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You did not state how many miles were on your vehicle. It may be almost due for a plug change.
Often a poor running engine is noticed because the transmission does not appear to shift properly when actually it is just reflecting the poor running engine as it shifts. I learned this from my late uncle who was a professional car mechanic and owned his own shop for over 40 years. So, the dealer may be correct but it would not hurt to have someone else run your engine on a diagnostic machine to see if it is firing correctly. I know the replacement spark plug cost is high, but this engine is used in so many Toyota/Lexus vehicles that many independent shops are very familiar with replacing its spark plugs. They will be less expensive than the dealer.
If you need to find a good shop in NH I suggest you try this link to the shops recommended by listeners of the Car Talk radio program ( the one with Click and Clack) https://www.cartalk.com/mechanics-files
The engine runs very smoothly at all speeds which made me doubt the plugs, which I did have changed. Just found out it was trying to be in 2 gears at the same time( does that make sense to anyone?) had tranny replaced because I'm going on a trip and didn't want to break down on road. There is 135,000 mi on it and I hope to go at least 80 -90 more. Thanks for the input.
I did go back to dealer because if something happens on the road at least I can go to a Lexus dealer and they will honor the 1year, 12,000 mi. Warranty. It's in a national database. It is a rebuilt tranny and I paid ( I hope you're sitting down) $ 6,300. I almost threw- up when he told me. Everyone tells me they screwed me, but the car runs great, rides great and I really like it. $ 6,300 isn't even a down payment for a replacement and I don't have car payments, so I bit the bullet and went for it, just hope it pays off.
I'm sticker shocked by the repair cost of your Transmission baily. My 2007 IS350 is currently having issue with the transmission not shifting out of 3rd gear and I'm looking for a more affordable solution. The current value a working 07 IS350 is as much the cost of fixing the transmission at the dealer!