Q: What does a vacuum pump do in a car?

There are both brake system and emission system related vacuum pumps. Both are part of essential vehicle systems and so their repair is definitely important but to answer your question as to the cost of the repair, not only would I need the specific code or codes that have set but the actual faulty component would then have to be pinpointed. The reason is faults in these systems could be due to something as simple as a corroded connection (having an inherently low repair cost) or, in another scenario, something as complex as the whole pump being faulty or maybe multiple faulty components. So, the cost depends on, first, which vacuum actuated system on the vehicle you are referring to and, second, within that system, exactly what is the failure? You can get a prompt, and least cost, resolution to this by requesting a check engine light diagnostic. The responding certified mechanic will pinpoint the issue and get this resolved for you promptly right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.