Code Po121 Issues

[hmrken68]

i have a 2008 silverado....i keep getting a code po121. the code only comes up when i let the truck in park. there is no performance problem. i have an aftermarket intake with k&n filter..also i have been able to turn the ignition off unplug the MAF sensor when the code comes up and the check engine light goes off....i was told by the local chevy dealer that code was due to the aftermarket intake and it would not be warranty ro fix..they also said it was because of the oil used on k&n filters, only i dont have oil on my filter and the MAF sensor is clean and free of any debris....any help would be wonderful..is there away to keep the code from coming up or does the intake need to be replaced with a stock one