'97 Tahoe with SES light but no codes

A friend of mine has a '97 Tahoe 5.7L mostly stock with a K&N intake. He has been having a problem with his truck when it sets outside in very damp conditions and the engine is cold. It will turn on the SES light and run rough like it has a miss fire. He always checks for codes but none show up. After the engine warms up it runs fine. Also if the truck stays inside or it is dry weather it runs fine. He has checked the distributer and the plug wires and all seems fine.

Any ideas would be appreciated

old plug wires will act up when the weather is rainy and alot of moisture in the air.but will act normal in dry weather. bet a set of plug wires will cure it.

If the service engine light is on there should be a code past history or present.
K&N intake can give you a lot of problems to.
Find a good engine capable scanne and check it out.