I have a couple recommendations for auto repair shops.
Two Good shops both honest and they stand by their work. However they are one city over from Diamond Bar, in Chino. (take the 60fwy east to Central go South, a Right turn off the fwy.)
Gabriel's Automotive at 13654 Central Ave (Near Schaefer) 909-464-9005
I have had extensive work done by Gabriel's on two different vehicles and have been totally satisfied each time. I do confidently recommend them. They have also provided courtesy shuttle service as well as a free loaner car when they needed to keep the car for longer periods.
Also There is also Kievits Automotive at 12949 Central Ave. 909-628-6107 (just south of Riverside) I have also had work done there and was totally satisfied. Although I have had more work done by Gabriel's
however that was solely due to Gabe's business hours fitting my schedule better.
But you can't go wrong with either!