I have a 92 305 tpi/auto combo and i just installed a new vehicle speed sensor, water pump, hypertech fan switch and a mr.gasket 180* t stat. my problem is when im crusing down the highway doing 70ish cars at full temp ,first line on the gauge, its about 72* outide and then my service engine light comes on. ran great but the light came on. Then I was driving home from work @ 1030 pm i had the heat on cause it was 55* and it never came on. i am gonna check my code this weekend. i think i need a new chip but i don't know. thanks.

 

Check the codes then repost, I don't see anything in your post that would require a new chip in regards to an SES light.

 

my formula does the same thing. i do not have a scanner. i hope its not the egr