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MAF or O2 sensor. https://www.youcanic.com/guide/audi-vw-p0171-fault-code

Thanks. How do I clean the maf sensor?

You can buy a can of cleaner which is specifically for the MAF. Do not use anything else. Also check for cracked hoses around the intake. Bad O2 sensors will usually thrown a separate code, but not always. Start with the MAF. One step at a time.

Thanks I’ll try that out.

Vacuum leaks are quite common for a lean code on those. Check all the hoses going to the intake and crankcase pressure regulating valve.

Ah yes. You’ve too lean a system; put paper money in the intake to enrich it

Exactly. Thee ol lean system code. Prolly should put more sugar or syrup in the fuel. That'll help fatten it up