EXCESSIVE FUEL CONSUMPTION-Lexus LX300

Hi, everyone.
A friend of mine is got an RX 300 Lexus truck, 2007, V6, with 107,524miles on it. Ever since he bought the car as a used one, it has being consuming excess fuel. Though, there is no black smoke coming out. We have tried scaning the car without success, because the CEL only displays on the cluster intermttently. We have tried checking for vacuum leaks, but got nothing. The plugs, air filter, vacuum pipes, exhaust pipes and catcon are OK. Although, the MAF sensor thermistor was very dirty, and has being cleaned. I am awaiting feedback from him, what the effects of the cleaned MAF sensor had on the car's fuel consumption.
What do you guys think could be the problem?Why won't the check engine light come on, so that we can scan it?
Appreciate your helps in advance.

scan for codes. the check engine light rarely comes on for no reason. See what codes may be stored in memory. you may need to try a different scanner. Some code readers won't show pending or one trip codes. They will only show hard codes.

excessive fuel consumption is ambiguous. what has the miles per gallon that you believe is excessive? Fueleconomy.gov says that the 07 rx350 4wd was rated at 19 mpg combined average. I know that many people don't get the same mileage as the reported fuel economy numbers because of their driving conditions and driving styles but how far off are your consumption figures compared to the new ratings from back in 2007?

If there are no codes or any scanned values that are out of the ordinary, the computer is programmed and calibrated to give you the best of performance and mileage within EPA guidelines.

Use a scanner and drive the vehicle under your normal conditions and look at you fuel trims. Some O2 sensors, when they get older, will slow down their reaction times. They get some residue built up on them or may even get contaminated. Catalytic converters sometimes get clogged with debris, damaged or contaminated and reduce their flow. A dirty MAF will usually show up as a lean condition on the back end because of reduced airflow readings.

Thanks for the reply. We have tried scanning the car, but the check engine light won't just come up. It comes up intermittently.
So, what do we do now?

try a different scanner, try scanning for mode 6. what kind of scanner are you using?