The first thing to do if a mechanic incorrectly diagnosis a vehicle is to speak with him, as it is possible that it is an honest mistake. Explain the situation, inform him of what the expectations were, and try to come to a fair compromise. If this does not work, however, other steps to take include contacting someone with higher authority, taking legal action, giving bad reviews and making complaints with consumer advocate agencies.
If the mechanic works at a chain or dealership, ask to speak to the manager or owner, if possible. If there is still no agreement, there may be a corporate office that can help. Many times, employees in a corporate office can help a customer come to terms with the mechanic. If the customer service department does not help, follow the chain of command through management.
If going up the chain of command does not help with a misdiagnosis, it is possible to take legal action by suing in small claims court. However, people who get to this level must decide if the time and money that goes into suing the mechanic is worth the outcome. Compare rates for attorneys before taking any step toward legal action, as the cost of this could be well over $1000 for attorney fees, as of 2015.
As an alternative, using social media, such as blogging or tweeting an experience with a mechanic, may also be effective if the mechanic does not take steps to remedy the mistake.