One of the operants here was the fact that Maggie went in to get the diagnosis. Unfortunately many otherwise scrupulous mechanics frequently try a snow-job on a woman, figuring she won't know the difference. Others who look as it they might not know the difference between a power wash and a power flush, or the fact that one won't cure much of anything. This probably has to be something of a male thing, which still doesn't excuse it.
I get very good service from carefuly chosen shops that often look like the Wreck of the Hesperus, but still do very good work.
I also used to get good muffler service from Midas, but that may have changed over the years.
For the other Yobbos, Better Business, if it operated in your area frequently does wonders, as does you local TV station's Expose Shoddy Situations program or newslady.
Dave