Q: fuel mixture is lean. must be adjusted

I’m guessing that you are getting a P0171 code or something similar. Yes, its telling you that the engine is running too lean, but there is not an adjustment that you can make. When this happens it is because of a malfunction that needs to be addressed. In most cases, a P0171 will come up because there is a vacuum leak admitting air to the engine. Any air that gets into the engine without being measured will lean out the system and cause problems. Look at all the rubber hoses and boots under the hood to be sure they are well connected and in good condition. There are a few other problems that can bring up a 171: a bad air mass meter or oxygen sensor, or a leak in the exhaust to cite a few. But your best bet is to have the car checked out before you start replacing expensive parts. Contact YourMechanic to have a technician come out to your home or office to ]check out your Audi](https://www.yourmechanic.com/services/check-engine-light-is-on-inspection) and let you know what it will take to solve the problem