P1518 Chevrolet -

Question

2003 Chevrolet Tahoe is showing this code: p1518. It runs ok until the defrost is turned on. Then it goes into Reduced Engine Power Mode until I turn it off. When I restart it, its out of the mode. It hasn't done it all summer until I had to use the defrost driving after dark. When its cold and I use the defrost it does it quite frequently. Have any suggestions?

Thanks,
Jeri Hall