99 explorere

helping a freind out, changed the spark plugs with bosh platnums, I think the p lugs were original, 105k on the motor. changed the air filter.
The truck keeps giving a lean left bank error on the odb11 check engine light.
I checked all vacumn lines and everything I can. they all looked great.
Anyone have any ideas, I'm leaning towards the rubber o-rings under the y looking plastic intake plenum.