2 sensor?

You should focus on a vacuum leak, which can cause the timing to be off. Pull the spark plug anyway and check the electrode. If it is rounded, replace all 5.

Also check the hose from the air box to the throttle body for air leaks.

Klaus
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1975 164 w/174,800mi (Sold) 1995 850T w/91,000mi, 1998 V70R w/129,000mi

Had all the vacuum hoses and elbows replaced about 1 year ago. I know the sound of a leak of air (as I've had them before) and there are no sounds of leaks.

The codes are specific to certain things. Are you saying I shouldn't focus on either?

I assume you have reset the codes after reading them. Is the O2 sensor still throwing a code? A mis-fire happening in rapid succcession will turn on the CEL, the O2 code may be a left over from a previous time.
A leak causing low manifold pressure may not be loud enough to hear over engine noises at highway speeds. I just suggest that you start with the cheapest things to fix first, rather than spending money for an O2 sensor first.

Klaus
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1975 164 w/174,800mi (Sold) 1995 850T w/91,000mi, 1998 V70R w/129,000mi