My 2006 5.4 with 211,000 miles has recently developed an occasional misfire or possible tranny slip upon slight acceleration. It's happened 3 time this week. It has not caused the CEL to come on. Will my local auto parts store still be able to access codes with a scanner?

 

Yes but there is a high threshold to set cel-- you would not want it to set a cel every little hiccup . . If you or somebody has one with mode six it will tell you the misfire computer counts per cyl .Those are counts towards a misfire cel being set, not each count being a misfire .
Torque pro app $5 /android smart phone /cheap blue tooth reader -amazon will work on many many cars . A lot of info besides that ,look under test results .
Basically the pcm sees whether the crank got a boost when that cyl was supposed to fire .
You will notice misfires going up a hill or in overdrive around 45mph, engine is low rpm but lugging .Plugs are a big deal on this engine . Replace all boots no matter what they look like , Blow all dirt from around well before pulling cops ,do it again in each cyl to keep dirt off piston . Leave no open holes without rags in them . Treat each cyl extended plug tip area with carb cleaner to dissolve carbon out of there ,it breaks plugs going in and out .
The newest plugs are mc sp546 .Do a hosp clean plug job ,keep hands clean . Clean all cop contacts . Use just a little dielectric grease on boot ends .Torque plugs 25 to 28 foot pounds with a T wench not by feel . Loose plugs are a big problem ,60k is now the plug replace cycle .
Keep the oil very clean in this engine no extended oil changes .