I have a 2002 Hyundai Elantra. The check engine light came on, we took it to our trusted mechanic. They replaced the catalytic converter. A week later the light came back on, different code. Says it is o2 sensor. He said the price, and I asked if it is something I can do myself. He said it should be ok for me to replace it. I paid a second diagnostic fee, and replaced the o2 sensor myself.
Now another week later, the check engine light is back. It has the same code, for upper o2 sensor, that I replaced with Bosch o2 sensor.
My question is if we take it back to the same mechanic is it fair for us to pay another diagnostic fee? I feel like they should have caught this the first time, or it is at least related to the first diagnostic and second fee we paid.
If you don’t specify the codes each time and what kinds of things count as the “diagnostics” you paid for then no one can say much about it.
Sometimes a “diagnostic” literally just means reading the codes. As long as you’re not in California you can have your codes read for free at many auto parts stores.
You asked if it was “fair.” All business set up their own standards for fairness, and these standards apply unless a customer negotiates in advance. If I ever specify a repair to a mechanic (e.g., replace the veeblefetzer) I will insist he may not charge me for the diagnostic fee that is his normal shop standard.
Note that by reading your code and advising you on a self-repair, the mechanic lost your repair business on this transaction.
In this case the mechanic performed a service for you and thereby earned his fee, fair and square. He ought not be held responsible for whatever happens thereafter.