Few shops will ever blindly peform a repair diagnosed by someone else with out a disclaimer to cover their butts.
Nor should they. Several years ago, I had a very intermittent failure in my Sienna, an evap leak failure. It would fail some times and not others, sometimes not fail for weeks.
I played with it for months. I knew if I took it to a mechanic, the most likely result would be a bill for diagnostics coupled with no trouble found, and do not wish to pay lots of money for no fix.
Finally, a man on the Sienna club had the same exact problem, but he had two Sienna’s. He swapped parts until he found it was the charcoal canister, there are some pressure operated valves there and he guessed there was a sticky one.
I took my car to Toyota, explained I was a retired technician, and was sure enough of my diagnosis I simply wanted the part replaced, no wasting my money in an impossible cause for an intermittent with little chance of seeing it fail.
They did indeed have me sign a waiver that I took responsibility for the diagnosis, which I was glad to sign.That, diagnosing embedded microprocessor problems, is what I did for a living for over 30 years… They replaced the canister assembly, and it has not so much as blinked since then.
When my brake computer showed a failure last year, I had to let them do the diagnosing because I had no clue.
I just remembered something that happened in the days of the VW fastback and squareback. An engineer had one, and it would really start and run hard when very cold.He would take it to the dealer, and they would let it sit in the warm shop so it would be nice to work on, and then report NTF.
He finally tired of this stupidity, and did some investigating on his own. There was a temp sensor which controlled fuel richness. He put a potentiometer on the dash, and had a manual choke, problem solved.