Everyone knows the feeling of taking your car to the shop and waiting to hear what catastrophic things need to be repaired. The worst part of it is, if you don’t know about cars, you don’t know if the repairs are truly necessary.

You can’t learn everything there is to know about cars without seriously studying them. However, it’s easy to learn what the common scams are and how to avoid them by paying close attention. Here are four scams that auto shops run to rip you off.

1. We went ahead and fixed “X”.

Reputable garages always get authorization before making repairs. It’s the only way to be fair about it.

Unfortunately, there have been reports of garages that make repairs and then demand payment from the owner. If a mechanic makes unauthorized repairs and refuses to return your car until you pay, call the police. Without a signed authorization, the garage has no standing to keep your car.

2. The oil change upsell.

Changing your oil occasionally is as non-negotiable as putting gas in your car. It simply must be done. That doesn’t mean you should be shelling out several hundred dollars every time you take your vehicle in for routine maintenance.

A common tactic that less conscientious garages use is to advertise an ultra-low price for an oil change. Once you’ve taken your car to them, they try to add on services or special products.

They may urge you to use synthetic oil, which is more expensive. Some shops even go so far as to have customers return to sign off on not using synthetic oil as if that would hurt your car. It will not. This is just a high-pressure sales tactic used to swindle more of your money.

3. The filthy air filter scam.

Hidden cameras have caught garages using this literally and figuratively dirty trick. They’ll tell you your air filter is filthy and needs replacing. If you acquiesce, they’ll replace it and say no more.

If you object, they may offer to show it to you. The problem is, they may show you a dirty filter that isn’t from your car. By inspecting what’s under your hood and taking a little extra time examining what is happening, you can catch these cons in the act. If you can’t confidently identify your air filter, you can at least keep records of when it is due to be replaced (usually every 15,000 to 20,000 miles).

4. The ever-changing estimate.

Most garages will print out an estimate and ask you to sign to authorize the repairs (see scam number one). However, if you’ve only gotten a verbal estimate, you may find that the final bill is significantly higher.

A fast-talking mechanic can certainly make a case for why the costs were higher. That’s why it’s so important to get an estimate in writing before the repairs are made. If there are blanks on the sheet, make sure they are filled in before signing. It could save you big money in the long run.