You own a high end luxury vehicle as do I, 2005 ES330, the dealership is the best place to get Lexus problems fixed. With my own vehicle, I had a thermostat fault code come up, and like you it would reset itself and come back. The code said it was the thermostat and I queried it with my tyre guy. He also said it is a thermostat code. I said but it isn't overheating. He told me that the computers in Lexus are so clever they are actually looking for the water to reach x degrees in x minutes. He replaced the thermostat, the fault never came back. Anyway back to your P0172 Error code.
Faulty front heated oxygen sensor
Faulty fuel injectors
Exhaust gas leaks
Incorrect fuel pressure
Faulty Mass Air Flow (MAF) sensor
The first thing I would try is that stuff that you add to your fuel to clean the injectors.
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> ... > IS Models > IS - 2nd Gen (2006-2013)Jul 6, 2013 - IS - 2nd Gen (2006-2013) - Pull codes P0172
/P0175, need help diagnoising - My
check engine light, VSC, and traction light all lit up yesterday. I pulled ... Title: 2GR-FSE ENGINE CONTROL SYSTEM: SFI SYSTEM: P0171,P0172
,P0174,P0175; System Too Lean (Bank 1); 2006 MY
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Check engine light Code P0172
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Jun 4, 2014 - Uploaded by RomanAutomotive
A common cause for
lean condition codes
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code p0172 on my 2005 Lexus GX470 Google Search