Most likely a missfire code caused by the gas you used. The ECU is supposed to compensate for the lower octane, but maybe you got a little water and/or bad gas. Your 740 would probably not be as affected by bad gas as your 850 (is a much cruder system in the 740).

I would never use less than 89 in my Volvos, and almost always use 93. I have sometimes (for a long trip) mixed 89 and 93 to get 91 octane, the minimum recommended by Volvo. You might try that to save a little $$. In 35 years of driving, I've never used less than 91 in any car I've owned. Performance and fuel economy is better, system is kept cleaner, and I've only ever had to do one valve job, and never a ring job (the typical problem caused by using too low octane fuel). If you ever get pinging (usually from low octane), remember that is the sound of independent explosions occurring at the wrong place and time within the cylinders (i.e., not a nice smooth explosion wave front). It is the sound of expensive damage to your engine. Pay me now or pay me later.