Please note Win960's (Shawn?) reply.

First off, he's quite right, only the FWD Volvo's had OBD-II. The B6304F's used in the 960's had the Bosch Motronic system and in 1995 I believe it was the M1.8 system which was OBD-I. The fact there are codes beyond what's shown in the FAQ would be due to newer engine management systems and EPA requirements. I've edited my post accordingly.

The 5-1-1 code description he found in Chilton does indeed seem a bit different from what I saw. I'm curious, so I did a bit more digging, including the 5-1-2 code for comparison.

What he found in Chilton:
Code: 5-1-1 Long-term idle air trim, idle
Code: 5-1-2 Short-term fuel trim
[although Chilton usually shows codes specific to a fuel system, they have been known to use their own interpretation of other Volvo specs for things like torque and clearances, so I don't consider them gospel]

What I originally found on the referenced ECU web-site, a generic list for Volvo's up to 1997 from a commercial ECU site:

Code: 5-1-1 Adaptive oxygen sensor control, provides leaner mixture at idle
Code: 5-1-2 Oxygen intergrator at maximum lean running limit

What I found on the web for Motronic M4.3, an OBD-II system used in 1995 Volvo 850's and which appears to be direct from Bosch:

Code: 5-1-1 Long term fuel mixture too rich at idle
Code: 5-1-2 Short term fuel mixture too rich

What I found from AllData for Motronic M1.8, an OBD-I system used with the B6304F in the Volvo 960's (including 1995, I believe) and which appears to be a photocopy of the Volvo literature:

Code: 5-1-1 Idle Long Term Fuel Trim Lower Limit
Code: 5-1-2 Short Term Fuel Trim Lower Limit

Now near as I can tell, for the 5-1-2 code (that you don't have) the descriptions are consistent (although it's interesting to note how they say it). For your 5-1-1 code, I think the last three are all in agreement, you're running too rich at idle. A stuck IAC valve or air leak (unmeasured intake) shouldn't affect any of this. So I'll still stand by my earlier comments and my opinion of Chiltons.

-Dave (not to be confused with a real expert, just goofing around at this)