Courtesy Quick Lube & Tune

Proceed at your own risk.
I called and explained that my car had failed emissions. «Bring it on in. We’ll take a look.» I took the car (whose check-​engine light was on) and the emissions report. They looked at the report, pulled the car into the bay, and about 30 minutes later Juan (mechanic) explained that the diagnostic that they’d just run would be $ 75. No one had mentioned this cost before running the test (a test that O’Reilly Auto Parts runs for free); they just ran it and charged me. He also recommended $ 386 worth of new parts, which he couldn’t guarantee but was «98% sure» would make the car pass emissions. I paid the $ 75 and went to Sid’s Automotive in Golden.
Without even looking under he hood, Sid’s read the report, explained that the parts that Courtesy recommended replacing don’t lead to the emissions problem I had, and suggested getting a new air filter for $ 12. I did. Emissions passed.
Either Courtesy knew what would make the car pass emissions and still tried to sell me parts/​labor I don’t need (i.e., dishonest), or they didn’t know (i.e, not as competent as I’d like). Maybe it was an honest mistake, but I’ll take my car elsewhere in the future.

These guys might look like a no frills shop but they rock. They are super friendly, honest and know their trade.