"Car dealerships often do this and charge you truly eyewatering prices for each wrong guess - despite being in a far better position than anyone else to do the job properly due to having all the knowledge and proprietary diagnostic equipment on hand."
And access to information about "silent recalls".
My girlfriend owned an Audi Fox that was about 2 years old at the time (late '70s). One day, we drove into town, shopped, and then tried to go home - car simply would not start.
Had it towed to the nearest Audi dealer (in Chicago area), they scratched their heads, finally replaced the fuel pump and billed us for that plus 3 hours of diagnostic effort. I looked at that, called bullshit, and asked to keep the old pump, which I boxed up and shipped back to Audi national headquarters to be checked, along with a copy of the repair invoice.
In the meantime, we started a 3 week road trip to the Rocky Mountains. Yeah, why would I be typing this if the car didn't fail exactly the same way at a campground 125 miles out of Jackson Wyoming? Had the car towed to the only foreign car dealer in town (Subaru, btw). When we rolled into the parking lot, a guy outside looked up, saw us, whistled at another guy, gave him a couple of minutes of instructions. By the time we were doing the sign-in in the office, they already had the car off the truck and into a bay.
Fifteen minutes later, the car was fixed! No parts, and they only billed the 15 min labor!
The head mechanic kept up on trends in car problems, and knew about the faulty connector design Audi had committed on the power circuit feeding the fuel pump, so he told the guy to simply pull the wires out of the connector and solder them together. Never had another problem (with ignition) on that car.
Oh, and when we got back from the trip, Audi had sent us a letter stating that the fuel pump was just fine, along with a check reimbursing us for the full, original, dealership "repair". About six months later, there was a formal recall of that connector.
And my then girlfriend had had the foresight to pay an extra $2.00 on her car insurance to cover emergency towing situations, so the repair in Jackson, Wy was our only expense - insurance covered the 125 mile towing charge. Clever girl!
She is still my wife - I'm glad she thought I was good enough to keep around. We drive a Subaru now.