check engine light due to gas cap being loose or worn,is what i was told. on 1992 Lexus LS400

check engine light is on .had it diagnosted I was told the gas cap was causing problem,I changed cap and light is still on.

2 answers
It takes two "drive cycle trips" to "turn off" the light. The computer looks at certain criteria before it will naturally turn off the light, if not a scan tool is required to turn off the light.
ventilation line restriction in gas line.