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The 700/900 FAQ section basically just has the OBD-I codes for late 80's/ early 90's cars. You can find the additional OBD-II Volvo codes here
You'll see that a Volvo OBD-II code of 5-1-1 means there is some kind of failure in the "Adaptive oxygen sensor control, provides leaner mixture at idle".
That sounds to me like the ECU is attempting to lean out the mix at idle, but the feedback from the O2 sensor does not indicate that a leaner mix was achieved. This does not sound fatal, but means your fuel consumption may be higher during engine idling. Apart from fuel consumption, the only other aspect I can think of might be failing a vehicle emissions test especially if that test happens to measure emissions output at idle (many only measure under load at higher revs).
I'm not sure what exact component might be causing this problem, but if anything I would be suspicious of the O2 sensor itself, possibly not burning deposits off adequately. Deposits on the O2 sensor itself may be a problem leading to slow reponse and an incorrect reading. Now if your mechanic is intimately familiar with this problem and says otherwise then maybe the ECU, otherwise I'd just take it as an educated guess. In any case, a new O2 sensor is a heck of a lot cheaper if you're going to throw parts at the problem. Is there any evidence that an aftermarket O2 sensor has been installed? Personally, I would doubt the ECU is at fault unless there is a poor contact at the main ECU connector or at the O2 sensor connector. If there is corrosion or a connector has been previously opened for some reason then that might be a possibility, otherwise wiring and contacts probably aren't an issue.
Here's some other things I'd want to consider. Engine deposits are both a major source and a significant indicator of marginal running problems and that would mean incomplete combustion and that would mean a perpetually high O2 sensor reading. There are many factors that might contribute to that. I can easily imagine an ECU attempting to fine tune the mix to the lean side in a dirty engine having problems. City driven cars are especially prone to engine deposits. What grade and quality of fuel are you using? Try at least a 92 octane [(R+M)/2 rating]. Try switching to another quality brand (additives do vary). Give your engine a long highway run at higher revs to help burn off accumulated deposits. How good are your spark plugs? Worn, fouled or incorrectly gapped plugs lead to incomplete combustion. How clean is your throttle body? A dirty throttle body (always important to check and clean) might inhibit idle air flow or be an indication of other problems. Could a stuck EGR valve be involved? Could an injector be leaking? Could the AMM be involved? Even something simple like has the air filter been changed in living memory? Then there's always the Swiss factor. Any chocolate stuck on the O2 sensor?
Hope that helps. Others may have a more definitive answer.
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