service engine soon light

why does my service engine soon light come on when i go over 65mph on the freeway? this is the only time it comes on. everything seems to be fine. does anyone have any ideas?

My 96 sierra used to do the same thing , only come on when on the highway......turned ouyt to be a bad O2 sensor

well my o2 sensor is brand new... any other i ideas?

well my o2 sensor is brand new... any other i ideas?
Does the light go out by itself or do you have to reset it?
Sometimes a loose gas cap will make that light come on.

What year is your truck. My 88 had 1 O2 sensor, my 96 had 3 and my 2000 has 4.

mine is a 90 and i think it only has one o2 sensor... the light stays on until i turn it off for a sec and when i turn it back on its off....

Possibly the EGR valve has insufficient flow?

Also, about the O2 sensor, new doesn't always mean good... I've got more than one bad part in a new box.

im going to assume that everything that has to do with smog is in good working order because i just had to fix it to pass... wich it did with flying colors.

kenny there are 100s of cars with one o2 what are you talking about

my 88 had the same problem and it ONLY showed up when I drove over 55mph. It turned out to be the Vehicle speed sensor. I don't know why this did the trick, but i realized that it only did it after I changed the tranny. It still worked but it wasn't quite right.

How about you pull the code..this will give you a wealth of information.

ok. where is the speed sensor? and how many do i have? and what do you mean by pulling the code? how do i do that

On a 1990? You take a little piece of wire (about 2-3 inches) strip ends, bend in a U, and jumper the dianostic terminal. turn ign. to ON, CES lite will flash 12 three times (I think) thats the "handshake" or system ready code, then it will start giving trouble codes in morse code (three times for each bad code) then it will finish with the same #12 three times. Example 12,12,12 32,32,32 12,12,12 that would be one trouble code #32 you look up 32 and see EGR malfunction...etc. It's been awhile since I had a OBD 1 ,so someone back me up please.