my engine misfires only when temperature outside is under 40 degrees.

You need to retrieve the trouble codes NOW. Do not drive truck untill you get code/codes. Otherwise you may have the expense of catalytic converters. That is much less expensive than going to a parts store to get the code/codes free.

the check engine light is from the missfire. you need to check the plugs, wires, injectors, coolant sensor and check for vacuum leaks. Roy