1998 Lexus LX 470

Engine Performance problem
1998 Lexus LX 470 V8 Automatic 43370 miles

we got a lexus that came in with the check engine light on with DTC P0120 and P0121 but when we restart the car, the check engine light is gone and the DTCs are no longer in the current DTC list. So we decided just to clear the codes. A month later, the same car comes back with the same problem. The owner said that the check engine light would sometimes light up and then disappear. We checked the sensors and wires and there seems to be nothing wrong with them. Has anyone encountered this kind of problem?
March 1, 2009.

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