DEAR CAR TALK: I have a 2003 Lexus ES300. The check engine light came on, so I took it to my mechanic. He scanned it, and the code said it was the evaporative emissions system.

He then did a “smoke test” and saw smoke leaking out of pinholes in the fuel filler neck – which he then replaced. I drove it for about 200 miles and the light came on again.

Same code. This time they replaced the evaporative canister. I drove for about another 200 miles, and again the light came on with the same code.

This time they replaced the vent control solenoid. Again, I drove for about 200 miles and the light came on. They explained that there are many parts in the evaporative emissions system.

Should I go to the dealer? Will they be able to diagnose it more accurately? What’s left to replace? – Fil

RAY: Well, clearly the problem is that you’re driving too far, Fil. You need a new lifestyle, in which you never drive more than 199 miles.

These guys are guessing now, unfortunately. Doing the smoke test was the right place to start, and if the filler neck was perforated, it made perfect sense to replace it. But maybe that wasn’t the only leak? Maybe that was just the biggest leak.

The question is, How are they approaching the problem now? If they have good diagnostic software, it should be suggesting next steps for them in terms of what to test.

But if they’re just guessing, that could go on until the next season of “Game of Thrones” is released – there are a lot of individual parts in the evaporative emissions system.

It could be a bad hose that’s running from the evaporative canister to the fuel tank. It even could be something as simple as a bad gas cap (let’s hope they tried that already). Or it could be a bad computer.

If you like these guys, you can say, “Hey, fellas, I think you owe me some free diagnostic work here, considering I already helped you make about two months’ worth of boat payments.”

And if they’re willing to keep trying stuff for free until they nail it, then you can give them some more time. I’d suggest they start by doing another smoke test.

If they still can’t figure it out, and they’re really good guys, they’ll take it to the dealer for you and get it diagnosed, and then fix it.

But if they’re unwilling or unable to continue, then I would take it to the dealer. They’ve worked on many more ES300s than your local mechanic has, and they may know what oddball problem tends to set off a check engine light in cases like these.

Good luck, Fil.

Got a question about cars? Email Car Talk’s Ray Magliozzi by visiting the Car Talk website, www.cartalk.com.