If you changed the spark plugs and wires on a 1994 Ford F-150 4.9 but now whenever the truck warms up the check engine light comes on what could be wrong?

the famous 300 , 6cyl

Just a couple of thoughts, This could be completely coincidental as it was just an ignition tune-up

1st . Try and unhook your battery for 2 mins and clear any stored trouble codes in ECM, as you said if comes on after N.O.T. (normal operating temp) and system runs closed loop so all sensors and controls are activated 2nd . Double check your new plugs for correct heat range, I have heard sometimes if range is off it could throw off your o2 sensor as the engine doesn't run effeciently or burn correctly. 3rd . Again might be coincidence that light is on, if your truck is running the same it's not a hard code and truck is not in "limp" mode running like crap.

if you plan on keeping your truck consider buying a code reader from a auto parts dealer, just a shot in the dark but hope it was of some help

Jason