How do mechanics know if a part in your car is aftermarket?

Is it usually because it looks newer than it should? Is it usually the brand name? I know this is a very general question.

The reason I ask is because my 2013 VW Jetta wouldn’t start after I got gas, so I got a new purge valve, and then three weeks later it wouldn’t start every now and then Just Because, and when I took it back to the same mechanic he couldn’t figure out why. So I took it to the dealer and they diagnosed the problem as a bad N80, which after I hung up and could no longer talk to the guy because it was now the weekend I Googled it and saw it was a purge valve, which was frustrating because I couldn’t tell him “But I just got a new purge valve” until Monday. Mechanics should always say “It needs a new blankety, also known as a blank” or at the very least use names instead of numbers for things. But I digress.

Anyhow, I called Monday morning and told the dealer “But I just got a new purge valve” and he said it was a bad aftermarket one. I did remember he said the word “aftermarket” in the list of things that was wrong with my car, so it is apparently the purge valve that was aftermarket. So now I am just wondering how they knew at the time it was an aftermarket, and how dealers generally know these things.

Serena , all parts can be called after market . They could have just said ’ bad replacement part ) . You really need to talk to friends , relatives or co-workers and find a good shop for your many problems with this Jetta. Jumping from shop to shop to dealer just seems to be confusing you .

Parts have labels or stamped/raised part numbers on them. If those identifiers don’t match up with OEM, then they know it’s not OEM.

That doesn’t mean it’s a bad thing or that your first mechanic ripped you off by not using an OEM part.

Sometimes parts (OEM and aftermarket) are defective out of the box. It’s not ultra-common, but it does happen. You just won the bad luck lottery this time around.

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They know because they install factory parts. They know what they look like. Aftermarket parts are not identical and/or have a factory part number.

Aftermarket parts that are not made by the original supplier to VW, cannot have a factory part number on them. They can look the same but often don’t. Cheap aftermarket copies of the original part are easy and cheap to buy. There are good aftermarket parts and there are bad ones. The VW mechanic see bad ones all the time.

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The term aftermarket is used very loosely by dealers and many people in this forum. Many think Aftermarket means anything that wasn’t bought at the dealer. Which IMHO is totally BOGUS. Spark plugs are a prime example. If it doesn’t say Toyota on the plug then many people think it’s aftermarket and shy away from it. But people in the know - know that Denso or NGK plug you buy at the local parts store is the EXACT SAME PLUG, except it doesn’t have the Toyota stamp on it.

Aftermarket is any part that’s NOT OEM (sold at the dealer or made for the manufacturer and sold elsewhere). But many people have a very narrow view that aftermarket is any part NOT sold at the dealer.

Most of the parts on any car are aftermarket even as the car exits the factory assembly line. Most car makers stamp out the sheet metal and build the engines. Everything else comes from a long list of suppliers.

My Lincoln Mark for example. Engine blocks from Italy a la Ferrari engine blocks, all lighting made by Sylvania, seats manufactured by I forget who, glass from PPG, plastic trim and door panels from somebody else, water pumps and alternators from there, starters from here, and so on down the line. VW is no different.

Sometimes the aftermarket part is manufactured by the same company that manufactures the so-called OEM part on your car as new. The only difference may be a VW logo or part number imprinted on it. The part from your local AutoZone may be from the same company as the part your car was born with.

You might be getting the wrong diagnosis from both mechanics, wouldn’t surprise me.