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Lexus RX behaving poorly

I have a 2000 Lexus RX 300 with 65K which has recently had its check engine light go off. Like a good owner, I took it to the local Lexus dealer where they promptly told me it was a failed knock sensor, costing $820 to repair. I agreed to the fix only to have the check engine light go off again 2 days later. Back to the dealer, and this time they read a code of the second failed knock sensor (on the other side of the engine) and an air/fuel sensor. They wanted $1100 to repair both, saying there were no assurances that this was indeed the problem. Both times the check engine went on, it was the exact distance from our house in nearly the same spot, both times in the morning at about 50 degrees, at about half a tank of gas. Is there something else going on here, or should I keep replacing sensors until I’m in the poor house? I suspect the car might not be getting enough fuel? In any case, there car has not been running rough as a result of any of this, just check engine lights going off. Please help!